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Post subject: Supersonic 22 bias - tubes mismatched?
Posted: Sat Mar 15, 2014 4:19 pm
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I'm looking at the bias of my supersonic. It's the first time I've done such a task.
I've got the schematic and I understand that, in basic terms, you set the voltage of TP40 and TP41 to 24mV using the bias pot.
So are the test points supposed to necessarily read the same? I get a couple of mV difference. I adjustment the bias such that TP41 reads 24mV, but TP40 is only on 21.4mV. Is this difference OK/normal? Or is it a sign that my power tubes are mismatched? Or that I've taken some measurements wrong?


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Post subject: Re: Supersonic 22 bias - tubes mismatched?
Posted: Sat Mar 15, 2014 5:39 pm
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A few milliamps difference is not bad, and yours are just over 10% difference. They could be better matched than that, but it isn't horrible. Try swapping the positions of the output tubes to see if you can get a closer match.

Often if you let the tubes idle for awhile, even play them for a bit, they can settle in a bit closer. So, recheck the bias after awhile and see if things are better.

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Post subject: Re: Supersonic 22 bias - tubes mismatched?
Posted: Sun Mar 16, 2014 6:19 am
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Difference up to 8 ma is better sounding , less steril than 0 ma.


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Post subject: Re: Supersonic 22 bias - tubes mismatched?
Posted: Thu Mar 20, 2014 2:54 am
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I'm very skeptical of tube matching. The properties of all components, including tubes, will drift as they age. I doubt a matched pair of tubes will stay matched as the tubes age. And who is going to guarantee that the resistors on the grid and suppressor are also going to match prefectly and age identically?

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Post subject: Re: Supersonic 22 bias - tubes mismatched?
Posted: Thu Mar 20, 2014 3:11 am
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mhowell , you are not wrong but if you start with unmatched tubes will they be worst with time ?

I don't buy / pay for matched tubes since few years and the tubes I received at home are always matched for me :lol:


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Post subject: Re: Supersonic 22 bias - tubes mismatched?
Posted: Thu Mar 20, 2014 9:53 am
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stratele52 wrote:
mhowell , you are not wrong but if you start with unmatched tubes will they be worst with time ?

I don't buy / pay for matched tubes since few years and the tubes I received at home are always matched for me :lol:

That's a good question and I don't know the answer.

A proper technician can correct me but presumably the goal is to minimize crossover distortion. I don't know that matched tubes can make that happen unless the entire push-pull circuit is balanced. Even with both sides of the entire push-pull circuit balanced, including the heaters, things will start to drift from day one. So what's the use?

What is matched in the tube anyway? Were measurements made at the plate, screen, and grid? Were the tubes tested in a tube tester or in a more real life circuit? How do we know the tubes match at various temperatures? Do the transfer characteristics match? I could go on but no sense in beating a dead horse.

If I were going to get matched tubes it would be from someone who explains how they match the tubes and charges a very small premium. For example, tubesandmore.com only charges $1.25 per tube and they explain "We match current production power tubes based on plate current and transconductance and consider them matched if they are within 2% of one another. We charge an additional $1.25 per tube for matching."

They charge $2.50 to match NOS tubes.

I'm skeptical that even that minimum sum is worth it but a buck or two is trivial so no real loss if it doesn't really help.

One final thing and I will finally shut up. :P
I would rather have quality unmatched tubes than crappy matched tubes.

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Post subject: Re: Supersonic 22 bias - tubes mismatched?
Posted: Thu Mar 20, 2014 11:50 am
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mhowell wrote:
stratele52 wrote:
mhowell , you are not wrong but if you start with unmatched tubes will they be worst with time ?

I don't buy / pay for matched tubes since few years and the tubes I received at home are always matched for me :lol:

That's a good question and I don't know the answer.

.


There is probably not one (1) answer . But I found mine :)


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