Why, oh why, now and this way..

October 28, 2007 12:15 PM

Cursing bad luck: One of the preamp tubes in my Peavey Classic 30 seemed to peter out during practice yesterday. Two hours before my first concert of the year. An awesome friend came in in the clutch, but now I'm puzzled as to what to do. (and have another show on Thursday)

So, playing. It was turned fairly high up (Pre at 6-7, Post at 6-7) (note that the scale is 1 to 12). I'm playing fairly loud (with a drummer, and a loop being sent to it), and within a few seconds I lose volume. We all stop, I look at the back of the amp, and one of the three preamp tubes is not showing its beautiful glow. Playing yields a barely audible, really sad sounding chime.

So, I live with it. The weird part is that today, I remembered that perhaps the other channel on the amp is usable (I think I'm incorrect in thinking this, because it isn't a 'true' two-channel amp) Anyway, the amp seems to work, although that tube still isn't glowing. I'm afraid to test it thoroughly, because I keep reading horror stories of this sort of thing (and I've seem some scary stuff happen with vacuum tubes in a photography setting).

So, wtf do I do? I'm thinking I should retube it, and Eurotubes is insanely hyped. At $61 for a retube of 7 tubes (my amp has 4 post, 3 pre), average price seems to be pretty good relative to other shops. And they can deliver in three days.

Or should I just buy one tube? Or am I set to go with this weirdly-working setup?

I'm disregarding bias for the time being because the C30 uses EL84s which supposedly don't need to be biased. But the thing is, I only have had the amp for about 6 months, and it was brand new. So, WTF? Tubes are supposedly usable for years on end, and these only lasted a half. Am I running the preamp too high? is there anything else that could have caused this (fuses?) that I'm not looking at?

Thanks for any guidance.

posted by tmcw to media & arts (13 answers total)

Preamp tube failures do occur but they're not usage-hours related the way power tube failures usually are; they're kind of random. You should never need to rebias because of a preamp tube adjustment.

I'd say just buy one tube, drop it in in place of the failed preamp tube, and quit worrying about it.
posted by ikkyu2 at 12:21 PM on October 28, 2007

I just finished building a tube amp, and 20 seconds into testing it for the first time, I suddenly lost all volume. One of my preamp tubes had stopped working (grid was shorted out somehow). Put in another tube and its worked great ever since.
posted by jpdoane at 12:35 PM on October 28, 2007

I'm with ikkyu2... replace the dud tube.

It's a good idea to get service literature (including a schematic) for the amp. In the future, it'll help with the next failure.

Buy your tubes on eBay. These days, that's how I buy most of them. Haven't gotten a bad one, yet.

On your other questions... the design should accomodate any selected amp settings. It may sound like hell, but there is no setting intended to break an amp.

Tube filaments would not be individually fused. Depending on how they are wired, one filament can refuse to glow and the rest be OK. SOunds like yours are wired in this manner (parallel). A loose wire is a possibility. Check this by moving a working EL84 to the same socket. If it glows, wiring is OK. If not, socket/wiring problems.

If it uses a lot of EL84's... think about getting spares?

Last, tubes do last a long time, and die from a variety of causes. Yours sounds like an open filament. It happens.

Good luck. (email (in profile) if you have more questions.)
posted by FauxScot at 1:18 PM on October 28, 2007 [1 favorite]

Related question - when I turn off the amp, there's a pop of varying volume. I think it's related strongly to the reverb level, but maybe not. Any ideas?
posted by tmcw at 1:40 PM on October 28, 2007

Okay, further intrigue - the user manual specifies four EL84s wth a 12AX7 driver, and then two 12AX7s in the preamp section. The one that isn't lighting is the one closes to the EL84s, so I assume it's the driver. eBay's selection is pretty inexpensive but the diversity and the knowledge required to sort it out is a little intimidating.
posted by tmcw at 1:46 PM on October 28, 2007

You can get one at Guitar Center for now. When you can plan in advance here are some decent tube vendors:
Tube Depot
Triode Electronics
Angela Instruments
Antique Electronic Supply
Tube World
Vacuum Tube Valley
Brent Jesse

I would definitely get some spare EL84s. Power tubes go.
posted by caddis at 2:03 PM on October 28, 2007

I am not an amp tech, but I don't think you'd get any sound at all if the driver was shot.
posted by timeistight at 2:07 PM on October 28, 2007

Eurotubes seems to undercut the prices of most places (well, tubesandmore is $2 cheaper, tubedepot is $10 more expensive) - is there any reason not to get them?
posted by tmcw at 2:22 PM on October 28, 2007

hmmm, I must have cut off part of my comment. You should definitely have some spare power tubes, the EL84s, on hand if you will need this amp for performance. A spare amp is a good thing too, but now we are talking real money.
posted by caddis at 5:42 PM on October 28, 2007

While Bob's prices are very good you don't need to get a complete set. Generally preamp tubes last a long time and don't need to be replaced along with the power tubes. But it is a good idea to have a spare set of power tubes on hand, especially for someone gigging.

So replace the dead tube, as you've already been told. My preference is Lord Valve. Good prices, top notch QC, and he definitely knows his stuff. Plus you aren't limited to JJ. For new production I like the Russian Tung-Sol 12AX7 and you really couldn't go wrong with that tube. LV won't sell EI 12AX7s anymore because of quality problems but if you happen to get a non-microphonic one they sound as good as anything produced in any era. Venturing into NOS* can be interesting and expensive; I basically feel that the "premium" NOS aren't worth what people pay for them but YMMV. Most of my tubes are used OS and I won't pay more for NOS than current production of a given type. The only exception was buying a used Mullard ECC83 from LV for $30 to see if was worth the hype. It wasn't.

* New Old Stock, generally referring to American and Western European tubes that have been out of production for decades, in some cases. Prices can be very high for for certain rare or high demand types.
posted by 6550 at 7:42 PM on October 28, 2007

People who should know, including a guy I know who's been writing reviews for pro audio magazines for 20+ years, tell me that 12AX7s are basically a commodity and that for use in a guitar preamp, the only thing you really care about is that the first tube in the chain (V1) is not a very noisy tube. (The same guy sent me a bunch of old Telefunken 12AX7's sourced from old tube-based EEG machines where noise really did matter, to use in my V1s, but I swapped them in and out with my Mesa and Groove Tubes and "Magic" tubes and could never hear any difference at all.)

I think I would base my buying decision on who would get the tube to me the fastest, and I would not spend more than $10 on a 12AX7 tube.
posted by ikkyu2 at 8:17 PM on October 28, 2007

Spending big bucks on NOS tubes for a guitar amp seems like a waste to me. Save those dollars for tubing a hifi or something. Guitar amps first of all distort and overdrive tubes by design. Subtle high fidelity nuances are lost in that. Also, that overdriving really abuses them, making spending big $$$ on NOS tubes even more expensive. There is no doubt that the NOS tubes offer sonic advantages and that after all the old tube factories were shut down in favor of sand that many important skills and trade secrets were lost. Yet, these days the NOS tubes have become a bit too dear it seems, except for rich audiophiles.

Basically, you want a tube which can take the abuse, yet still sounds great, and this applies to preamp, driver and power tubes. Some of the Russian tubes are known for this, but their is such chaos in that market now such that I am hesitant to recommend any particular brand as my information from last year is probably not relevant this year.
posted by caddis at 12:56 AM on October 29, 2007

"there"
posted by caddis at 4:52 AM on October 29, 2007