Matched tubes (or not) on cathode biased amp

[bluesholyman]

Does it make a tonal difference (I am thinking it would) when using matched power tubes on a cathode biased amp, versus using un-matched random tubes.

The specific amp in question is an Ampeg GVT15.

I am looking at swapping out stock tubes and wondering if matched or unmatched is tonally more desirable?

Thanks,
BHM.

[xtian]

I don't think cathode bias vs. fixed bias will matter to this discussion. In push-pull amps, having mismatched power tubes can create more harmonic distortion.

[bluesholyman]

Ok, so maybe I am just missing something. Biasing is more about running the tubes efficiently, than matching the paired tubes themselves...?

[Jerryz1963]

in cathode bias it does matter. my first amps were cathode bias and I was buying "matched" sets from a vendor. Only they weren't matched. The amp will distort sooner with unmatched tubes in cathode bias. You will get unpredictable results from one set to the next for that reason.

Then I got a tube tester and checked my "matched" tubes. I then matched them up myself and got much better sound quality as a result.

Fixed bias isn't nearly as critical because you can adjust the quiescent current and essentially force the tubes to match.

FWIW, screen resistors help particularly with cathode bias to keep things balanced. It's just something I noticed along the way. There are probably others who can explain why better than I. I use them as a matter of course. They don't have to be high value. It's my preference to control the distortion by design and not have it varying randomly based on what tubes I put in.

[mandopicker]

How close can tubes be and still be considered matched? 1, 2, ma - Make a difference? Great topic.

Thanks

[bluesholyman]

I normally buy tubes from bob at eurotubes, tend to trust his judgement. But I am wanting to do something quickly this time (rather than wait 3 days for the mail) and the locals don't have matched 6V6s, just singles....hence the basis for my question...

[vibratoking]

It is possible to match DC and/or AC parameters. If you are talking bias, then you are talking DC. DC matching may have little or nothing to do with the AC performance of the output section, such as gain and distortion. Just because multiple tubes have the same idle current does not mean that they have the same gm. Personally, closely matched tubes are more applicable to hi-fi than guitar amps. YMMV.

[Jerryz1963]

It is possible to match DC and/or AC parameters. If you are talking bias, then you are talking DC. DC matching may have little or nothing to do with the AC performance of the output section, such as gain and distortion. Just because multiple tubes have the same idle current does not mean that they have the same gm. Personally, closely matched tubes are more applicable to hi-fi than guitar amps. YMMV.

Would you mind commenting on tubes labeled as hard or soft? Do I have that right? I think there are three grades? it's aimed at guitar amp users/builders. That is, if it's not out of place in this thread.

[vibratoking]

Most girls prefer IT hard, but sometimes they want IT soft. It's all very confusing especially when there is no real specification given. Sometimes you don't even know what IT is. It's that way with tubes, too. Seriously, I have no idea what anyone that sells tubes means when they use those terms. You would have to ask each individual seller and I would guess that they can't specifically define what criteria is used when applying those descriptive names. Real data would be the most informative, but tube sellers rarely supply any of that.

[Jerryz1963]

I normally buy tubes from bob at eurotubes, tend to trust his judgement. But I am wanting to do something quickly this time (rather than wait 3 days for the mail) and the locals don't have matched 6V6s, just singles....hence the basis for my question...

Now, I'm an amateur, so take this for what its worth, but I have subbed in the 6P3S for 6V6 and they sounded good in an AUDIOPHILE amp. Exactly what you'd get in a guitar amp, I'm going to guess possibly less distortion. They might be harder to drive. I did read somewhere that NEW 6V6 are more like 6L6 than NOS 6V6, so, if that's true, then the 6P3S would be just as good if not better, than NEW 6V6. It's my understanding NOS 6V6 are easier to drive than the 6L6 family. At typical 6V6 B+, the cathode resistor value of 250 ohms is going to be just about right with the 6P3S. They might pass more current and your PS may not be up to the challenge. I overengineer so that's not an issue for me.

The only reason I mention them is because I bought a bulk lot of them and have never (in my limited experience) seen tubes match so closely. I ran all 50 of them through my tester and the majority of them came up within 2 to 4 units of each other on a scale of 100. I was impressed with the consistency. If a guitar amp is less critical in terms of matching, you just might like the result. They are certainly tough tubes and, when implemented properly, sound fine.

[gingertube]

Depends--
If each output tube has its own cathode bias resistor and bypass cap then NO.

If the push pull pair share a common cathode bias resistor and bypass cap then a matched pair is ESSENTIAL.

If building a new amp or repairing an old amp always implement the individual resistor + bypass cap for each output tube.

To convert
R value for each tube is X2 what was used for a pair.
Bypass cap for each of those new resistors is 1/2 what was used for a pair.

Cheers,
Ian

[bluesholyman]

Ian,

Thanks for the info - very useful. I am changing out tubes in an Ampeg GVT15 (new production, around 2011) - not sure if the bias resistor and cap are common - anyone know?