807 Tube Amplifier

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In the summer of 2002 I decided to build a pair of mono block parallel single ended DIY tube amps , using the 807 tube. It all started with my decision to use the less known tube 807 , a tetrode whose features and sounds I was well aware of, from a specific DIY project (with one tube per channel wired in triode) made by Christos Spathopoulos. Christos 807 amp played very smoothly in the whole spectrum (perhaps the best sound I had heard until then ) but, unfortunately, it has not enought power. So I focused to build a tube amp with two parallel 807's per channel in a cathode follower connection, wired in tetrode - class A, fixed bias (using negative voltage in screen), according to the characteristics of RCA and SCT.
For nearly two months worked designing the amp, having in mind to solve two major problems. First, it was my idea to build the power supply using tubes as rectifiers, and the second concerned the bias circuit for the final tubes. Finally, I decided to use two independent lines for powering the system (using GZ34 for the drivers and 5U4G for the power stage). I also built the Bias circuit, based on two SPECTROL 22K micro potentiometers for adjusting the bias (one for each tube ). Each tube 807 is biased at -15.5 volt (grid). Plate voltage is about 505 volts, and screen voltage is 228 Volts. Next step was to construct the chassis. I designed the chassis in auto cad and a CNC machinery did the whole work. The whole construction is hard wired with one exception of a small hand crafted board for the power supply capacitors. All of the most critical path of amplifier have been wired using silver-Teflon cables (especially the high voltage paths). 
The use of a NOS pair of RCA 807s tubes improved the clearness a lot, instead of the westinghouse 807s I used before. The driving section uses the 12ΑΤ7 (general electric) (one per channel) and the pre-amp is the well known PV10A Conrad Johnson.
Important notes regarding the power Transformers :
The power transformers used in my 807 parallel SE project are old transformers from a vintage philips amplifier. Here are the data regarding all the Power transformer sections.
Because I had to reduce the construction cost I preferred to use these old power transformers. If you are planing to buy new power transformers you can change the T3 to 450-0-450 (in this case R1=50K is no needed) Also for the T8 30Volts is all you need.
I have to mentioned that Christos Spathopoulos triode SE807 amp was a derivative work based on RH807 by Alex Kitic.

From Wikipedia, the free encyclopedia

^ R.C.A. Air-Cooled Transmitting Tubes, Technical Manual TT3, R.C.A. Manufacturing Company Ltd. Harrison, New Jersey, 1938

^ "Ham Speak – Know the Lingo" . QRZ.com . Retrieved 9 May 2012 .

^ Midwestern Journal of Language and Folklore . Department of English and Journalism, Indiana State University. 1978. ...amateur slang such as handle (the name of an operator), lid (a poor operator), rig (radio set, equipment), 807 (a can of beer – from resemblance to a now obsolete transmitting tube), and XYL (ex-young lady: wife).



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Left, British Emitron brand 807
Second Left, Canadian Westinghouse brand 807
Second Right, Dutch Philips brand military 807/ATS25

Pin 1, Heater
Pin 2, Screen grid, g2
Pin-3, Control grid, g1
Pin-4, Cathode-beam plates
Pin-5, Heater

The 807 is a beam tetrode vacuum tube , widely used in audio- and radio-frequency power amplifier applications.

807s were used in audio power amplifiers , both for public address and Hi-Fi application, usually being run in push-pull pairs in class AB1 or AB2 giving up to 120 watts of usable power. The plate voltage limit is 750 volts and the screen grid limited to 300 volts. Because of the 300 volt screen grid voltage limit, the 807 cannot be triode connected for high power applications. Failure to observe this precaution will cause screen grid failure. Less commonly a single 807 was used in a pure class-A, single-ended audio output stage delivering about 10 watts.

The 807 is fully rated to 60 MHz, derated to 55% at 125 MHz in Class C, Plate-modulated operation, thus they were popular with amateur radio operators (radio hams).
In this application a single 807 could be run in class-C as an oscillator or amplifier which could be keyed on and off to transmit Morse Code in CW mode. For voice transmission on AM a final amplifier with one or more 807s, up to about four, could be connected in parallel running class-C. Connecting multiple 807s in parallel produced more power to feed to the antenna. Often the modulator stage (simply a transformer-coupled audio amplifier for A.M., with the secondary of its output transformer in series with the anode supply of the final amplifier), was also constructed using 807s. Many hams found multiple paralleled 807s a cheaper alternative to a single larger valve, such as a single 813, as many military surplus 807s became available cheaply after World War II. In Australia 807s are affectionately referred to as " stubbies " because they are almost as ubiquitous as that common Australian beer container.
The class C operational values in the info box at the right are for anode modulated A.M. operation ; for CW operation a maximum anode voltage of 600 is permissible, whereby the anode current increases to 100 mA and the anode/plate dissipation rises to 25 watts. The screen voltage is the same, at 300, but its dissipation rises to 3.5 watts.
37 watts of R.F. power is produced from 220 mW of drive but only a 50% duty cycle is allowed. The maximum allowable negative control grid, g1 excursion allowable is -200 volts and average control grid current is 5mA in both A.M. and CW modes. [1]
Later versions could be used on CW with a supply voltage up to 750 V and a current of 100 mA to produce 50-55 watts of output power.

The electrically similar 6L6 was not favored by hams because high transient voltages on the anode when operating in class C could cause a flashover between pins 2 and 3 on the octal base, whereas the 807 had the anode connected to a top cap, physically distant from all the base pins.

The 1624 ( VT-165 ) is an 807 variant with a directly heated filamentary cathode operating at 2.5 V, 2 A.

The 1625 ( VT-136 ) is an 807 variant with a 12.6 V heater and a 7-pin base. These tubes were used as RF power amplifiers in some of the SCR-274 and AN/ARC-5 "command set" transmitters of WW2 . Postwar, 1625 tubes flooded the surplus market, and were available for pennies apiece. Surplus 1625s found some commercial use, notably the use of a pair as modulator tubes in the Heathkit DX-100 amateur transmitter.

The HY-69 is an 807 variant with a 5-pin base and a directly heated filamentary cathode operating at 6.3 V, 1.6 A.

The 5933 / 807W is a ruggedized military version of the 807. It uses a shorter, straight-sided T12 bulb, which provides better element support for improved microphonics and shock/vibration resistance.

The ATS-25 is a military version with ceramic base.

The Г-807 (G-807) is a Soviet/Russian version. The 6П7С (6P7S) is similar to Г-807, but with an 8-pin octal base.

The 807 also found some use as a horizontal output tube in early TV receivers, particularly those manufactured by DuMont . The 807 design (with some " value engineering " to reduce production cost) was the basis for the first application-specific horizontal sweep tubes such as the 6BG6G and 6CD6G . The redesign mainly involved the omission of some of the internal RF shielding, and the substitution of a bakelite Octal base for the micanol or ceramic 5-pin.

In turn, these low cost sweep tube derivatives found some use as RF power amplifiers in homebrew amateur radio transmitters in the 1950s.

Ham operators in the US sometimes use the term "807" to refer to bottles of beer due to the shape of the tube. [2] [3]

24 W, (plate dissipation = 16.5 W max )
300 V (screen dissipation = 2.5 W max )
90-119 mA (AB1), 90-240 mA (AB2),(zero to max signal)
Philips Valve Data Book, Philips Electrical Industries, 69-73 Clarence Street Sydney, Radio Valve Application Division, 1958
Radio Valve Data, Eighth Ed. Iliffe Books Ltd., London, 1966

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