Can a single ended el84 amp be used to match tubes?

[59Vampire]

I scored a badcat min i 2 and was wondering if the bias meter could be used to measure current for matching the multitude of tubes i have. i blew up my fixed bias amp a year or so ago!

forgive me if i stupidly asked this question in the past.

[mike shaw]

I asked the same question elsewhere a couple of years ago and the answer I received was 'no'.

[rockon1]

No expert but I cant see why you couldnt use a fixed bias single ended amp to measure the current draw of the tubes. Bob

[SamBooka]

I am probably talking out my a$$ it will get you in the ball park most of the time. The problem is no one really defines what "matched" means, especially when you are dealing with vendors on ebay.

Bias meters will tell you what the current draw is but not things like transconductance which I believe most reputable vendors match to (although even some very well know dealers seem to match new prod tubes by mA (or at least if they are matching to something else they dont say what)). I also think that that value would not accurately reflect perfomance of a tube in AB.

If I wanted to match tubes for personal use? Yeah.. that would probably be good enough for me (deaf as a post). If I were reselling I would have serious ethical issues using a SE amp.

Hopefully someone knowledgable will chime in. Take what I say with as many grains of salt as your blood pressure will allow.

[Blue Strat]

No expert but I cant see why you couldnt use a fixed bias single ended amp to measure the current draw of the tubes. Bob

Fixed bias yes. Cathode bias no. Most single ended amps are cathode bias aka "self biasing" which means that they attempt to bring the plate/cathode current to the same point. This means that a lot of tubes that don't match will give approximately the same reading.

[Blue Strat]

I am probably talking out my a$$ it will get you in the ball park most of the time. The problem is no one really defines what "matched" means, especially when you are dealing with vendors on ebay.

.


Probably because few of them know anything about tubes, tube matching, or electronics.

The few that do clearly specify how their tubes are matched. Then you have to know enough to realize whether their methods are valid or not.

[rockon1]

Fixed bias yes. Cathode bias no. Most single ended amps are cathode bias aka "self biasing" which means that they attempt to bring the plate/cathode current to the same point. This means that a lot of tubes that don't match will give approximately the same reading.

Hence I wrote fixed bias. Written fixed bias might of made that point clearer. : > )

[Blue Strat]

Hence I wrote fixed bias. Written fixed bias might of made that point clearer. : > )

I was just trying to clarify the point since there's a lot of mystery and misconceptions regarding fixed and "self biasing" which YOU know aren't the same, but a lot of other people aren't so sure about:)

[mike shaw]

Fixed bias - yes. Just make sure you adjust the negative bias voltage to a value and use the same value for all the tubes you test. You want to keep it as consistant as possible.

[59Vampire]

These are the responses I thought. I knew that cathode biased gives similar but wasnt exactly sure with the one tube variety. An matching to transconductance doesnt mean much in my . Thats more of a qualitative match than an electrical current flowing match. Would you agree?

[rockon1]

You could always buy a cheap fixed bias amp like the Crate VC50 I bought for 179$.(It actually sounds pretty good too.) FWIW- As far as matching is concerned I think matching current draw alone is perfectly acceptable. Also for 12-14 watt tubes it may be more important to have them closer matched than 25-30 watt tubes since 1 ma is a bigger % (if you get my drift) especially if they are running higher pv. Bob

[Old Tele man]

short & simple answer: no, not with a SE amp, because:

...when "matching" two (or more) tubes, the idea (ideally [pun]) is to have them all respond identically, as much as possible, under the same operating bias-, screen- and plate-voltages.

...the SE amp can only accommodate one output tube at a time, so "side-by-side" testing under the same operating bias-, screen- and plate-voltages cannot be done.

...and, since each tube typically loads down the plate voltage slightly differently, the voltages for one tube will generally NOT match those of subsequent tubes plugged in and tested.

[rockon1]

short & simple answer: no, not with a SE amp, because:

...when "matching" two (or more) tubes, the idea (ideally [pun]) is to have them all respond identically, as much as possible, under the same operating bias-, screen- and plate-voltages.

...the SE amp can only accommodate one output tube at a time, so "side-by-side" testing under the same operating bias-, screen- and plate-voltages cannot be done.

...and, since each tube typically loads down the plate voltage slightly differently, the voltages for one tube will generally NOT match those of subsequent tubes plugged in and tested.

Da#n Im having a hard time following this.... Wouldnt each tube you plugged in be tested at the exact operating conditions? Bob

[Old Tele man]

Da#n Im having a hard time following this.... Wouldnt each tube you plugged in be tested at the exact operating conditions? Bob...no, because *if* any tube responds even slightly differently to the bias voltage, the tubes' differing plate current will simultaneously pull-down the plate voltage yielding NONE EQUAL operating conditions.

[rockon1]

Ahhh..I think I get it. In a two tube amp it still seems like it would be difficult though since you would need to measure the PV of each set of tubes installed too?? Bob

[Old Tele man]

...when both tubes-under-test are operated from the exact same screen- and plate-voltage sources (such as happens to the two output tubes on either side of a parallel-push-pull, ie: Twin 4x6L6, etc.), then they can be measured and matched to each other because that "matching" occurred under the same operating bias-, screen- and plate-voltages.

...ironically, even though the 'other' two tubes on the other side of the same parallel-push-pull OT would seem to also be operating under the same operating bias-, screen- and plate-voltages, and thus making all 4 tubes matched, that's typically not so because the DC-resistances of the two different sides of the OT windings are almost never perfectly matched, so the plate voltages to each side are seldom equal.

...so, conditions on side-A are equal, and the conditions on side-B are equal; but, both side-A and side-B are seldom equal to each other.