Do I need to use matched power valves in a cathode-biased amp?

[Dave_Mc]

Hey everyone

I've got a Fender Excelsior here which I'm getting a lot of rattle from- MusicStore suggested I should try swapping the preamp valves as they might be microphonic (I already knew that, but wanted their blessing in case they tried to claim doing that had voided the warranty). No problem, I have several working preamp valves here.

I figured when I was at it, I might as well swap the power amp valves, too (unless that definitely won't solve the problem). Problem is, I don't have a matched set of 6V6es- but I do have two (unmatched) JJ 6V6es that I could try.

Far as I'm aware, the Excelsior is cathode-biased, so that means no biasing (right?). Do I need to use matched power tubes? I've heard conflicting things, and as there are several electronics gurus here whose opinions I trust, I thought I might as well ask.

Here's the schematic (which I'm assuming is correct), if anyone wants to check it's actually cathode-biased.

https://docs.google.com/file/d/0B94M...dlk/edit?pli=1

Thanks very much for your help

[ICBM]

Yes, cathode-biased, with a common cathode resistor which means that matched valves are a good idea.

Reasonably well-matched valves are always a good idea really, even in fixed-bias amps with separate adjustments or in cathode-biased amps with individual resistors. Just because valves can be made to match for idle current by adjusting them individually doesn't really mean they will perform the same under signal conditions. But that depends whether 'the same' is going to give you the best sound, in a guitar amp... it might not. The most important thing about matching the valves at idle (either inherently or by separate biasing) is that it minimises hum.

[Dave_Mc]

Thanks ICBM, i was hoping you'd see this. I'll not chance it (also there's a fair bit of background noise, so minimising hum would be useful).

Follow-up bonus question- JJ or Tungsol (or something else?)? I've tried JJs, and they're quite nice, but they have a bit of a hard edge when driven that I'm not sure I like (of course, maybe all 6v6es are like that). Supposedly Tungsols are a bit creamier, which I might prefer, but I haven't tried them...

[ICBM]

Just try it, the worst that can happen is that it will hum.

I don't like JJ valves much either, although I admit they're reliable by modern valve standards. The power valves aren't as bad as the preamp valves, the ECC83s in particular are horrible (in my opinion).

[Dave_Mc]

oh ok so it won't do any harm then?

i don't mind JJs in brighter (or more marshally) amps but i'm not convinced by them in more fendery things.

[ecc83]

oh ok so it won't do any harm then?

i don't mind JJs in brighter (or more marshally) amps but i'm not convinced by them in more fendery things.

No, no harm. You can get a matched pair of Ruskies from Langrex for a tenner. Nocaster has used them.

FWIW I did some tests on "matched" pairs and quartets of EL84s and EL34s a couple of years ago and found better matches OUTSIDE the sets than within them!

Dave.

[jpfamps]

As stated above, it would be better to use matched valves.

The Tungsols have a more "classic" 6V6 sound in my view when compared to the JJs.

I like the JJ-6V6s a lot, but have had trouble with filament rattle in combos.

NOS 6V6s are available at reasonable prices, but are a bit of a crap shoot in my opinion.

6V6s were used in tons of gear (TVs, car radios etc), and unlike many valves, were made by virtually all the major manufacturers, and are quite inconsistent.

[Dave_Mc]

^ thanks. that's what i was hoping regarding the tungsols versus the JJs- I don't particularly dislike the JJs, but they are quite aggressive-sounding.

No, no harm. You can get a matched pair of Ruskies from Langrex for a tenner. Nocaster has used them.

FWIW I did some tests on "matched" pairs and quartets of EL84s and EL34s a couple of years ago and found better matches OUTSIDE the sets than within them!

Dave.

haha, i could well believe that

Thanks. Not sure i'll bother, though- I have new "matched" tungsol 6v6es on the way, so I might just wait until they get here. (Also to get the other "unmatched" 6V6es I'd have to open another couple of amps to get them out and I'm not sure I can be bothered when i have new ones on the way, anyway.)

I'll try the preamp valves, though, i have a couple of JJs lying around.

EDIT: ok, tried the preamp valves.

rattle is cured- awesome

Now the tremolo's not working and there's a weird blatty/farty/almost cutting out type of distortion when i really crank the thing. I'm guessing I just didn't put the valve in right or something like that.

[musicegbdf]

Just had a look at Langrex and they are still doing the "russian military" 6v6 as a matched pair for £10...! So a fiver a valve. I have some in my spares box...

[ICBM]

Now the tremolo's not working and there's a weird blatty/farty/almost cutting out type of distortion when i really crank the thing. I'm guessing I just didn't put the valve in right or something like that.

No, but it might be a fault with one of the new valves. Swap out the power valves for the old ones first, and see if the distortion goes away. If it doesn't, replace the preamp valves one at a time with the old ones, or one of them.

The tremolo isn't a bias-mod type so that probably rules out the power valves as the reason it isn't working, so I'm guessing it could be a preamp valve.