answers to questions about matched tubes.

[crazyzeke]

Informative post. As I recently learnt, the bias pot adjusts both tubes simultaenously, whereas I once assumed it adjusted the amount split between them. If your tubes aren't matched closely enough then the bias pot can't be useful, as one tube will always be out of range, whether it be on the hot or cold side. And crossover distortion being quite UGLY sounding, and limiting your dynamics, it's wise to get a good set.

What do you think of the JJ Electronic valves sold by Bob over at Eurotubes?

[skottissue]

The TV-7 series of testers are decent and well built testers, and like all testers they are far from perfect. They do test transconductance but the meter goes from 0-120. This is not micromhos, however you can figure out the micromhos and there are charts available for download online. On a TV-7, if the tube seller says, for example, that a 12AX7 tests 62/60 with a minimum good value of 56 you can figure out the micromhos and whether or not the tube is balanced or a pair is matched. The minimum good value on a TV-7 is 65%. So in this case 56 on the meter is the same as 65%. Using this you can figure out what theoretical 100% is. If you do some research with keywords "TV-7" and "conversion chart" you can find more information on how to understand TV-7 test results and conversion charts that allow you to translate a reading into micromhos.

[gregsguitars]

I never used matched seats ,I want my amp to work and pull ,may not get as much life from the tubes , but it is about the tone....

[Bill Ashton]

Just wondering...

"Back in the day," when we needed tubes in an amp we replaced them one at a time, after maybe having tested them at the local drug store or Radio Schack...sometimes it was 'maybe its time to change the tubes'...they were a very cheap item. Nowhere did anyone ever speak of matched tubes or even biasing after replacing tubes.

By "back in the day" I am refering to that wonderful time in the mid to late Sixties...I was there and I remember, what does that tell you ...but my amps always ran, never broke down, always sounded good as did most of my friends...

Maybe, maybe they were biased at the factory, but I doubt it was ever done again unless a trip to a tech...who probably would have been a TV repairman...got it done.

[Bill Ashton]

Thanks "7-Zero," that is a well thought out response and makes a lot of sense...

Yeah we were always talking RCA-GE-Westinghouse-Sylvania-Amperex...probably forgetting some...but they were all domestic-branded tubes, with just the occasional Japanese or British thrown in.

[wahchickawahwah]

This is a very helpful thread. Thanks for posting.

What's the story with RE-biasing? I remember being told when I first started playing through tube amps that I should take them to a shop to be re-biased every time I replaced a power tube, but I ceased doing that once it became prohibitively expensive, and haven't noticed any problems.

[Bill Ashton]

In a BF Fender amp, the "hum balance" control on the back panel is not the same thing as a bias control, correct?

[Jay]

No, it's not the same at all and there shouldn't be a hum balance control on a 60's black face Fender.

[Bill Ashton]

'K, then I am mistaken...

[crazyzeke]

'K, then I am mistaken...

You weren't mistaken. Your original question mentioned doubt about the function of a hum balance control on a BF Fender and Jay confirmed it. Perhaps it's an aftermarket modification added by an end user?

[amplituden]

Just to be clear, you guys know way more about this than I do.

However, I was told by a very reputable tube amp repair technician/boutique amp builder ( Kevin O'Connor http://www.londonpower.com/" onclick="window.open(this.href);return false; ) that the whole "matched tube" thing was hooey.

When I bought my Fender Bassman 100 head, I noticed after playing it for a while that there were only three power amp tubes in it. (I was 18, what can I say?!) I was stressing out about having to replace all the power tubes because I didn't have any money and I talked to him about it. He said, just go in to a guitar store and buy a 6L6 and stick it in. Don't worry about it, it will be fine.

And it was...

Since that day I have replaced single tubes with no consequences that I have noticed, that was approximately 1994.

Thoughts?

[crazyzeke]

Just to be clear, you guys know way more about this than I do.

However, I was told by a very reputable tube amp repair technician/boutique amp builder ( Kevin O'Connor http://www.londonpower.com/" onclick="window.open(this.href);return false; ) that the whole "matched tube" thing was hooey.

When I bought my Fender Bassman 100 head, I noticed after playing it for a while that there were only three power amp tubes in it. (I was 18, what can I say?!) I was stressing out about having to replace all the power tubes because I didn't have any money and I talked to him about it. He said, just go in to a guitar store and buy a 6L6 and stick it in. Don't worry about it, it will be fine.

And it was...

Since that day I have replaced single tubes with no consequences that I have noticed, that was approximately 1994.

Thoughts?

Every time I've replaced a duo/quad of power tubes for a matched version from a higher quality vendor, I've noticed an increase in complimentary harmonics/dynamic range and a significant reduction in crossover distortion once the bias is set correctly. So I've always been happy to pay the premium to get strigently matched tubes. It's the one GAS type thing I do buy into. Sure, they'll drift a bit with hours of use, but they still outclass cheaper, unmatched tubes.

Bear in mind I have near-perfect pitch so I'm inclined to notice this kind of thing. And as I understand it, even current draw from all the power amp tubes is good for longevity of other components in the amp too.

[eggwheat]

I always bias by ear and eye, never use any test equipment(unless something is acting funky)..and have never paid anyone to do it.

Did my friends '68 JMP 50 yesterday...tubes in..listen..sounds great!..ran tubes for about an hour playing it. Dead quiet, tubes running nice and cool..job done..no adjustment necessary.

Although you should be aware of the danger factor as there is some high DC in tube amps.

[Jay]

I always bias by ear and eye, never use any test equipment(unless something is acting funky)..and have never paid anyone to do it.

Did my friends '68 JMP 50 yesterday...tubes in..listen..sounds great!..ran tubes for about an hour playing it. Dead quiet, tubes running nice and cool..job done..no adjustment necessary.

Although you should be aware of the danger factor as there is some high DC in tube amps.

While this defenitely works fine if you have a good ear, folks who may not know what to listen for might be inadvertently setting their tubes and amp up to fail. To my ears a tube that's biased really cold clearly sounds terrible. Unfortunately a tube that's biased anywhere from a little bit cool to hotter then hell has an infinitely more subtle change in tone and without getting some idea of the current draw where you like the sound a premature tube failure could easily occur—maybe not an expensive repair in and of itself but when that failed tube starts taking out other portions of the amp the repair bill can add up quick.

[CZ101]

I think in Peavey Classic 30s (and maybe the whole "classic" series) - each power tube functions independently in series, so at least for the Peavey Classic 30 (I only know because this is the amp I use) - the quad matching can be important (unlike EL84 amps like those you speak of where you can achieve "half power" by removing either the outer two or inner two tubes)...

On that note, I have found I can get some interesting variation by mixing and matching different EL84s w/ my Classic 30.. Because it's my only amp and I can't afford to blow it up, however, I tend to stick with a "matched" quad of JJ EL84s (but I would love to try some better NOS tubes some day)..