Mixed or Matched 6L6s?

Chris Bauer · From: Nashville, TN USA

As every post of mine in this section amply demonstrates, my knowledge of electronics falls somewhere south of the kindergarten level... So here's two more completely naive questions on power tubes (6L6s in this case):

1. Since different 6L6s have different sonic characteristics, is there any potential sonic value in mixing brands or is that simply asking for an impossible biasing job (or other electronic mayhem I'm not even informed enough to imagine)?

2. In an amp using four power tubes, is there any issue with using two sets of matched pairs versus a matched quad set?

Tom Gorr · From: Three Hills, Alberta

Ive ran specially selected 6v6s and el34s as two sets of matched pairs in the same amp.

ive used two different brands sets of 6L6s in the same amp.

Lots of things are possible if you really know your amp and how to spec tubes for selection.

in general its not necessary to have a matched quad as long as the two sets of matched pairs are still in the proper bias zone. For the small extra fee...it keeps life simple.

As for mixing different tonal colors...a quad of great tubes will probably always sound better than two mismatched pairs that compensate for each others weaknesses.

In general...however....if you don t
have a certain level of understanding..your results will be random.

Tubes are expensive...and its easy to spend a thousand bucks learning what you like and dislike and what works and doesnt.

Once you have a great set of tubes in a great anp...money is better spent on optimizing the speaker and cabinet selection.

Mark Fowler · From: Minnesota, USA

That's why it is really nice to have individual bias adjust for each tube like on high end amps but even having bias adjust for each pair would allow you to run different 6L6 tube set each side correctly bias like an old Mesa Boogie amp. Smile

Mark

Gil Berry · From: Westminster, CA, USA

A question for you real experts out there. I know that biasing a set of tubes such as 6L6's would make both tubes identical at whatever signal point the bias was set, but unless the gain curves are identical for both tubes then at another input signal level I would expect the output current to differ. Perhaps not a big deal of difference, but enough so that a push-pull setup might distort the midpoint? Or in a parallel setup wouldn't one tube be pushing more than it's counterpart? If that's the case, is there really any need for biasing the pair if they are pretty close to begin with?

George Kimery · From: Limestone, TN, USA

Since I now have two 112 Valve Kings and I am now a "tube" guy, I need to be educated. What is involved in "biasing" tubes? Seems to me that tubes are set in concrete and there is no adjustment on them that I can see. Does the biasing have to be done by a tech, internally?

I have noticed that they seem to sell the 6L6's in either sets of two or four. Does this mean the set is already biased, and is "matched" and "biased" terms that mean the same thing? And what happens if the tubes aren't biased properly? Just generally poor performance? I can't tell any difference in sound between the two amps I have. So, does this mean either they are both biased correctly or both are not biased correctly? Is there any test I can run myself that will let me know if my tubes are correctly biased (or matched?).

This whole biasing thing is new to me, so I hope somebody can bring me up to speed in layman language.

Thanks for any help.

Gil Berry · From: Westminster, CA, USA

George, I am not an expert, so my comments are certainly subject to review and outright dismissal by someone who is, but, I did study electonics for several years before the advent of the integrated circuit. An amplifier tube, such as a 6L6, consists of a "heater" to make the electrons boil from the surface of the cathode. A high positive voltage on the "plate" of the tube attracts those electrons. In between the cathode and the plate are control grids, which help or hinder the flow of electrons across the tube. When tubes are hooked up so that two of them have the same voltages and signals applied and are feeding the same output (they are in parallel with each other) it's necessary they be "balanced" or "biased" so that both tubes conduct equally, else one tube does most of the work and will burn itself out prematurely. Also, if two tubes are connected in a push-pull circuit (one tube conducts when the signal voltage is positive, the other when the signal voltage is negative), then the output of both tubes is reformed to match the original sine-type wave at the output device (usually a transformer) except that now the signal is greatly amplified. In this scenario, if the tubes are not "balanced" (biased) half the signal would be amplified more than the other half causing distortion and a shift of the point at which neither tube is conducting (zero signal input). Ok, I hope this explains a bit of what biasing is for. As for how it is done, this is by controlling the voltages applied to the tube grids and/or screens to attempt to make both tubes conduct with the same current at identical signal inputs.

Mark Fowler · From: Minnesota, USA

As far as I know your Peavey is fixed bias non adjustable so would need to change resistors in the bias circuit if it was not within 60-70% level.

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