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EL34 Pushpull imbalance

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Hello, very helpful people,

I’ve been working on Mr. Cariou EL34 PP amp for the few last months. This is my 3rd amp. I’ve read that it was a good design and quite straight forward. Since I write this message, it would seem that it’s not as simple as expected for the newbie I am.

Here is the problem: an intriguing imbalance is the splitter stage (ECC82 / 12AU7). I join a pic of the readings I get on my amp compared to the published readings. For most of those, I’m very close to theory, but the left side of the splitter is grossly unbalanced. Specs say I should get 250 – 250 V, I get 283 – 288 for the right side (final adjustment is easy). The left side read 214 – 363 V!

I rechecked the wiring, resistor and caps readings, found nothing. I even swapped 12AU7 tubes right and left channels; the left channel is STILL unbalanced!

Question: what am I missing here? Any suggestion on how to isolate the problem?

Pic link :

http://pelfra.hostarea.org/el34.jpg

Thank you people for your help.

A desperate newbie.
 

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If you've checked the plate and cathode resistors, and swapped tubes, it's has to be the tube socket. I've experienced this even with new sockets, especially with newer manufactured tubes, the pins are a lot skinnier. Maybe put a slight bend in the tube pins on the offending triode and see if you get a connection. Also make sure the wiring is correct and solder joints are good, use a magnifying glass to be sure. Good luck and let us know what you find. This is frustrating but usually it's something really silly.

Craig
 
EL 34 imbalance - results

Hello,

Many thanks for your help regarding my EL34 PP problem. I have tested each of the suggestions I've received on this forum. Here are the results:

1) All parts are ok and layout is checked. No wiring error. No leaky cap or bad resistor. As llwhtt suggested, the problem was with the tube socket. Just by moving the tube on it's socket shifted the readings from balanced to unbalanced. I bent the pins a little and the new readings are right on target. So yes, something as simple as tube to socket contacts may generate weird problems.

So thanks to every one of you for your very helpful comments.

Best regards, Francois.
 
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Be sure to replace that socket soon, next tube replacement is likely to cause the same problem.

I am struck by the amazing similarities between this design and the earliest version of the one I used to sell as an upgrade to the Dynaco ST-70.. (Nearly 20 yrs ago) Guess there are only so many ways to skin a cat.. :devilr:
 
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