I've got a large mV difference between my two E34L's

[TheShaggyFox]

It's about a 6.0 mV difference, I swapped the tubes and one tube just gets this 6.0 mv higher reading than the other. I'm assuming this is just the tubes fault? If so, what tubes should I get that will get closer readings? Or is this pretty normal? Thanks!

They're JJ E34L's by the way.

- Mike

[tonepilgrim]

That's just a slight mismatch between the tubes. A little more than one typically sees in "matched" tubes, but not a show stopper in my opinion. I've had worse. Bias it up on the hotter tube, let the colder one fall where it may, and let 'er rip. As the tubes burn in, those values will drift a bit anyway (recheck the bias in a few weeks and adjust). Your next set of tubes will probably be a little closer matched.

[TheShaggyFox]

That's just a slight mismatch between the tubes. A little more than one typically sees in "matched" tubes, but not a show stopper in my opinion. I've had worse. Bias it up on the hotter tube, let the colder one fall where it may, and let 'er rip. As the tubes burn in, those values will drift a bit anyway (recheck the bias in a few weeks and adjust). Your next set of tubes will probably be a little closer matched.

Cool thanks for the reply! I've been playing the amp since June now, at first I believe the mismatch was about 4 mV, now it's gone up to 6 mV. I set the hotter tube to about 39.5 mV and the other is at 33.5ish mV. I'm a little bummed, but I guess this gives me an excuse to buy higher quality tubes at sometime in the future.

[PCollen]

It's about a 6.0 mV difference, I swapped the tubes and one tube just gets this 6.0 mv higher reading than the other. I'm assuming this is just the tubes fault? If so, what tubes should I get that will get closer readings? Or is this pretty normal? Thanks!

They're JJ E34L's by the way.

- Mike

6.0 mV difference is too much. Consider this....if your plate voltage is 400 Vdc, and you are calculating 38 mA on one tube and 44 mA on another, that converts to a 15.2 watt and a 17.6 watt on the two tubes , respectively. Certainly within tolerances, but the bias spread is too wide, IMO. Now, if you have
42 mA and 48mA, the spread is still the same and you have one tube biased at 16.8 W and the other is biased smokin' hot at 19.2W. I'd get another set of tubes, for sure.

[demonufo]

6.0 mV difference is too much.

With respect, I totally disagree. Use your ears man, sometimes a spread like that can even sound better than perfectly matched tubes (whether it's all to do with Venus and Mars being perfectly aligned with the moon or whatever, I have no idea, but once or twice I have found mismatches sounding much more alive than matched tubes, and have NEVER gone wrong on me.

Now, if you have 42 mA and 48mA, the spread is still the same and you have one tube biased at 16.8 W and the other is biased smokin' hot at 19.2W.

Then bias the average slightly cooler. No big deal.

Either use a scope and bias by finding the crossover notch (and then backing off a tiny bit), or bias on the safe side and tweak by ear.

However, if you are concerned after everybody's thrown their 2c in, then get them changed by all means. I would say that doesn't quite qualify as a set of matched tubes, it's just that I don't think that's necessarily a bad thing. Of course, there's every chance that in your amp, a mismatch that wide may sound horrible. But always swap them round to see if you can hear a difference too, since Marshalls are usually somewhat unbalanced anyway, you might find that the mismatch works better hotter on one side, than the other.

To change, or not to change is totally down to what you're comfortable with, but biasing carefully, you should be perfectly OK with what you have.

[TheShaggyFox]

Thanks for the replies guys! Even though the mismatch is pretty big, I jammed the amp at practice yesterday at about 3 on both Vol I and Vol II (I jump channels) and ran it with a Catalinbread Dirty Little Secret clone that I got... This amp just destroys. I'm just pretty OCD about the amplifier so in the back of my head I keep thinking about how the tubes aren't matched up that well, but as long as it sounds the way that it does I have no complaints.

[Roe]

leave it if the amp sounds good. try to swap the tube (place the hottest tube in v5)

[kdmay]

I must admit, when I biased up the Mullards I got with the kit, they were about 4ma apart. I didn't think much of it, but when I cranked her up something just didn't quite feel right.

I then tried a set of Winged C's I had in another amp and they biased to within 0.5ma of each other. Now, I don't know whether it was the closer match or the Winged C's were just a superior tube, but the sound was night and day. My 1987 now absolutely rocks and I am totally happy.

Just my experience, I'm not experienced enough in this domain to put it down to one or the other, but you'd have to say that the matching had something to do with it.

As everyone says though, your ears are the ultimate judge.

[demonufo]

Just my experience, I'm not experienced enough in this domain to put it down to one or the other, but you'd have to say that the matching had something to do with it.

No you wouldn't!
Mullard re-issues = crap (and production varies a whole lot too!)
Winged C = the best EL34 available today IMO.

[Roe]

3-4mV difference often sounds best