Unmatched power tubes- Anything to worry about?

[edward]

I love threads like this because I know precious little about tubes and their respective circuits.

But I've rolled tubes in the past, heard some great differences, heard zero differences, and only twice in my life (um, I'm old) ever heard genuinely bad tubes (they rattled with higher volume), and only once had a power tube fail (fortunately during a rehearsal, and I did have a spare 6L6, yesss!).

All this to say I've gone the "ocd" route, NOS game, etc, LOL! And the wheel has come full circle and have now come to the simplest conclusion: don't obsess! Gear should be in the ballpark, plain and simple. The rest is up to me ...well, and the band if I've got one. So I still have my spares, but have long been done with obsessing over this or that tube.

I'll catch flak for this, but will say it: maybe ol' Randall Smith and other fixed-bias guys have a valid point: get it right at the onset, establish a workable range, and leave the rest of it to you and where you set your knobs!

Edward

 

[VICOwner]

Very cool! Thank you.. Yes, the tube on the left labeled Sylvania has the hole on the bottom of the base and the one on the right labeled GE does not. So, the one on the left is possibly an XF2 or XF3 Mullard and the one on the right is an XF4 Mullard? This is a great education, so thank you again.

Edit: I took a closer look at the tubes. The Sylvania one is labeled XF two with the Blackburn code. The GE on the right, I’m not so sure, but here is the remnants of a code. There is nothing additional on this tube besides the silk screening. So this is an XF4?

The GE is not a Mullard if it doesn’t have the hole in the locating pin. The ribs on the plate structure is also throwing me off. Not a Mullard there.

 

[skhan007]

The GE is not a Mullard if it doesn’t have the hole in the locating pin. The ribs on the plate structure is also throwing me off. Not a Mullard there.

Gotcha. OK, the GE tube is something else, but sounds excellent when paired with the Sylvania Mullard. I guess at the end of the day, if it sounds good, it is good.

 

[Timbre Wolf]

I know it's cool to act like we don't care - very rock 'n' roll - and then pull out that old back in the day adage. But I'm not afraid to be that nerdy voice of dissent (one of 3, according to the poll).

In response to the title question Unmatched power tubes- Anything to worry about? Yes! One of the tubes can be drawing too much current, and could head toward early demise after redplating. Make absolute certain to bias the hottest tube at the maximum dissipation limit,or lower, instead of biasing to the lowest, or even to average.

 

[Joe Robinson]

I know it's cool to act like we don't care - very rock 'n' roll - any pull out that old back in the day adage. But I'm not afraid to be that nerdy voice of dissent (one of 3, according to the poll).

In response to the title question Unmatched power tubes- Anything to worry about? Yes! One of the tubes can be drawing too much current, and could head toward early demise after redplating. Make absolute certain to bias the hottest tube at the maximum dissipation limit,or lower, instead of biasing to the lowest, or even to average.

What about a cathode bias circuit, such as an AC30?

 

[skhan007]

I know it's cool to act like we don't care - very rock 'n' roll - any pull out that old back in the day adage. But I'm not afraid to be that nerdy voice of dissent (one of 3, according to the poll).

In response to the title question Unmatched power tubes- Anything to worry about? Yes! One of the tubes can be drawing too much current, and could head toward early demise after redplating. Make absolute certain to bias the hottest tube at the maximum dissipation limit,or lower, instead of biasing to the lowest, or even to average.

Yup, that’s exactly what I did. The hotter tube is at 32 mA @ 498 v DC, which is about 63%. The second tube is much, much lower at about 18.5 mA.

 

[Timbre Wolf]

What about a cathode bias circuit, such as an AC30?

Redplating can still happen in cathode-bias circuits.

 

[HotBluePlates]

What about a cathode bias circuit, such as an AC30?

Redplating can still happen in cathode-bias circuits.

Everything is worse in a cathode-biased circuit, because you're guaranteed the over-hot tube(s) will run even hotter.

That's "biasing to the average" that @Timbre Wolf warned against in his post.

The caveat is cathode-biasing pushes all the tubes towards an average idle-point.​

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The cooler tube(s) idle warmer and the hotter tube(s) idle cooler. Most likely, this will result in the over-hot tubes idling higher than their rated dissipation.​

 

[JB6464]

Simple answer, unmatched power tubes are perfectly fine so long as the highest output tube is within the bias range.
Now will it sound the best, that's totally up to the player's ear.

 

[AintNoEddie]

Still I assume the one biased hotter would wear out faster than the other one?

 

[Moderato]

no

 

[HotBluePlates]

Still I assume the one biased hotter would wear out faster than the other one?

The best we could honestly say is, "not sure."

Who here has done any kind of MTBF testing of mismatched output tubes, and compared the results to MTBF testing of matching tubes? Where is the data to show the mismatched tubes fail earlier?​

Absent any defects that cause one of the tubes to die early, both will probably last a long time. We like to equate tube-wear to physical things like tires (as happened earlier in this thread). But what we really have is more like, "how long with this rechargeable battery last? Will it stop holding charge if I use it _____ way?"

That's because the tube operates by releasing electrons from the cathode, and replenishing those electrons via a connection from cathode-to-ground.​

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It's somewhat uncertain how long this electro-chemical process will continue, and at what future point the cathode will deteriorate enough that the tube's performance shows "wear."​

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We might think the cooler-running tube will last longer, such that at some time in the future the tubes will have drifted far apart. However, it may take many years before we get to that point. Will we care that one tube gave us "many years" and the other might give us "many years, and then some"?​

​

The old reference books from the 1950s talking about tube-matching and used an example where the two output tubes were completely different types (one being half as strong as the other, to simulate a very worn-out tube). The bottom-line conclusion was the mismatch generated less clean output power, and a bit more distortion than perfectly-matching tubes.

 

[AintNoEddie]

@Moderato @HotBluePlates just to be sure, I was not referring to faster wear-out because of the mismatching. My question was based on the claim (not mine) that hot biased tubes wear out faster than cold biased ones. If that was true, this would mean an uneven wear for mismatched tubes ("hotter" tube needs to be replaced sooner).

 

[Dontchaknow]

It’ll be fine, back in the day I doubt people paid attention to all that. Everyone always overthinks this stuff. If it works than it’s fine.

See this a lot as a bit of "real musicians don't worry about it", but how many amps back in the day were considered dogs or just didn't quite have it, etc vs others of the same model that were awesome and how many times was that because of less then optimal operating condition? Of course we will never know, but you get the point.

 

[Geetarpicker]

I’d check the amp cranked in a room with lights off to verify there is NO red plating. In my experience with old Marshall 100 watters a badly matched set of output tubes can be hard to tame bias wise. Sometimes just juggling the tubes around socket to socket can find a workable situation. That said check the numbers, but also check in the dark as well.

 

[HotBluePlates]

@Moderato @HotBluePlates just to be sure ... My question was based on the claim (not mine) that hot biased tubes wear out faster than cold biased ones. ...

Yeah, that is what I was thinking about. I'm not sure the experimental data exists proving that... it's just something we assume all the time.

There are too many "Exceptions to the rule":

Play a Class AB or Class B amp LOUD, and the tubes wear out faster because they're working the cathodes very hard.​

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Play a Class A amp LOUD, and it's not much different than idling. The Plate (though not the Cathode) actually runs cooler when driven hard because of the power transferred to the speaker (not the same situation as Class AB or Class B).​

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"Hot Biased" is meaningless on its own: idling at 75% or idling at 300%? Somewhere between? What does it mean?​

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The idle bias doesn't tell us what the amp is doing when driven, and presumably we are playing through the amp more than we are leaving it on/idling.​

And so that was the series of thoughts that lead me to conclude, "We just don't know; not enough info."

 

[AintNoEddie]

Yeah, that is what I was thinking about. I'm not sure the experimental data exists proving that... it's just something we assume all the time.

There are too many "Exceptions to the rule":

Play a Class AB or Class B amp LOUD, and the tubes wear out faster because they're working the cathodes very hard.​

​

lay a Class A amp LOUD, and it's not much different than idling. The Plate (though not the Cathode) actually runs cooler when driven hard because of the power transferred to the speaker (not the same situation as Class AB or Class B).​

​

"Hot Biased" is meaningless on its own: idling at 75% or idling at 300%? Somewhere between? What does it mean?​

​

The idle bias doesn't tell us what the amp is doing when driven, and presumably we are playing through the amp more than we are leaving it on/idling.​

And so that was the series of thoughts that lead me to conclude, "We just don't know; not enough info."

Thanks, appreciate your thoughts on this!

 

[ibanez-stratplayer]

I know Ill catch it for this, but, gee in 1968 on the way to a gig you stopped in a 7-11, grabbed a single 6L6 out of the bottom of the tube tester cabinet, paid yer 2-3$ stuck it in the amp and had a ball. No meters, no current measurements or adjustments.
Or if you were really classy maybe a Lafayette or a Radio Shack.
Just sayin’

Exactly. Now fast forward 50 some years later and one phsycho decides to start a war and we're all wondering if we're ever going to be able to find tubes again