Not my idea, is from Dynavox. Even worse with the Radford ECC81, only -0,4V.In view of grid current, 0,5 V is too low for my taste.
By lowering the 470k screen resistance you need more negative voltage to get the same anode current.
Mona
Not my idea, is from Dynavox. Even worse with the Radford ECC81, only -0,4V.In view of grid current, 0,5 V is too low for my taste.
Forget this comment. You ARE building a nice amp. Much better then dynavox !Why do you say this?
Now I have all the components and I would like to take away the whim of hearing him play.
Not your idea, but you did suggest it.Not my idea, is from Dynavox. Even worse with the Radford ECC81, only -0,4V.
By lowering the 470k screen resistance you need more negative voltage to get the same anode current.
Mona
ok,Forget this comment. You ARE building a nice amp. Much better then dynavox !
To recapitulate:
-disconnect NFB in case it's oscillating
-remove the couplings caps to the EL34 just to make sure it will power up
with no problems. Then adjust bias and see if it stays on. power off
- reconnect coupling caps ( at least one of them, remove the ECF82 tube and
power up. If successfull there is something fishy with the ecf82 neighborhood.
With different resistors at the cathodes the voltage between the triodes is not halfway no more. With only 30V left for the bottom one you get only -0,4...-0,5V on the grid to get some current flowing.Not your idea, but you did suggest it.
I don't know how you reached -0.4 V for the ECC81 in the amplifier of TS. When looking at the curves I think it will be around -0.7 V for the lower triode.
I think that when I wrote my post #2, the schematic that is now attached to post #1 didn't show 1K as the value of cathode resistor of the upper triode yet. I assumed it would be something like 510, so almost the same as the cathode resistors of the lower triode.
unfortunately last night I was not well (and even today I'm not well) and I was not able to do all the tests.Forget this comment. You ARE building a nice amp. Much better then dynavox !
To recapitulate:
-disconnect NFB in case it's oscillating
-remove the couplings caps to the EL34 just to make sure it will power up
with no problems. Then adjust bias and see if it stays on. power off
- reconnect coupling caps ( at least one of them, remove the ECF82 tube and
power up. If successfull there is something fishy with the ecf82 neighborhood.
6U8 is identical with ECF82. And ECF82 was what radford used.Actually the STA25 has a regulation pot that the STA35 does not have but the valves are also different (in the annex a comparative image of the two diagrams)
before starting the project I had read in old contributions on the internet that the radford schemes technically would not have worked but instead they did it very well even though using a lot of NFB compensated by the excellent quality of the output transformers;
this story reminded me a lot of the famous story of the hornet "The wing structure of the hornet, in relation to its weight, is not suitable for flight, but he does not know it and flies anyway."![]()
forgive me, my mistake6U8 is identical with ECF82. And ECF82 was what radford used.