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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 27th Feb 2012, 9:11 pm   #1
Neil Purling
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Default Amplifier question

You are working on a amplifier. This example should have 6V6's in push-pull & you don't have the original matched pair of 6V6's.
Can you make a brief test with a un-matched pair?
I have several 6V6's. I don't think I have two from the same maker & envelope type, never mind a matched pair. A pair I can get locally at a price or get a Reflektor pair on-line from a FSU republic.
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Old 27th Feb 2012, 9:20 pm   #2
paulsherwin
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Default Re: Amplifier question

You can use an unmatched pair indefinitely unless the amplifier is spectacularly badly designed. I bet you won't be able to hear the difference.
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Old 27th Feb 2012, 10:00 pm   #3
Herald1360
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Default Re: Amplifier question

If the match is really awful, it might be worth tweaking the cathode bias resistors to get the same anode current in each valve. This will keep the output transformer away from possible saturation problems.
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Old 27th Feb 2012, 10:48 pm   #4
PJL
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Default Re: Amplifier question

If the design uses a common cathode bias resistor you must ensure they are matched for quiescent current or you will exceed the plate dissiption on the strongest valve and it may thermally runaway resulting in a damaged output or mains transformer. If you have a few, best bet is to put a couple of dmm's in the anode paths and try them out. I wouldn't bother with a properly matched pair which I beleive is strictly same batch and within 10% gm and quiescent current.
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Old 28th Feb 2012, 7:51 am   #5
Craig Sawyers
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Default Re: Amplifier question

With output vavles it is more important to match the anode current at the operating point to keep the current balance in the output transformer correct. Mismatch biasses the core. But generally 10% match will be fine in practice. Plug them in and use a meter to measure the anode current of each - that is the proof of the pudding.

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