Federal Build: From Octals to Miniatures

[schmidlin]

Thanks Schmidlin, lots of inspiration. What meter did you use? Is it a 1ma, or smaler?
20 db att on the output sounds like a lot, but if it works...

joe

Yep 1maDC, and 20db is on the backside: it's a lot and it works fine (so I hear).

 

[bernbrue]

Old thread, but the topic of this thread describes  exactly what I did.

I'm working on a federal build with miniature tubes. 2x 6BA6, 6AV6 and 12AU7. The unit passes audio but with distortion.  First I pulled the 6AV6 tube to check the amplifier only, but distortion is still there.

My first thought: the 6BA6 tubes can't be used as a direct replacement of the 6SK7 without changing the circuit. I suppose the 6BA6 needs a cathode resistor (68R).

Second thought: the voltage for G2 might be too low (11.8 V).

Third idea: cathode resistor (240R) for 12AU7 (both triodes in parallel) is too low. I suppose 600R - 1,2K would be right.
Help is very much appreciated.
Bernd

Edit: wiring error, unit passes clean audio, though overall gain is not that much. Meter doesn't work. Increasing 12AU7 cathode resistor added a bit of gain but I think 240R is fine.

 

[Heikki]

What is the plate current in 6BA6 tubes? I think it should be around 1mA for each 6BA6. If it's much lower you might need to increase the screen voltage by few volts. Also what's the plate current in 12AU7?

 

[bernbrue]

Hi,
I'm still tinkering around with the unit. It passes audio and there is no distortion. Thanks for answering.
Bernd