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Post subject: Mismatched vs Matched Power Tubes
Posted: Thu Jan 03, 2019 10:26 am
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I've been going through a box of decent used (mostly RCA) 6V6's that I tested some time ago. There's no matched pairs so I've been trying mismatched pairs in a silverface Deluxe Reverb to kill the boredom. I've gone up to 5ma's difference so far. They actually sound better than the matched pairs I also have - to my ears. Any thoughts?

UN


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Post subject: Re: Mismatched vs Matched Power Tubes
Posted: Thu Jan 03, 2019 12:28 pm
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Here’s my thought on this, plate/cathode current within 5mA is close enough. Matched transconductance within 10% is a bonus. Even as high as 7 to 10 mA out usually works fine.


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Post subject: Re: Mismatched vs Matched Power Tubes
Posted: Thu Jan 03, 2019 3:05 pm
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If you had a perfectly matched set and a "close enough" set installed in the same exact amp side-by-side, what exactly would that 'bonus' amount to? Extra clarity? Warmth? Mojo?


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Post subject: Re: Mismatched vs Matched Power Tubes
Posted: Thu Jan 03, 2019 5:43 pm
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The closer the tubes are matched the better they work together in your amp giving you the optimal sound quality and tube life. The tube with more plate current will be dissipating more power and getting hotter than the tube with less plate current. This means the hotter tube will wear out quicker than the cooler tube leading to an earlier loss of clarity and definition. The amps bias should be checked after some hours have been put on a new set of tubes because the bias can change. Some tube suppliers offer burn in time on their tubes to help settle them in and this makes them less likely to drift from the initial bias setting.


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Post subject: Re: Mismatched vs Matched Power Tubes
Posted: Fri Jan 04, 2019 2:32 am
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roadhog96 wrote:
Here’s my thought on this, plate/cathode current within 5mA is close enough. Matched transconductance within 10% is a bonus. Even as high as 7 to 10 mA out usually works fine.



Perfectly matched tubes = sterile sound.
5 to 8 ma between tube are very good.

Upnorth2,
Your ear are right when you say ;

" I've gone up to 5ma's difference so far. They actually sound better than the matched pairs I also have - to my ears. Any thoughts ? "


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Post subject: Re: Mismatched vs Matched Power Tubes
Posted: Sat Jan 05, 2019 9:42 am
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Part of the reason you hear a difference between the matched and unmatched pairs is the inherent mismatch of the output transformer primary windings.
The voltage of the ot is dependent on the turns ratio. The two primary windings have to have the same number of turns.
However, one primary winding is laid on top of the other. So the inside primary is closer to the core and is inductively more efficient.
The outside winding is physically larger and each turn of wire is slightly longer. Taken together, the winding has a higher resistance. This reduces current flow compared to the inside winding.
I have seen instances where factories have split the windings to overcome these disparities and brighten the tone, but it's rare.
The phase inverter plate resistors are mismatched 100k/82k to help with phase balance of the transformer, but its only compensation, not error correction.

Musically, unbalanced waveshape is more appealing than balanced sound. Thats one of the draws to Class A amps.
Some Peavey heads have a back panel control to turn the drive to the bottom power tube down, resulting in very asymmetric waveshape.

So the question I have is when you get that golden mismatch, does is sound the same when you swap the power tubes side to side?
That is, is the mismatch you're creating compensating for OT variations or creating a more appealing asymmetry?
If it still sounds just as good, its the asymmetry you're hearing.
If its off peak in different positions, then you're mismatched tubes are compensating for OT variations.


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Post subject: Re: Mismatched vs Matched Power Tubes
Posted: Sat Jan 05, 2019 7:01 pm
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TimsAudio wrote:
So the question I have is when you get that golden mismatch, does is sound the same when you swap the power tubes side to side?


Thanks for the good info, I'll document the next time I go through them.

UN


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Post subject: Re: Mismatched vs Matched Power Tubes
Posted: Sun Jan 06, 2019 7:22 am
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"Matched" tube may also have less output stage hum ---- as the (+) and (-) waves are more closely symmetric.


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