807 single ended tube amps
807 parallel Single Ended Amplifier
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.
My Parallel single ended 807 vacuum tube amplifiers at a glance :
- two 807 tubes / channel (parallel single ended)
- two independent power supply stages per Channel
- full tube rectifiers PSU
- pure class A
- No feedback
- two independent lines for tube 807 biasing
- CNC chassis design
- hand made output transformers P&K audio
807 Tube amplifier building
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).
Driving - pre amp
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.
- T1 = 220V mains (you should change this to 110V for US or 120V for Canada)
- T2 = 5V / 3A
- T3 = 600-0-600V 250mA
- T4 = 10V / 4A (actually you need 7-7.5 Volts)
- T5 = 6.3V / 3A T6 = 5V / 3A T7 = 280-0-280V / 200mA (150mA is enough)
- T8 = 72V (30V is all you need)
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.