Tube Matching

[ajeffcote]

Just put a pair of NOS RCA 6V6's in my amp.It is fixed bias,adjustable via a pot.I set the bias at about 70% idle.For one tube.The other at the same setting is idling at about 52%.I could,and may eventually,put another pot in so I can set each tube seperately.
The only disadvantage I see to running a mismatched pair is that one will have a shorter lifespan unless I swap their positions occasionally.Maybe if they are closely matched the break up point will be closer and they will sound different,but you get into preference there.
I guess my question is how closely matched is matched?

[Diablo]

Swapping positions won't change tube bias - the same bias voltage is applied to both tubes. The hot tube on the left just becomes the hot tube on the right if you swap positions. I would just use the tubes and you may find their bias drifts through use. They may even start to drift closer to each other.

[defaced]

I guess my question is how closely matched is matched?

Everyone does it differently. Matched for one supplier is not the same criteria as matched for a different supplier.

The answer to bias is answering yes to these two questions:
"Does it sound good?"
and
"Is it within spec?"

If you like they way your setup sounds, and we know it's within spec, then you're good to go.

[ajeffcote]

Swapping positions won't change tube bias - the same bias voltage is applied to both tubes. The hot tube on the left just becomes the hot tube on the right if you swap positions. I would just use the tubes and you may find their bias drifts through use. They may even start to drift closer to each other.

I realized what I said about position after the fact.But I was too lazy to change it.
They do sound good the way they are.

[Old Tele man]

...something to remember, the OT-primary windings are seldom perfectly matched, and thus can account for anything from 1V- to 10V-differences in plate voltages...actual voltage measurements are the only way to know for sure.

[Robert M. Martinelli]

...something to remember, the OT-primary windings are seldom perfectly matched, and thus can account for anything from 1V- to 10V-differences in plate voltages...actual voltage measurements are the only way to know for sure.

Very good point, and this brings us to another point : Even carefully matched tubes ( which, BTW, will tend to cut even harmonics in an AB1 design ) will be matched only for a few hours/days of operation as the slightly different working conditions they face will wear them at different rates....Just another reason not to worry too much if the tubes are matched, say, within a 10% tolerance IMHO.....

My advice would be not to over-analyze the "match" issue and trust your ears...If it sounds good it probably is...

Hope this helps

Best regards

Bob

[MWJB]

Too much mismatch on plate current will kill fidelity...you might like that though?

[ajeffcote]

What I like to hear when I play are clean,fat notes that just start to break up when I bear down,clean up when I back off.So if one tube is on the verge of saturation,the other is still cold,without being matched more closely.The sound now when the tubes are pushed into distortion is good.But I hardly use that much distortion.
Maybe they will drift closer with time,but right now I'm seeing a difference of about 5 Mv.

[MWJB]

+/- 5mA is typically within tolerance for a "matched" pair, when discusing 6V6/6L6/EL34 etc, it's unlikely that one will be saturated & the other "cold, more a case of the balance of push pull will be slightly out of whack, taking the edge off fidelity.

Many amps come from factories with a greater degree of misatch than that, you only have a problem if you can hear one. Many respected tube vendors would accept 5mA as being within tolerance...though there may be applications where a closer match may be required, but not many.

[jbltwin1]

bias mis-match

SOME people prefer a little mis-match. I'm one to check balance when I install them but unless they are REALLY out of balance(one pulling 50mA and the other 20, I'm not likely to screw with them. 5mA is actually pretty good in todays world.

[ajeffcote]

After burning in for about 36 hours they are about 4 Ma's apart.And they sound good.I'm still going to install another bias pot so I can adjust to whatever my preference maybe.
Thanks for all the replies.I welcome and appreciate the wealth of knowledge you all have shared with me.I've learned a lot since I started prowling around this forum!

[stokes]

Closer than 5ma's may be desireable in a Hi-Fi application,but for guitar amps I find they have a bit better tone,particularly at the "edge" of breakup as you say you like.
Adding another bias pot would require a second supply,it would be easier to add a "balance pot" ala the Fender Silver Face bias set-up.

[Enzo]

Amen. Fidelity is not the purpose of a guitar amp.

[jrfrond]

Some of you here (like me) will remember the days when you bought tubes from a supplier and tossed them in, and all was good. The concept of matching sold a lot of tubes, and for the most part, that is still all it is good for, as far as guitar amps go anyway.

If you do buy matched tubes, remember that the only thing being measured is gamma, and nothing else. Also, matching is not an indication of quality or lifespan.

[stokes]

Some of you here (like me) will remember the days when you bought tubes from a supplier and tossed them in, and all was good. The concept of matching sold a lot of tubes, and for the most part, that is still all it is good for, as far as guitar amps go anyway.

If you do buy matched tubes, remember that the only thing being measured is gamma, and nothing else. Also, matching is not an indication of quality or lifespan.

Yeah,I remember those days.Nobody even bothered with bias either,but I cant say I miss those days.I was 14 years old when a local TV repairman taught me to bias my power tubes,making me a very popular 14 year old to all the hippie musicians in the neighborhood.Although precise matching isnt all that important,I cant say I agree that just installing random pairs or ignoring bias is a good idea.