A 6J5-807, single ended, class A1 amplifier

Dimitrios Tsifakis (dtsifakis AT gee-mail dot com)


The 807 has always been my favourite valve. I have used it on a number of RF transmitters and modulators and I thought it was only fair if my hi-fi audio amplifier uses it too. There are plenty of good designs out there showing different audio amplifiers using 807. The Williamson design is a famous instance. My desire was to design something simple from scratch, so I turned to the 807 datasheet looking for single ended designs. There are many datasheets out there but the one that is by far the most complete is the STC one.

Triode or Tetrode?

The majority of hi-fi valve ethusiasts will rave about the quality of the triode valve. The 807 is not a triode, but it could be connected in triode mode by connecting the screen grid directly to the anode. In this case, the maximum voltage that can be applied to the plate will limited by the maximum allowable screen voltage which is 300 V. That's a low value and that's the reason why the 807 wouldn't be an ideal valve to use in an ultralinear design. The datasheet shows that a triode strapped 807 is not expected to produce much more than a watt of audio. The tetrode connected 807 produces over five watts. I have found that given the sensitivity of my speakers (89 dB) and my listening room and preferences, five watts is more than enough.

While the valve enthusiasts will argue that one triode watt is plenty and they would prefer that to anything else, I was of the opposite opinion. In the STC datasheet, there are tables with suggested configurations for tetrode operation. In the tables above, we see that the 807 will produce more power if used as a tetrode. So what's the difference? The difference appears to be in the distortion of the final amplifier. The tetrode amplifier is expected to have more harmonic distortion, as seen below. In general, even harmonics (2nd, 4th, etc) are said to be more "pleasing" to the ear, while odd harmonics (3rd, 5th, etc) are believed to be an unpalatable form of distortion. One probably has to experience this and form their own opinion. In any case, I decided the described amplifier will produce a healthy dose of even harmonics!

Driving the 807

Given that this is a class A1 amplifiers, the driving requirements are simple. In Class A1, there is no current flowing on the 807 grid. A resistance-coupled voltage amplifier seems to be all that is needed. In the 6J5 datasheet, we get some instructions on how to do this. The schematic is as follows:

The proposed values for the components of the 6J5 amplifier are seen in the schematic below. They result in a voltage gain of about 18.

Final schematic, as built

Design comments

Listening

One could write all sorts of subjective comments on the performance of an audio amplifier, but I will resist that. In general, the amplifier sounds pleasing to my ears. Is it because of the even harmonics or because I like the looks of it? Or is it because I built it? In any case, the result is positive and that's what it's all about!

Photos

References