revised 8/21/07

Moorhead tubes have long been popular with collectors despite the limited and incomplete data available on many types. This article will attempt to describe all tubes made by Moorhead, including electrical specs, and offer a few insights into the nature of this early electronic entrepreneur. Since many Moorhead tubes have serial numbers it was decided that a survey of characteristics related to serial numbers could be useful, and such an ongoing survey was made with over 350 tubes examined and logged thus far. In many cases the survey data made it possible to define approximate production quantities of serialized types.

Otis B. Moorhead was born January 9, 1893 in San Francisco. Not much is known about his early life but he claims to have started experimenting with wireless by the age of eight.[1]. While in his teens he held various positions in wireless telegraphy both on land and sea. In 1914 he was a radio inspector for Marconi Wireless and worked with a Major Allen Forbes, then an assistant inspector for Marconi.[2] As one of the perks of being an inspector, Forbes mentions that they would often eat all the fruit in the ship's radio shack during their inspections.

In 1915 Moorhead worked evenings in the DeForest booth at the Panama-Pacific Exposition selling audions and saw them as an opportunity to make money. DeForest charged high prices for his audions and only sold them as part of his detector units. He told Forbes that he was going to manufacture tubes, copied after DeForest, and that DeForest was such a poor businessman that he would never sue them.[3] Forbes put $200 into the venture, and $500 came from a man named Kendrick. It's not known how long Forbes and Kendrick remained in business with Moorhead, but they certainly provided the capital to help get him started. An office was set up on the 4th floor on 650 Mission Street in San Francisco with a P.O. Box for an address.[4] Apparently Moorhead was more concerned about a suit then he admitted to Forbes as some early sales were made through a Real Estate office to hide his location.[5]

Also working in the DeForest booth at the 1915 Exposition was Elman Myers, a DeForest employee at the time. Myers was later to become known for producing the RAC and Myers tubes, also without benefit of a DeForest license. Forbes mentions that Moorhead probably met his wife, Barbara Myers, while working at the booth.[6] The implication is that Barbara Myers was related to Elman Myers, perhaps his sister.

According to B.F. McNamee, a former chief engineer for Moorhead Laboratories, Moorhead and E.T. Cunningham collaborated on the initial design of a gas detector tube.[7] This occurred in early 1915, but Cunningham did not advertise his Audiotron until October of 1915.[8] They soon went their separate ways, but this early union explains the great similarity between the Moorhead tubular and the Cunningham (Audiotron) tubular. This early affiliation is mentioned in the first Moorhead ad[9] of July, 1916, which states in part: "The former manufacturers of the Audiotron are now manufacturing a new and better Tube."

Moorhead and Ralph Hyde, a former G.E. lamp factory superintendent, produced the first Moorhead tube, the tubular Electron Relay, in 1915. The production facilities were crude, the exhaust manifold only handling 10 tubes that would be on the pump all day.[10] This tube was not advertised and the exact details of construction not known, but presumably was similar to the Electron Relay advertised the next year. Forbes states that sales were good but tube quality poor. Enough of these tubes were sold to attract the attention of DeForest, who brought suit in early 1916 seeking an injunction against Moorhead to stop manufacture of the Electron Relay. Since the validity of the DeForest Audion patent was being challenged in the courts by Marconi, an injunction was not granted but a bond was required as a condition of continued production until such time as the Audion patent dsipute was settled. After posting the required indemnity bond, the first Moorhead ads appeared. It should be pointed out that at this time Moorhead was only 23 years old.

Fig. 1

Fig. 2

In August 1916 the first ad to picture the Electron Relay ran in The Electrical Experimenter.[11] This tube was claimed to be an improved version and could be had with either single or double filament. A double filament type is shown in Fig. 1. As far as is known this tube had no marking, and as such has been very difficult for collectors to identify due to its similarity to the Audiotron. However, examination of many Moorhead and Audiotron tubulars reveals some distinct variations in construction. The key difference is shown in Fig. 2, a close-up of the press area. The top tube is typical of Audiotron, having a narrow press while that on the bottom has the wider press of Moorhead. Another key to this early tubular is the overall dimensions. The Aug 1916 ad contains a large clear photo of the tube, and with the aid of calipers a length to width ratio of 5.24 could be measured. The tube in Fig. 1 has a ratio of 5.27, a near exact match. Later Moorhead and Audiotron tubulars have a typical length to width ratio of about 3.7, a considerable difference. The one good filament in the tube shown drew 1.1 amps at 4 volts, higher than the typical .94 amp draw of later tubulars. At this time Moorhead was advertising under the name of Pacific Research Laboratories.

Fig. 3

Fig. 4

The September 1916 ads showed a new tube, an an external grid type called The Moorhead Tube,[12] shown in Fig. 3. This tube usually had the Moorhead name in a thick disk plate and was only made in a single filament version. One example has been seen (Fig. 4) with a thin unmarked plate. Samples seen had paper labels with the notation "cells" and a handwritten number or number range, apparently a factory test of optimum plate voltage. Apparently Moorhead decided to side step the DeForest patent issue with a non-infringing design. However, his advertising told a different story. In a sales brochure for The Moorhead Tube he states that new designs were necessary to improve upon his Electron Relay. He also said that after a period of use the vacuum changed and caused a lack of sensitivity. He also acknowledged that the Electron Relay was not stable in operation, a trait shared by all early gas detectors including the DeForest Audion. Another interesting statement was that when the first filament burned out a gas was generated which reduced the sensitivity of the tube.While this construction avoided the DeForest patent, performance suffered. Even so, Moorhead claimed in advertising brochures[13] that this new tube reduced static and had increased sensitivity.

Fig. 5

Fig. 6

Fig. 7

The external grid tube soon faded from the scene and January of 1917 saw the introduction of the new Electron Relay,[14], again "improved" over earlier models. The arguments given for the change to the external grid tube were conveniently ignored. This new tubular was similar to the 1916 version but was somewhat shorter and larger in diameter (Fig. 5). ER was stamped into the plate, and is the earliest Moorhead that collectors are likely to encounter today. This was the first of the Moorheads to have a serial number etched into the glass, but only about 15% of the samples seen were actually serialized. Unnumbered tubes usually had a black or red paper label, while the serialized ER's have MOORHEAD etched in the glass. Available facts indicate that the serialized ER's are most likely post-war production. See the section on the Marconi/Moorhead Type D for more information. The 1917 ER was normally supplied with a dual filament, but a few have been seen with a single filament. Fig. 6 shows such a tube. The large picture shows a close-up of the press area. The second filament support lead has been cut off flush at the press and only two leads have been brought out. No advertising has been seen indicating the availability of a single filament version. An interesting variation of the ER is shown in Fig. 7. This tube has a candelebra base and ER stamped into the plate. This candlebra based tube has also been seen without the ER stamped in the plate. About this time the company name was changed to Moorhead Laboratories.

The image of Moorhead at that time was that of a young ambitious huckster, knowingly infringing the DeForest patents and given to making grandiose claims for his products. A paper presented to the IRE[15] in February of 1917 indicated a reasonably good grasp of the mechanics of tube construction, but little understanding of the importance of element sizes and spacings. His position with respect to the DeForest patent was similar to that of Cunningham. He argued [16] that since the DeForest Audion patent specifically stated that conduction in the DeForest device was through a gaseous medium, high vacuum triodes were not covered by the Audion patent. He claimed that his tubes were highly evacuated and conducted via electron flow through a vacuum. Perhaps he tried to achieve a high vacuum, but all tubular ER's tested by the author showed some gas content. In a later paper for the IRE[17] he freely stated that the Electron Relay "belongs to the class of detector tubes depending on the presence of traces of gas for their action".

The entrance of the United States into the war brought manufacture of tubes for amateurs to a halt but gave Moorhead a chance to achieve some legitimacy for his operation. Under government protection against patent infringement suits, Moorhead contracted with both the British and the U.S. to supply tubes for the military. It is interesting to note that Cunningham, the other major unlicensed tube builder of the period, did not obtain similar government contracts despite the fact that the Cunningham Audiotron was being sold in much larger numbers than the Moorhead tube.

The tubes that Moorhead contracted to supply to the military were much more sophisticated than the relatively crude unbased tubulars he had been making. These new tubes were based and required to meet quite stringent mechanical and electrical specs.[18] The specialized machinery required was mostly not available from normal commercial sources. The bulk of this equipment had to be designed from scratch or improvised by modifying existing devices. Many of the operations required rapid completion of processes that previously had only been accomplished by tedious laboratory methods totally unsuited for mass production. Moorhead is given credit for much of this equipment design[19] as well as training methods necessary to properly use such equipment. In addition these tubes were required to be high vacuum devices and had to be evacuated to a much higher degree than the tubular detectors. According to Radio News, Moorhead held patents on a chemical exhaustion process[20], and the military tubes were the first Moorhead types to be gettered. However, a search of the early patent records did not disclose any patents held by Moorhead or his company related to chemical exhaustion. Perhaps he only held a patent application which was not granted for some reason.

Fig. 8

The British Army had been using a hard receiving valve known as the type R that was fashioned after the French type TM. The TM had been designed in 1915 for the French Military and was probably the first mass produced tube. Over 100,000 were made before the end of WWI. The four pin base used on the TM was utilized by the British, and later became known as the European B4. Moorhead supplied the British with a quite similar tube with spherical bulb and British base.[21]. The major difference was in the plate structure, which was vertical in the Moorhead version.(Fig. 8) Moorhead later stated that he made the R with a vertical plate rather than the horizontal British style because he had not been supplied with drawings during the design, only specifications which did not show this feature. The bulb was marked MOORHEAD R with a serial number and usually had a light gold getter. The filament ran at a nominal 4 volts and .7 amps, and was pure tungsten at .061mm in diameter. The tube would handle 400 volts on the plate for test. Serial number data indicates that around 45,000 or so were made.

Fig. 9

Fig. 10

A transmitting tube was also made for the British Army (Fig. 9), marked MOORHEAD B.[22] The B was similar to the R but used a horizontal plate and had a grid of 22 turns, twice the turns used in the R. This tube could handle about 500 volts and had a notch in the press between the plate and filament leads, probably to increase the internal breakdown voltage. The grid was molybdenum rather than the nickel used in the R tube and the filament was run at 6 volts, drawing .85 amps. The filament was .058mm in diameter and contained a small percetage of thorium. The bulb showed a light gold getter. This tube had a spherical bulb, British base, and unique marking, a circle with a large B in the middle. MOORHEAD was marked around the inside upper arc of the circle and SAN FRANCISCO was marked around the inside lower arc. Under the B in small letters was written USA. Under the circle was a serial number. The sample shown here has a paper label around the lower base which states that the tube was licensed under the Fleming patent. Serial number data suggests that about 20,000 of these tubes were made, but the British B seems to be a exceedingly scarce item today. A variation with a Shaw base is shown in Fig. 10. This Shaw based version is unmarked and is perhaps an electrical prototype.

Fig. 11

This same internal design was used in another tube marked MOORHEAD VT 32. This tube was identical to the B except for the bulb, which was a tall cylindrical style (Fig. 11). For the British, the VT designation would normally indicate a tube made for the British Air Force. However, the number VT 32 would not seem to fit into the WWI British number sequence so the actual usage is unclear. The US Army number VT-32 was unassigned and apparently reserved in deference to the British usage of an American produced tube. The VT 32 tubes usually had light gold getters and carried no serial numbers. Some VT 32's have B marked on the press. The lack of serial numbers makes production estimates difficult, and the production quantity is unknown. One magazine article [23] stated that Moorhead had made 50,000 tubes for the British during the war. Known production of the Types R and B would account for that total. However, since there seems to be more surviving samples of the VT 32 than the R or the B, it is the authors belief that the VT 32 was made after the end of the war and thus not part of the 50,000 . Moorheads own brochure of 1919 shows both the R and the B but makes no mention of the VT 32.

Fig. 12

The other wartime tube was the S.E. 1444,[24], made for the U.S. Navy. This tube was marked MOORHEAD, S.E. 1444, S.F. CAL, and was serialized. The design was essentially the same as the British R except for the Shaw base and cylindrical bulb. A gold or reddish getter was normally used, but some were made with clear bulbs (Fig. 12). There was no marking on the base or press. Logged serial numbers indicate a total production of about 25,000 tubes. In a San Francisco newspaper article[25] dated Nov. 16, 1918, Moorhead said that he was about to leave for Washington to close a deal with the government for an additional 75,000 tubes. The reality of this sale would seem to be confirmed by an article in Pacific Radio News [26] for March 1920 which stated that Moorhead had supplied the US government with 100,000 tubes. A June 1920 article[27] in the same publication said that Moorhead was at that time building transmitting tubes for the Army. These statements were probably just the result of company exaggerations, as there appears to be no known samples with appropriate serial numbers to begin to confirm such a total.

Despite the wartime contracts the company was in financial trouble and Moorhead admitted to being in debt to the tune of $100,000 plus in November of 1918.[28] Moorhead blamed this condition on his business associates and his lack of involvement in the business side of the company. He reorganized the company and took control of the financial affairs as well as the production aspects. Armed with patents or patent applications he held pertaining to a chemical exhaustion process, he negotiated a complicated set of agreements with Marconi and DeForest. Under these agreements, finalized in June of 1919,[29], Moorhead would manufacture tubes to be marketed by Marconi, and Moorhead became the only legal manufacturer of receiving tubes for sale to the public in the United States. This was quite an accomplishment as only three years earlier was being sued for patent infringement.

Fig. 13

Fig. 14

Production of these new tubes started about September of 1919. One of the tubes made for Marconi was the Moorhead type A,[30], also known as the Marconi VT Class II. This was a hard tube, near identical in construction to the S.E. 1444. (Fig. 13) Some of the early A's were marked MOORHEAD AUDION, SAN FRANCISCO, but most were marked MOORHEAD LABORATORIES, SAN FRANCISCO. Most were also marked A and all had serial numbers. In addition, some were also marked PATENTED NOVEMBER 7, 1905 and/or SOLD ONLY FOR AMATEUR AND EXPERIMENTAL USE. The early A's had numbers up to about 6000, and no base marking. At this point the serial number jumped to 200,000 and about half of these later A's had Marconi and DeForest markings stamped into the base. Contrary to stated information, most A's, including the very earliest, did not have the class number ink stamped on the base. The A marked on the bulb was the normal designation, but a small number of tubes had CLASS II stamped on the base rather than A on the bulb. In some cases a paper label stamped CLASS II was attached to the bulb. The A can be found with both clear and gold bulbs, and many were marked A or 2 on the press. Some A's, about 15 to 20%, had spherical bulbs (Fig. 14). Despite higher figures quoted elsewhere, the serial number data would indicate a total output of only about 15,000 tubes.

Fig. 15

Fig. 16

Another tube produced for Marconi was the Moorhead type B[31], also known as the Marconi VT Class I. The B was mechanically the same as the A but employed a low vacuum and was used as a detector.(Fig. 15) The performance characteristics of this tube were somewhat similar to the RCA UV200 introduced in late 1920. Critical adjustment of both filament and plate voltage was required to obtain a sensitive operating point. Tests run by the author on several samples showed the optimum filament voltage to vary from 3.9 to 4.6 volts. By using a high voltage "violet-ray" device the gas in the tubes was made to fluoresce and gave a light green glow. This color indicates the presence of argon, the same medium used in the UV200. The bulb marking details are the same as listed for the A, except for the use of the B designation. The bulbs were all clear, and almost none of the B's had the Marconi/DeForest logo stamped in the base. Many B's had 1 or B marked on the press, and like the A some were made with spherical bulbs (Fig. 16). However, the percentage with spherical bulbs is quite small, on the order of 2%, and therefore are rather rare. Also like the A only a small number had the class number stamped on the base rather than B on the bulb (Fig. 15). The total number made was about 20,000. For some reason more than twice as many A's have survived to the present than B's.

Fig. 17

Fig. 18

A transmitting tube made under the Marconi agreement was known as the Moorhead Type C. This tube was electrically the same as the British B but used a vertical plate assembly with the same 22 turn grid (Fig. 17). The notch in the press used on the British B was not used on the Type C. On one sample examined the bulb was gold and marked C MOORHEAD LABS S.F. and carried a serial number. The press was marked C and the base was plain. A meaningful production estimate for the Type C was not possible, but based on the very small number of surviving samples the quantity must have been rather low. No advertising for the Type C has been seen, but a full description is given in an April 1921 paper[32] presented to the IRE authored by Moorhead and E. C. Lange. After the Marconi arrangement was terminated this tube was replaced by the Type TT. A variant of the Moorhead C was the Moorhead Special (Fig. 18). This tube is mechanically the same as the C. The sample shown here has a clear bulb and no serial number. The bulb is marked MOORHEAD SPECIAL. The press is marked C and the base has no marking. No information at all has been found for this tube, but the lack of a serial number suggests that it may have been an experimental or prototype version. Tests run on both the British transmitting tubes and the US versions showed the British tubes to have significantly higher Gm, see the electrical data at the end of the article. However, since only one tube of each type was available for test these measured differences may not be indicative of the overall production. In the 1921 IRE article Moorhead discussed these tubes as being electrically equivalent.

The Marconi Type D[33] was to have been the same as the earlier unbased Electron Relay but no samples so marked have been seen. However, various facts suggest that those unbased Relays with MOORHEAD and a serial number etched in the glass might be the Type D. A check of a couple of these serialized tubulars revealed that the filament current had been reduced from .94 amps to .65 amps, about the same as the based tubes. However, it is also possible that these serialized ER's are the tubes briefly advertised in May of 1920 after the Marconi license had expired.

Marconi Types E, F, and G[34] were to be the same as the A, B, and C but having spherical bulbs and British bases. No samples of these tubes have been seen, and no advertising has been found. Since the American market for British-based tubes would have been almost nil, the presumption is that if any of these tubes were produced they were made for export to England. It is likely that these types were never put into production. Moorhead had serious quality problems in the manufacturing of types A and B, with Marconi complaining of high reject levels at their incoming inspection. It probably required all of Moorhead's resources to try and meet the production schedules for the US tubes, with nothing left for British based tubes.

In the paper delivered to the IRE in 1921, Moorhead is listed as General Manager, and Lange is listed as Chief Engineer. The technical expertise shown in this joint paper is far beyond that of the 1917 paper. It is likely that this expertise came mainly from Lange or other engineers rather than from Moorhead himself. Lange's tenure as Chief Engineer was apparently a brief one as B. F. McNamee claims that he was Chief Engineer from 1920 to 1922.[35].

1920 was not a good year for Otis Moorhead. In January Marconi notified Moorhead Labs of the cancellation of their license, effective July 1920. The financial position of the company was very poor, and the majority stockholders voted to reorganize. As a result Henry Shaw became President and DeForest acquired a significant interest in the company.[36]. Moorhead was kept on as Chief Engineer, but had to assign his patents to the corporation. Thus Moorhead was now just an employee of the company he had founded. Given that during this period Moorhead had absolutely no legal competition in the receiving tube field, it is difficult to understand the poor financial showing. Newspaper and magazine accounts[37] of the company reorganization used terms like "chaotic" and "very badly messed up" to describe the company financial status. It would appear that like many entrepreneurs, Moorhead had more talent as a pitchman than as a solid businessman. However, it should be remembered that tube manufacturing was always something of a black art, an enterprise where minute and sometimes unmeasurable differences in materials or processes could cause seemingly inexplicable problems in the final product. It was not uncommon even in modern times for a major tube company to run off hundreds, perhaps even thousands of useless tubes from a new line in order to debug and fine tune all the parameters involved. This was an expensive and time-consuming process, and Moorhead had neither the money nor the time in his particular situation to fully optimize his product before the stockholder action described.

It should be pointed out that due to the Marconi license cancellation in July of 1920, the only true Deforest-Marconi-Moorhead tubes were the types A through D already described. Many collectors have been confused on this point because many of the these types did not carry the Deforest and Marconi marks, while many of the later types did have those legends even though they were not legally entitled to be so marked.

Fig. 19

The last Marconi VT ad ran in April of 1920. In May new ads for the reorganized Moorhead Labs announced the Moorhead VT.[38] Also in May some ads showed the old tubular Electron Relay,[39] while other ads listed, but did not picture, the DeForest Type 20 detector.[40] The Type 20, if actually sold, remains a mystery. The tubes shown in Saga of the Vacuum Tube as being Type 20's are now believed to be the DeForest DV6 and DV6A of 1923 vintage. The DV6 can be found with two types of marking. The earlier style has DeForest etched into the top of the bulb and PAT APL'D FOR on the base, see Fig. 19. The later version has no bulb marking and has DEFOREST AUDION and patent numbers on the base. There has been some speculation that the tube with the earlier marking is in fact the Type 20, and this theory seems quite plausible. However, no hard evidence has been found to prove or disprove this assertion.

Fig. 20

Fig. 21

The Moorhead VT (Fig. 20) was identical to the Moorhead/Marconi A of 1919 and the VT serial numbers continued on in sequence from those used on the A. About 85% had gold getters, the balance being clear. The first 3000 or so were marked MOORHEAD VT. The next 13,000 were erratic in marking. The later VT's were generally marked VT and LICENSED FOR AMATEUR OR EXPERIMENTAL USE ONLY. About half of the VT's had either A, 2, or VT on the press. Despite the fact that the Marconi license had expired, more than half of the VT's carried the Marconi-DeForest logos on the base. About 40,000 of the VT tubes were sold. A small number of VT's have been seen with nickel shaw bases of the type used on the early DeForest DV-6. These tubes had no markings and were probably prototypes or experimental versions.

A new tube was announced in June of 1920, the based Electron Relay.[41] The ER was a gas detector, and early versions had an internal structure much like the tubular ER. The same type spiral copper grid and aluminum plate were used, the only significant difference being the tabs extending down from the plate and bent against the press (Fig. 21). In this construction the filament and grid were attached only at the press, leaving the other ends free to move if jarred. About 4000 ER's were built this way. To improve this condition a mica spacer was added at the top of the plate to help anchor the filament and grid. An additional 7,000 tubes were sold with the mica spacer. All of these tubes had clear bulbs and serial numbers, and were etched ER on the bulb but curiously did not carry the Moorhead name. There was no marking on the base or press. All previous tubes described used platinum leads in the press, but the based ER used dumet for the filament leads and platinum for the grid and plate leads. A small number of these ER's can be found with all platinum press leads.

Fig. 22

Fig. 23

At this time the ER was modified. The plate and grid were changed to nickel and their height was reduced to half the old dimensions, the plate diameter staying about the same. The filament was changed from a straight wire to a self-supported hairpin type. The mica spacer was eliminated. While this new design may have reduced the manufacturing cost, it was a step backwards in mechanical stability. The elements, especially the filament, were once again tied only at the press resulting in a very microphonic situation. The first 500 or so used four and one-half grid turns and some had the plate supported at one side only (Fig. 22). The balance of the production had seven grid turns and plate supports at both sides (Fig 23). The first 25,000 came with either shiny or black plates and were marked ER on the bulb. The rest had black plates and mostly carried the standard license notice in addition to ER. None had the Moorhead name or anything on the press. The first 13,000 had plain bases, but the remainder had the Marconi-DeForest logo. About 40,000 of the short plate ER's were made. Filament current of the based ER was inconsistent. The six samples in the authors collection varied from .62 to 1.03 amps at 4.0 volts. Like the earlier Moorhead/Marconi Type B all six samples fluoresced light green under high voltage stimulation, indicating the presence of argon. The degree of fluorescence varied considerably between the samples. Because of the age of these tubes it is not possible to say whether these differences are due to original manufacturing variations or partial adsorption of the argon over the years.

During the reorganization of the Moorhead Laboratories two distributing companies were set up, the Atlantic Radio Supplies Co. and the Pacific Radio Supplies Co. Starting in August of 1920 the tubes were advertised as the A-P Electron Relay and the A-P Amplifier-Oscillator.[42]. The cartons the tubes were packed in were marked A-P but the tubes themselves never had an A-P logo.

Fig. 24

In December of 1920 the A-P Transmitting Tube was first announced,[43] although it appeared in parts supplier ads as early as September of that year. Very similar to the VT in appearance, the Transmitting Tube (Fig. 24) was the replacement for the Type C made for Marconi. This tube was initially only licensed for use in DeForest equipment, and rated for 12.5 watts "capacity". Later ads claimed it was capable of 5 watts output. The restriction to DeForest gear was eventually lifted, (or ignored) as later ads[44] mention usage in Magnavox amplifiers. The Transmitting Tube was marked TT on the bulb, along with a serial number and usually a license notice. Some were marked 2 on the press, and the base carried the Marconi/DeForest logos. Like the earlier transmitting types a molybdenum grid was used. However, examination of two TT tubes revealed that unlike the earlier tubes an 11 turn grid was used rather than the usual 22 turns. Serial numbers on the TT tubes are inconsistant. Two samples are known with 400,000 series numbers, and two others have 200,000 series numbers. The serial number spread of observed samples was not large enough to define a production quantity, but a number in the hundreds might be a reasonable estimate.

Fig. 25

The influence of DeForest can be seen in the next type to be released, the A-P rectifier. First shown in May of 1921,[45] ads pictured the rectifier as being nearly identical to the DeForest "Singer" power tube, except for the omission of the grid. The author can find no evidence that the A-P rectifier was ever made quite as shown in the ads, and all samples seen take the form of the tube shown in Fig. 25 From a design standpoint a tube with the large filament-plate spacing of the Singer tube would be almost useless as a rectifier (at its rated voltage) due to the very high forward voltage drop that would result. This tube had a clear bulb marked only with a serial number. The base generally had DeForest markings only, but some samples have been seen marked both DeForest and Marconi. The press had dumet leads and was not marked. No published data on filament ratings could be found, but tests on several samples indicated that about 8.5 to 9.5 volts was needed to obtain the rated output current of 75 Ma. An estimate in the low thousands is made for total sales.

Fig. 26

Fig. 27

Shown in Fig. 26 is the Moorhead Series "D" Electron Relay.[46] The Series "D" has been seen with both a candelabra screw base and a four pin metal base (Fig. 27). However, two four pin versions examined by the author may have been rebased, so the candelabra base may have been the only type supplied by Moorhead. This tube had an internal structure very similar to the DeForest CF-185 except for the one piece plate. The filament rating (6 volts, .84 amps max) was identical to the CF-185. At the rated current the filament ran a brilliant white color, so the use of an oxide coating seems unlikely. Under test the tube in Fig. 27 showed the typical waveforms of a gas detector and required about 30 volts on the plate. No advertising or other information of any kind has been found for this tube, and the date of manufacture is not known. Manufacturing is presumed to have occurred after the affiliation of DeForest in late 1920. By this time the candelabra base was nearly obsolete for new designs, and it seems unlikely that any appreciable number of the Series D Electron Relays were made. This assumption is supported by the fact that very few surviving samples of the Series "D" are known. This tube should not be confused with the Moorhead-Marconi Type D which was probably the same as the unbased 1917 ER.

Fig. 28

An undated sales brochure from 1918 or later describes three tubes not familiar to the author, which are based on the WW1 valves supplied to the British.[47] The Type R has an internal structure similar to the British R but is in an unbased tubular bulb of about 1" diameter by 3" length, see Fig. 28. The bulb has a conventional tip on one end and the four leads exiting the other end. This tube had a low vacuum and was intended for use as a detector with 15 to 40 volts on the plate. A tube of similar construction but pumped to a high vacuum was called the Type RH and could be used with 50 to 400 volts on the plate as an oscillator or amplifier. The third tube was called the Type B and was intended for transmitting with up to 500 volts. This tube was unpictured but was apparently based as it is described as being "exactly the same as those being furnished the United States Navy at the present time." The Navy tube referenced was most likely the S.E. 1444. The ad stated that the tubes were licensed under the Fleming Patent, but this was probably not true. The brochure also showed the standard tubular ER. A plausible scenario might be that these tubes were offered on a small scale after the end of the war but before the agreements with Marconi and DeForest had been reached. Interestingly these tubes were offered by the old Pacific Laboratories Sales Department rather than the Moorhead Laboratories.

Fig. 29

An undesignated tube that appears to be a variant of the VT is shown in Fig. 29. The base carries the usual DeForest/Marconi markings. This tube came boxed in a carton of the same type used for the VT, but of smaller cross-section to fit the bulb used. No specific information is available for this tube, but the bulb is certainly unusual for Moorhead. If the R or RH described in the previous paragraph were to be based the result would be similar to the tube shown. This same internal structure and bulb has been seen with a British base. The bulb was marked Moorhead, but had no other ID or serial number. There was a paper label on the base with a Fleming license notice, the same as used on the British B transmitting tube.

Fig. 30

Another unusual version of the VT is shown in Fig. 30. This unbased tube has two standard VT assemblies, one at each end of the tubular bulb. The presses are marked VT. This marking would probably date the tube to about 1920, as this is the earliest observed use of VT on Moorhead presses. The tube is fairly large, measuring about .93" x 5.6". Only a few of these tubes are known to exist, and they were probably experimental in nature and never marketed.

Fig. 31

The next Moorhead item to be described here is a rare and interesting tube custom made for a Seattle businessman named Paul Hackett. This tube was an external grid power type using a standard Edison screw base and a large copper mesh external grid. The tube measures about 6.5 inches in length by 1.5 inches in diameter. As can be seen in Fig. 31, the tube had a clear tubular bulb and no marking of any kind. The general internal structure is similar to the 1916 external grid receiving tube, just on a larger scale. Two of these tubes were made for Mr. Hackett, who was involved in pre WW1 speaker design. Mr. Hackett had developed a large horn speaker known as the Wonderphone and had installed groups of them at race tracks and baseball fields for PA use. He traveled to San Francisco around 1916 to see Moorhead to obtain tubes for possible use in audio power amplifiers. As a result of this trip Moorhead produced the two external grid power tubes. It is not known if Mr. Hackett was successful in developing a power amp using these tubes. Mr. Hackett was a close friend of Warren Green (who supplied this information) and gave the two tubes to Warren at some later date. Electrical data for this tube was not specified, but the filament voltage was determined to be about 25 volts, at which condition it drew 2 amps. Note that the tungsten filament is coiled like a light bulb to get enough length to support the 25 volts. At 50 watts input for the filament the tube ran rather hot. Attempts were made to check for function but at fairly low plate voltages the tube showed indications of gas and the testing was abandoned. The tube shown is from the authors collection and the other tube was given to the Seattle Museum of History and Industry.[48] The fact that both of these tubes have survived and have the provenance stated has to be considered as near miraculous.

Since the license from Marconi had expired in July of 1920, the tubes sold under the A-P name were infringing the Fleming patents. RCA now held these patents and advised Moorhead Labs of their infringement. An agreement was reached in January of 1921[49] which granted to Moorhead Labs a temporary license to build a specified amount of tubes. Under this license Moorhead Labs continued to sell tubes until mid 1922, at which time the company ceased operation. Old stock of Moorhead tubes was available from surplus sources as late as the early 30's, when they could be found on Cortland Street in New York City.

Fig. 32

In late 1922 or early 1923 Moorhead and several associates established the Universal Radio Improvement Company in Alameda, California, at 1513 Lincoln Avenue.[50] A newspaper article described this operation as having a large plant capable of producing 1000 tubes per day.[51] This was a gross exaggeration, the facilities there consisted mainly of a laboratory. This organization is known for the Moorhead/A-P Solenoid Tube. A German by the name of Huppert, a former X-ray technician, was responsible for the design of this tube. As shown in Fig. 32 this tube was a form of external grid device but used the metal shell to give an additional connection to the solenoid winding. The solenoid winding, which replaced the normal grid, had a return lead that went to F- through an external coupling capacitor. This arrangement allowed the input signal to cause a current to flow through the solenoid winding, enhancing its function by as much as 100%.[52] These tubes were used only as detectors, and would not work as amplifiers. According to tests made in early 1923 these tubes were inconsistent in performance and not as efficient as standard types but did have a cleaner output.[53]

Fig. 33

Fig. 34

Fig. 35

Fig. 36

Moorhead died in January of 1923 but the company stayed in business. Later in 1923 the company was reorganized and the name became the A-P Radio Laboratories. Operations were then moved back to the old Moorhead plant in San Francisco. Henry Shaw was apparently the main backer.[54] Some of the Solenoid tubes were sold, but A-P decided to market conventional triodes. Serial number data would suggest that only a few thousand of the Solonoid tubes were sold. By October of 1923 a new Huppert tube known as the Two-in-One tube had been designed.[55] This tube had a dual plate and grid structure and a center tapped filament tied to the shell. This arrangement allowed the possible use of one section at a time, or both sections at once for normal operation. These tubes were made with both nickel and brass bases (Fig. 33, 34). The filament drew .25 amps at 6 volts when using both sections. A-P Labs also made a '99 type tube rated at .06 to .08 amps with 3-4 volts on the filament. These tubes can be found with various markings. The tube in Fig. 35 has a nickel base and is marked NEW "A-P" TUBE. Others have been seen with brass bases and marked Type 306M. These were also made with standard UV bases (Fig. 36) with the same electrical ratings. Advertising for A-P Labs seems to be nonexistent, and these tubes were not licensed. Apparently Moorhead had died without substantial means as A-P Labs employed his widow as an assembler in 1924.[56] Also working part-time for A-P in 1924 was Charles Litton (founder of Litton Industries), who at the time was a graduate student at Stanford.

A-P Labs was a low budget operation in constant financial difficulty. Thoriated tungsten for the filaments was hard to obtain and they could only afford to buy in small quantities. The result was frequent production shutdowns and inconsistent tube quality.[57] In 1925 financial problems forced the company to temporarily shut down. The company reopened later in 1925 but after a short time was taken over by the QRS Music Company of Chicago.[58] Presumably the tubes marketed by QRS starting around 1925/1926 were made at the San Francisco plant. The QRS receiving tubes featured the same dual element construction as used in the Two-in-One tube of A-P Labs. The QRS Tube Division was taken over by Raytheon in 1928 as part of a settlement against QRS for infringing the Raytheon gas rectifier patents.[59] This was apparently the end of the line for the San Francisco plant.

Moorhead died at the age of 30 on January 31, 1923, as the result of complications from a fall in his home. He left two children and his wife Barbara. Examination of a letter Moorhead wrote to a friend indicates an outgoing individual with a sense of humor. Other information shows a different side to his character, a quick temper and impulsive nature that could have made him difficult to work with. In any case his untimely death cut short his further tube development which surely would have come about with the expiration of the Fleming and DeForest patents.

The table below lists electrical data for all Moorhead tubes known to have been made in any significant quantity. For those tubes for which published data could be found the published data is shown. For the other tubes data was obtained from tests performed by the author. The test sample size for some of the scarcer types was small (in some cases one) so the electrical data in the table should be considered as approximate and subject to some variation.

The information in the S/N RANGE SEEN column is taken from the tube survey mentioned at the beginning of the article. This tube survey is ongoing, and from time to time the number range changes as new data surfaces, and this table is updated. The author was not alone in this effort and wishes to thank Will Jensby, Lauren Peckham, Brother Patrick Dowd, Alan Douglas, and many others whose help was crucial in compiling this data.

ER (1916) 4.0 1.1 20-45 .75"D x 3.95"L
EXTERNAL GRID 4.0 1.1 15-35 .85" to .88"D, 3.05" to 3.70"L
ER (1917) 4.0 .94 20-45 .85" to .95"D, 3.60" to 3.70"L
ER (1920)/TYPE D?* 4.0 .65 20-45 .90"D x 3.25"L

        * Serial number range seen was 173 - 254.

R (BRITISH) 4.0 .70 40 -1.3 180 1842-43535
B (BRITISH)** 6.0 .85 400 -4.5 405 17994
VT 32** 6.0 .85 400 -4.5 405 NONE
SE 1444 4.0 .70 40 -1.3 180 34-26244
4.0 .70 40 -1.3 180 168-5941
4.0 .65 20-40 --- --- 223-10241
C** 6.0 .85 400 -4.5 260 264
VT 4.0 .70 40 -1.3 180 207547-247180
ER (TALL PLATE) 4.0 .70 20-40 --- --- 500252-502935
504649-510775 (MICA)
ER 4.0 .60-1.0 20-40 --- --- 511058-551279
TT 6.0 .85 500 400251-400333
RECTIFIER 9.0 3.15 75 Ma 28-531

        ** The test conditions used for the transmitting tubes were arbitrarily selected to provide a plate dissipation of 3 to 5 watts. Tests were run on a total of four samples, two British and two US. Averaged results are shown for the British tubes and the US tubes.


[1] Moorhead and His Valve, (brochure), Aug. 1919, p. 4

[2] Transcript of interview with Major Allan Forbes, Oakland, Ca, Nov. 1961

[3] Transcript of interview with Major Allan Forbes, Oakland, Ca, Nov. 1961

[4] Transcript of interview with Major Allan Forbes, Oakland, Ca, Nov. 1961

[5] Floyd A. Paul, "B. F. McNamee and his Moorhead & Kennedy Associations," SCARS Gazette, Feb. 1987, p. 4

[6] Transcript of interview with Major Allan Forbes, Oakland, Ca, Nov. 1961

[7] Floyd A. Paul, "B. F. McNamee and his Moorhead & Kennedy Associations," SCARS Gazette, Feb. 1987, p. 4

[8] O. B. Moorhead,  "The Vacuum Detector Patent Situation," Pacific Radio News, May. 1917, p. 203

[9] Advertisement of Pacific Research Laboratories, Elect. Exper., Jul. 1916, p. 193

[10] Floyd A. Paul, "B. F. McNamee and his Moorhead & Kennedy Associations," SCARS Gazette, Feb. 1987, p. 4

[11] Advertisement of Pacific Research Laboratories, Elect. Exper., Aug. 1916, p. 273

[12] Advertisement of Pacific Research Laboratories, Elect. Exper., Sep. 1916, p. 355

[13] The Moorhead Tube, (brochure), 1916, p. 2

[14] Advertisement of Pacific Laboratories Sales Dept., Pacific Radio News, Jan. 1917, inside front cover

[15] O. B. Moorhead, "The Manufacture of Vacuum Detectors," Proc. I.R.E., Dec. 1917, pp. 427-432

[16] O. B. Moorhead,  "The Vacuum Detector Patent Situation," Pacific Radio News, May. 1917, p. 203

[17] O.B. Moorhead & E. C. Lange, "The Specifications and Characteristics of Moorhead Vacuum Valves," Proc. I.R.E., Apr. 1921, pp. 99

[18] O.B. Moorhead & E. C. Lange, "The Specifications and Characteristics of Moorhead Vacuum Valves," Proc. I.R.E., Apr. 1921, pp. 95-129

[19] Moorhead and His Valve, (brochure), Aug. 1919, p. 14

[20] "Radio Vacuum Tube Litigation Settled," Radio Amateur News, Aug. 1919, p. 77

[21] Moorhead and His Valve, (brochure), Aug. 1919, p. 5

[22] Moorhead and His Valve, (brochure), Aug. 1919, p. 5

[23] "Moorhead Laboratories to Reorganize Corporation," Pacific Radio News, Mar. 1920, p. 295

[24] Moorhead and His Valve, (brochure), Aug. 1919, p. 14 Ibid., p. 4

[25] "Close Big Contract," San Francisco Bulletin, 16 Nov. 1918

[26] "Moorhead Laboratories to Reorganize Corporation," Pacific Radio News, Mar. 1920, p. 295

[27] "The Reorganization of the Moorhead Laboratories, Inc.," Pacific Radio News, Jun. 1920, p.389

[28] Moorhead and His Valve, (brochure), Aug. 1919, p. 22

[29] "Radio Vacuum Tube Litigation Settled," Radio Amateur News, Aug. 1919, p. 77

[30] Gerald F. J. Tyne, Saga of the Vacuum Tube, 1st ed., (Indianapolis: Howard W. Sams & Co., 1977), p. 172

[31] Gerald F. J. Tyne, Saga of the Vacuum Tube, 1st ed., (Indianapolis: Howard W. Sams & Co., 1977), p. 172

[32] O. B. Moorhead & E. C. Lange, "The Specifications and Characteristics of Moorhead Vacuum Valves," Proc. I.R.E., Apr. 1921, pp. 95-129

[33] Gerald F. J. Tyne, Saga of the Vacuum Tube, 1st ed., (Indianapolis: Howard W. Sams & Co., 1977), p. 172

[34] Gerald F. J. Tyne, Saga of the Vacuum Tube, 1st ed., (Indianapolis: Howard W. Sams & Co., 1977), pp. 172-173

[35] Floyd A. Paul, "B. F. McNamee and his Moorhead & Kennedy Associations," SCARS Gazette, Feb. 1987, p.4

[36] "Big Radio Deal Closed," Pacific Radio news, Jun. 1920, p. 589

[37] "Moorhead's to be Reorganized," San Francisco Examiner, Jan. 18, 1920; "Moorhead Laboratories to , Mar. 1920, p.295

[38] Advertisement of Pacific Radio Supplies Co., Pacific Radio News, May 1920, inside back cover.

[39] Advertisement of Pacific Radio Supplies Co., Pacific Radio News, May 1920, inside back cover.

[40] Advertisement of Pacific Radio Supplies Co., Elect. Exper., May 1920, p. 81

[41] Advertisement of Pacific Radio Supplies Co., Radio Amateur News, Jun. 1920, p. 711

[42] Advertisement of Pacific Radio Supplies Co., QST, Aug. 1920. p. 67

[43] Advertisement of Pacific Radio Supplies Co., QST, Dec. 1920, p. 99

[44] Advertisement of Atlantic-Pacific Radio Supplies Co., QST, Dec. 1921, p. 129

[45] Advertisement of Pacific Radio Supplies Co, Radio News, May 1921, p. 833

[46] Gerald F. J. Tyne, Saga of the Vacuum Tube, 1st ed., (Indianapolis: Howard W. Sams & Co., 1977), p. 179

[47] Moorhead Vacuum Valves, (brochure), undated

[48] Phone conversations with Warren Green, Mercer Island, WA, Nov. 1995

[49] Gerald F. J. Tyne, Saga of the Vacuum Tube, 1st ed., (Indianapolis: Howard W. Sams & Co., 1977),

[50] "Local Radio Inventor Died This Morning", clipping from unnamed San Francisco newspaper, Jan. 31, 1923

[51] "Grant Radio Plant Comes to Alameda", San Francisco Examiner, Jan. 22, 1923

[52] Extract of notebook of W. Hanscomb noting results of testing approximately 40 Moorhead (Huppert) "solenoids", p. 53, May - July, 1923

[53] Extract of notebook of W. Hanscomb noting results of testing approximately 40 Moorhead (Huppert) "solenoids", pp 51-58, May - July, 1923

[54] Letter from Frank A. Polkinghorn to Mrs. Robert A. Morgan, Mar. 1970 (Frank Polkinghorn was Chief Engineer of A-P Labs in 1924 and early 1925.)

[55] Extract of notebook of W. Hanscomb, Oct. 1923

[56] Letter from Frank A. Polkinghorn to Mrs. Robert A. Morgan, Mar. 1970 (Frank Polkinghorn was Chief Engineer of A-P Labs in 1924 and early 1925.)

[57] Letter from Frank A. Polkinghorn to Mrs. Robert A. Morgan, Mar. 1970 (Frank Polkinghorn was Chief Engineer of A-P Labs in 1924 and early 1925.)

[58] Letter from Frank A. Polkinghorn to Mrs. Robert A. Morgan, Mar. 1970 (Frank Polkinghorn was Chief Engineer of A-P Labs in 1924 and early 1925.)

[59] "Raytheon Absorbs Q.R.S. Radio Tube Division", Radio Engineering, Aug. 1928, p.45