Matching power tubes

[drmmrr55]

I know this has been probably asked 100's of times, anyway, I have a 2 channel Stage Right amp with 4 El84's, and was wondering if a power tube goes bad, can I change just a matched pair, without having to change all 4 tubes? I assume that one pair is for channel 1, and the second pair for channel 2. As you can tell, I'm not real knowledgeable about tube amps. I just take to the shop when I need power tubes. (I don't know how to bias)

 

[Synchro]

I know this has been probably asked 100's of times, anyway, I have a 2 channel Stage Right amp with 4 El84's, and was wondering if a power tube goes bad, can I change just a matched pair, without having to change all 4 tubes? I assume that one pair is for channel 1, and the second pair for channel 2. As you can tell, I'm not real knowledgeable about tube amps. I just take to the shop when I need power tubes. (I don't know how to bias)

If you can find a duo of EL 84s that match close to what you have, you might be ok, but the quad is serving both channels of the amp, so the quad has to be balanced, at least to a reasonable extent.

 

[drmmrr55]

If you can find a duo of EL 84s that match close to what you have, you might be ok, but the quad is serving both channels of the amp, so the quad has to be balanced, at least to a reasonable extent.

Thanks, I Kinda thought that might be the case!

 

[bettyseldest]

If I understand a little of how tube amps work, the channels are only treated separately in the preamp. Your EL84 power amp valves take everything that the preamp pushes at it.

By all means put an unbalanced pair in the amp to confirm the problem, but unless you are lucky enough to have randomly selected matching valves, you will need to replace the set of four.

 

[radiocaster]

Since I assume you don't have a tester, you might not be correctly guessing which one is bad. The other pair may also have drifted so it's no longer balanced. Plus you would need to bias it, unless it's cathode biased.

 

[SnidelyWhiplash]

A lot of EL84 amps are cathode biased. There is no bias pot for adjustment.

 

[Synchro]

Thanks, I Kinda thought that might be the case!

There are “matched sets“, and the again, there are “matched sets”, if you know what I mean. My Twin had a matched quad of 6L6s when I got it, but when I checked the bias current on each tube, I found that one of the four was not really all that well matched. The tubes were healthy, in fact, very healthy, but the matching wasn’t all that close. It could have been run that way until the end of time, but I think that the amp sounds a little better with the quad I have in there now.

If your amp is currently working properly, I wouldn’t give it a second thought. The current supply chain problems with tubes will abate, and when they do, it might be worthwhile to buy a matched quad of EL 84s and maybe keep a few 12AX7s around, but I wouldn’t lose any sleep over it. Most amps I’ve owned were fine, right out of the box and unless you are beating that amp mercilessly, you’ll probably get years of service out of the OEM tubes.

 

[Greg70]

The 4 power tubes in a typical guitar amp work in a push pull arrangement. The signal being amplified is a wave that goes positive and negative. One pair of your tubes amplifies the positive side and the other pair amplifies the negative side. An amp with 2 tubes uses one tube for the positive and the other for the negative. If the positive and negative signals are different you will end up with an asymmetric waveform that may or may not sound as good. You will also get what is called crossover distortion. This is distortion that occurs when the signal "crosses over" from positive to negative and vice-versa. While we guitarist talk about liking distortion, usually what we like is harmonic distortion. Crossover distortion can be less appealing to the ear.

In my experience, I haven't found matched or unmatched to really sound any different. However it does become an issue when you go to bias an amp. Most amps only have one bias pot for the entire set of power tubes- you can't adjust the bias of an individual tube. So you often have to compromise with your bias and find a setting that works for all of them. The closer matched that the set is, then the less you have to compromise.

So finally getting around to answering the original question, you should rebias when replacing any of the power tubes. You can certainly replace one tube or a pair as long as you can get the bias within a reasonable range for each tube. If you replace with a matched set then the odds are much better that you can get every tube operating at its ideal bias. If you're taking it to a tech for service, then the tech is most likely going to suggest that you replace with a matched set so that he or she can get set the bias up really good.

 

[dan40]

I couldn't find any technical info on your particular amp but as the fellas mentioned above, most of these EL84 powered amps are cathode biased and do not require the bias to be set when replacing tubes. A well matched quad is recommended though.

 

[Wally]

Not all EL84 amps are cathode biased.examples of fixed biased EL84 amps would be the Fender Pro Jr. and Blues Jr. amps. (;^)
An analysis of the circuit would tell a decent tech what was what there. Pictures of the circuit board might reveal it here.

 

[Henry Mars]

Personally I would replace the quad set with a new matched set. 45 years ago I would have given you different advice. With the way they make tubes today, if your amp is on duty regularly I would replace them once a year anyway.

 

[marshman]

More than a few "newer" EL84 amps are properly fixed, with no bias pot to adjust it. Unlike the bigger bottles, the 84s were engineered with the expectation of cathode bias (they were considered light duty tubes) and from what I've read, there just wasn't much more power to be wrung from them if used in fixed rather than cathode bias.

That said, in my non-scientific observations, tightly-matched tubes seem to generate more volume before distortion sets in...wether that's something you like or not I can't say.

As noted, the odds of your amp being true stereo are pretty long, which means the 2 tubes to the left are working together and the 2 tubes to the right are working together. If you can determine which (if indeed any) tube is malfunctioning, removing it and one from the other pair and replacing with a matched pair should keep at least a bias balance, as opposed to only replacing one, which might have a pretty different bias than its "mate" on the other side.