The 6SJ7 is not a direct sub for the 6J7, but the circuit can be tweaked in order to make it work properly.. You want a gain of about x80 (38dB) in the first stage, and feedback ratio from 6V6 plate of ~200:1, and transformer secondary of ~4.25:1. Do not forget to take into account the effect of the dc current from the plate feedback and the secondary feedback resistor on bias. Note you can also ac couple the plate feedback making things simpler and first stage bias entirely cathode bias.
Just a quick update aimed mostly at a couple of friends who heard this amp yesterday driving a pair of L Cao 8 inch full range drivers on open baffles. (And expressed an interest in building copies of their own)
The amp currently is using 5Y3G, 6V6G (Kenrad) and French made 6J7 in aluminum cans (not Neotron, but I don't currently remember what they are) The quality of the tubes is kind of important.
The amp remains exactly as described after the last changes, and the original A23 bias batteries are still installed and working fine. Output tubes are operating at 50mA..
I never changed the output transformers as the amplifier has worked really well with certain speakers including some fairly early Altec A-604s.
I'm not at all embarrassed by the amplifier ever when I take it to a shoot out where it gets swapped in for another. It generally acquits itself well as long as the speakers are a reasonable match for a 5W amplifier. Generally pentode operation is preferable and in most instances I would probably recommend not including the triode/pentode switch.
Strangely enough this thread is the only real history of the build of this amplifier.. I never formally documented it, something I think about being embarrassed to have embarked down this path; 5 years on I can say that was an unfounded concern.
The amp currently is using 5Y3G, 6V6G (Kenrad) and French made 6J7 in aluminum cans (not Neotron, but I don't currently remember what they are) The quality of the tubes is kind of important.
The amp remains exactly as described after the last changes, and the original A23 bias batteries are still installed and working fine. Output tubes are operating at 50mA..
I never changed the output transformers as the amplifier has worked really well with certain speakers including some fairly early Altec A-604s.
I'm not at all embarrassed by the amplifier ever when I take it to a shoot out where it gets swapped in for another. It generally acquits itself well as long as the speakers are a reasonable match for a 5W amplifier. Generally pentode operation is preferable and in most instances I would probably recommend not including the triode/pentode switch.
Strangely enough this thread is the only real history of the build of this amplifier.. I never formally documented it, something I think about being embarrassed to have embarked down this path; 5 years on I can say that was an unfounded concern.
Hi Kevin, I am very interested in using battery bias in my next project which is a SE el84 amp with 12ax7 driver a la Magnavox. How do I select the proper battery and its voltage? I really enjoyed reading your account of the 6v6 se amp. best regards
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