Help in biasing tube amp

[Vivek Rao]

I put together the Sweet Peach FU50 tube amp getting the parts from China - a fully built unit was dismantled and transported in phases after which I put this together here. The amp is still running-in but I have noticed that the balance is slightly more to the right channel. I wonder if this has something to do with the tube biasing... Perhaps due to the rebuild it got shifted slightly.. Can any FMs guide me here to help me rebias the tubes. I could bring the Amp over to any of the senior FMs in Bangalore. Need help and guidance please. Googling has not helped me much...as I need to tap out the voltage signals directly from the board... View attachment 19973View attachment 19974

 

[Fantastic]

Nice looking amp.
Did you switch ( interchange ) the speaker leads from left to right on the amp and confirm that the signal is now higher in the left channel ? If so the amp is responsible.
Changing bias slightly should not achieve what you are describing. AFTER the tubes have cooled down , try exchanging the tubes from right to left channel. First transfer the small tubes and test it. If no difference than transfer the power tubes. You should be able to spot out the problem area.
Are both channels currently having the same output tube bias setting ? ( I'm guessing you don't know !)
Looks like a solid state power supply. So you have one shared tube and two as a drivers . 6N3 *2 +6N1, power tube: FU-50 *2 . If there is no negative feedback, the input tube could have a mismatched section. But I doubt it....though it's possible.
Do you have a circuit diagram ?

 

[Vivek Rao]

Thanks for your feedback. I tried to swap inputs and still found the right stronger. My feeling was that the vocals were slightly weaker...hence the doubt on biasing. I have currently already swapped the output FU50s. Still need to evaluate completely.

The circuit diagram is at below link - German HIFI site.. I observed 2 variable pots as you also find in the pictures at the link. I am presuming this is for Biasing...


S

 

[GeorgeO]

Vivek

Why not talk to Philip at Lizzner and ask him to help 080-2269360? He is quite familiar with this kind of gear. He just recently repaired an EL 34 pp amp of mine.

 

[Vivek Rao]

Thanks George will do so... Switching the pre amp 6N3s has not helped nor the output FU50s. So now perhaps bias or something more rudimentary a la Volume pot imbalance... Will get in touch with Philip...

 

[Fantastic]

Vivek, you swapped the tubes except the input tube which you can't as there is only one of it. You also swapped the 'inputs'. Did you switch the speaker connections at the amp ? Playing left out of the right speaker and right out of the left speaker . If the problem stays at the same place maybe your speaker has a problem ?

PS: I just had a look at the circuit diagram. It's not very clear but looks like it has negative feedback to the input tube. So tube mismatch should not be a problem. Do switch the speaker leads at the amp's output terminals and see what happens.

 

[Vivek Rao]

Thanks Fantastic. Yes you are right on the input tube. I will do more tests including the speaker swap as you mention. will update the results.
Meanwhile I have now the guideline from the chinese manufacturer the process to be followed for the output tube biasing. That is also in my list of TODOs. Will update.

 

[Vivek Rao]

after some sleepless nights all checks done now. Biasing was OK. checked out the volume pots also OK. Speakers were OK - checked on 2 sets.
Then on a hunch checked out the output transformers - measuring the resistance at the speaker output terminals - Voila ! there was a difference in the resistance between right and left. When i switched to the 8 Ohm mode the resistances swapped between R & L! it was clear that the transformer manufacturers had messed up the connections on one of the sides. Swapped it back and all is good ! Ah the nice tube sound....
Waiting for the burn in to complete now.

 

[GeorgeO]

Congrats! Enjoy the music.