EL34 Pushpull imbalance - diyAudio
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Old 15th October 2008, 08:08 PM   #1
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Default EL34 Pushpull imbalance

Hello, very helpful people,

Iíve been working on Mr. Cariou EL34 PP amp for the few last months. This is my 3rd amp. Iíve read that it was a good design and quite straight forward. Since I write this message, it would seem that itís 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, Iím very close to theory, but the left side of the splitter is grossly unbalanced. Specs say I should get 250 Ė 250 V, I get 283 Ė 288 for the right side (final adjustment is easy). The left side read 214 Ė 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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Old 15th October 2008, 08:22 PM   #2
DougL is offline DougL  United States
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Check both the values of the cathode resistors on the bad side.
One could be either too low or high, altering the balance of the circuit.

Also check that both grids are at 120v.

HTH

Doug
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Old 15th October 2008, 08:28 PM   #3
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Hi

It looks like the grounded grid side of the LTP splitter doesn't conduct at all.
Check grid voltage at that grid, I would guess that you have a short to ground there,or the decoupling cap is shorted/very leaky (strange but worth checking)

God luck

/Olof
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Old 16th October 2008, 12:02 AM   #4
llwhtt is offline llwhtt  United States
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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
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Old 16th October 2008, 12:04 AM   #5
llwhtt is offline llwhtt  United States
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Forgot to say since the plate voltage is pretty much at B+ that side of the tube is not conducting, for whatever reason.

Craig
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Old 16th October 2008, 12:19 AM   #6
llwhtt is offline llwhtt  United States
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Just thought of another thing to check, are both heaters lit up? Check the pin 4&5 connection.

Craig
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Old 16th October 2008, 07:10 AM   #7
mikvous is offline mikvous  Greece
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Shouldn't there be a cap between ECC83 and ECC 82?
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Old 16th October 2008, 11:29 AM   #8
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally posted by mikvous
Shouldn't there be a cap between ECC83 and ECC 82?
No, it's a standard DC couped design (ala Mullard)

Fpelleti, I would try replacing the 470n cap which is connected to the 2nd ECC82's grid. If it has become leaky it will put that triode into cut-off.
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Old 23rd October 2008, 03:49 PM   #9
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Default 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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Old 23rd October 2008, 04:08 PM   #10
kevinkr is offline kevinkr  United States
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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..
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