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Old 24th June 2004, 07:11 AM   #1
riku is offline riku  Finland
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Default Sovtek 2A3 tolerances / defected tube?


Just finished 2A3 SE amp project and now I have difficulties to have correct bias voltage / quiescent current to second channel. Biasvoltage is 35V and it should to be 47V. So current is much lower and so B+ goes abot 25V higher than it supposed to go.

I'm using Steve's EZ 2A3 SRPP schematics here. I swapped 2A3s between two channels and I got same measurements here as well.

So is this tube just in its manufacturing limits or is it broken? The base and glass part is bit loose, so damages during transportation etc. might be possible.

In general, what are the possibilities to get equally behaving tube if I buy a new one?

As a newbie at least I learned in future to buy mached pairs only...


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Old 24th June 2004, 09:58 AM   #2
arnoldc is offline arnoldc  Philippines
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hi there, i built the same amp...

anyway, i've had some problems with a couple of Sovtek 2A3s as you described. some even marked as matched pairs but after three days of operation they drift. luckily, the source replaced them, no questions asked.

by the way, does the discrepancy change places too when you switch the tubes?
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Old 24th June 2004, 10:54 AM   #3
riku is offline riku  Finland
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Thanks for info, arnoldc.

I'll return the problem tube back to store and make order for few extra ones. Fortunately these are reasonably priced. Any experiece from another brands, NOS and current production models?

Problem follows with the tube. With the good one bias voltages etc. are as marked in schematics in both channels.
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Old 24th June 2004, 11:51 AM   #4
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If you don't order matched pairs/quads in the first place, can you then expect to have them matched nonetheless?

Powertubes like the 300B, 2A3 etc, will always have dispersions and it would be wise to either order them matched (not that that is going to be the solution for all amps but it's a start) or/and include some provision for individual biasing on the amplifier.

For a SET amp I wouldn't get too anal about it though.
Using a common B+ for both channels isn't the beest idea in the first place so I'd look into that first of all.

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Old 24th June 2004, 06:26 PM   #5
riku is offline riku  Finland
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Sorry my inaccurate explanation...

Plan was to order mached pair and change that odd one. In a good luck one within closer to normal values. Or at least have one witch is better attached in its base. For tube novice like me it was quite a surprice that values might change this much.

I don't have common B+ for both channels, so there is chance to tune PSU individually.

I was in sort of an understanding that too low bias lowers the ability to drive tube and lower current then again lowers the bass reproduction in output iron (too big aircap).

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Old 25th June 2004, 04:06 AM   #6
SHiFTY is offline SHiFTY  New Zealand
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If it has a loose base, it may well be damaged. MAybe it has a poor connection, or some vacuum has "leaked out"

Get a few and match them in circuit.
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