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Post subject: 6L6 duet not quite matched
Posted: Fri Nov 27, 2015 3:02 pm
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I installed a JJ 6L6 duet in my Prosonic today and, when using a bias probe, found that they weren't quite as matched as the vendor would have me believe. They were off by about 6mA between them. In the end I biased it such that one side dialed in at 36mA and the other side at 42mA - a tad hot but not out of the usable range.

Does anyone with a deeper knowledge of this know how significant the 6mA difference is? I know the tubes will continue to function with one working harder than the other and they may fail sooner than they should, but in the grand scheme how significant of a difference is it?


Thank you


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Post subject: Re: 6L6 duet not quite matched
Posted: Fri Nov 27, 2015 6:35 pm
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Such a difference would not pass muster with me and I'd insist upon a partial refund of my money or an outright exchange. A matched pair (or quartet) of output tubes should measure within 1.0 or 1.5 mA of each other.

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Post subject: Re: 6L6 duet not quite matched
Posted: Fri Nov 27, 2015 9:49 pm
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I would try swapping the tubes positions first and see if the higher current stays with the tube or with the position. :idea:

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Post subject: Re: 6L6 duet not quite matched
Posted: Sat Nov 28, 2015 2:47 am
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On Gerald Weber amps's book, he write that a mismatch under 8-10 ma is more musical than a perfect match witch could be sterile.

I never pay for a matched pair or quad since I read that and hear no difference and they are always under their specs when received.

I see sometimes tubes could be mismatch with some bias / plate voltage and good match with other bias / plate voltage .


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Post subject: Re: 6L6 duet not quite matched
Posted: Sat Nov 28, 2015 10:10 am
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Each of us is entitled to our own opinions.

Arjay

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Post subject: Re: 6L6 duet not quite matched
Posted: Sat Nov 28, 2015 10:57 am
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If you're paying for a matched pair of new JJ, they should be closer than +-3mA. Most of the matched JJ pairs I get are within +-1.5mA.
Follow shimmy's advice about swapping positions.
As to whether it makes any difference, well, the opposing polarities of the transformer primaries are supposed to cancel residual ripple in the power supply.
This relies on a perfect balance of current through the idling power tubes. It's usually only important for recording situations.
A 4 tube power stage needs to have the opposing pairs matched as well so current sharing can deliver maximum power without one tube runng out of steam before its twin.
But for all the care we take to match tubes and phase inverter drive, the output transformer primaries are the main mismatch in the power stage. The two primary windings are required to have the same number of turns. However, when one is wound over the other, the outside winding will be a longer wire with more resistance. Since it is further from the core, it also has less inductance than the inside winding.
These differences can be reduced by splitting the windings into sections. These are then staggered between the secondary windings. I've seen this in Fenders and Marshalls.
Still, it's difficult to acheive any kind of actual balanced operation out of a standard output transformer, regardless of tube match..


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Post subject: Re: 6L6 duet not quite matched
Posted: Sat Nov 28, 2015 11:30 am
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What he () said!

:wink:

Arjay

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Post subject: Re: 6L6 duet not quite matched
Posted: Sat Nov 28, 2015 1:04 pm
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Thanks for everyone's input.

I had ordered the duet years ago when I wasn't quite as knowledgeable about tubes as I am today (still got a long way to go though!). I am now questioning whether or not I requested that the pair be matched way back when I ordered them. I may not have. Regardless, I am picky enough that I may order a new duet, test the position swaps if they too are off, and reinstall/re-bias. I like this amp plenty enough to do so. Overall I agree that they should be closer in values than they are.


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Post subject: Re: 6L6 duet not quite matched
Posted: Sat Nov 28, 2015 1:48 pm
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divebomb wrote:
I like this amp plenty enough to do so.


+1

The Prosonic was a great amp but misunderstood by many. But for those players who took the time to explore the optional rectifier modes, it was a real sonic treat. I nearly bought the head version myself back when they were available.

Arjay

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Post subject: Re: 6L6 duet not quite matched
Posted: Sat Nov 28, 2015 6:27 pm
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And the cleans! They are fantastic. People rarely talk about that aspect since the gain is so unique on this amp. You need to re-EQ coming out of the drive channel due to the shared EQ, but the cleans are outstanding. In fact, I had done a wall-to-wall retube and re-bias on my DRRI the day prior the Prosonic, and when I A/B'd the clean channels on each, I believe the Prosonic had an edge. It's hard to describe, but "slightly more robust" might be the phrase. The differing speaker configurations make it more difficult to really compare the two overall.


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Post subject: Re: 6L6 duet not quite matched
Posted: Sat Nov 28, 2015 10:14 pm
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divebomb wrote:
It's hard to describe, but "slightly more robust" might be the phrase.


+1

Thus are the nuanced differences between a 6V6GTA and a 6L6GC. The bigger (and more powerful) bottle exhibits an enhanced bottom end.

Arjay

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