I am still relatively new to tube amps, so I haven't had either my VK or the AC30 actually quit on me (yet). But I know one day it could happen, possibly either in performance or practice.

So what exactly is the best plan of action here? Or if we need to get specifics, what should I do? Should I just replace them before they blow, if so what signs could let me know it is about time to change them? But in the case of having to do it on the spot, let's get a few parameters here:

Using AC30, no back-up amp, have an extra matched quad. Power tube blows during practice, but a performance is shortly after, and I have just enough time to do a bit of amp-work. Amp stops producing sound correctly, but lights are on. Amp is cathode-biased.

How would I go about diagnosing which power tube is the culprit? What visuals could I look for? If I can see which tube it is, should I just replace that one for the sake of being speedy even though the tubes are not perfectly matched? If visuals are indistinguishable, would it just be better to put in the whole spare matched quad? Or could I try swapping the whole quad with one of the spares until I find the culprit the hard way?

Another question is what should I do with the other (3 in the case of the AC30) power tubes after the whole ordeal is over. As well as, it might be better for me to just go direct for the time being rather than try to do anything under these conditions.

Thanks for any help you can give. c:
Last edited by Will Lane at Oct 16, 2015,
You could get away with it with that amp but swapping the entire quad is preferable.
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Right. It would be just long enough to get through the performance.

Some chaps on TGP recommended that I just specify a new tube quartet of the same specification. TubeDepot marks the tubes they send so I can do that. As well as just buying an octet.
If you know the quiescent current of your current ones, that is certainly one way to go.

Or you could get yourself another amp running two EL84's like an AC15 or 18W Marshall to use the leftovers in.
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Yamaha SBG500
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100

Cathbard Amplification
My band
Only two visual clues I know of, a dead tube can just be doing nothing, not lit up, or it can be red plating, in which case it will shut off and do nothing when it fails and you'll see no indication it is on at all. While red plating it will be bright red instead of the usual red glow they have while operating normally. Sometimes tubes will shine blue, I've never seen anyone say this is a big problem, some work that way for years.

To find which one is gone, swap out one at a time and check it.

You can swap all at once, probably the best way to go, replacing just the bad one will probably get you through a gig without problems.

I carry spares in a small case I fixed up with a layer of foam rubber top and bottom, and pockets cut with an xacto knife for several power tubes and the preamp tubes just fit pretty well in the humps in the foam without cutting out spots for them.
Hmmm...I wonder what this button does...
I have only ever had one tube go out to where it made the amp sound bad , it happened to be a 40 year old tube in a 40 year old amp (my 1973 Fender Twin Reverb ) ...... out of 7 tube amps only one tube failure from all , on gig's I'm worried about reliability I take a back up as you won't have time for a tube roll during a gig
This is my plan:

The power tubes are matched (I'm assuming since the stock tubes that came in my AC30 are in there).
I ordered a complete set of tube from, I think, thetubestore.com (or something), then ordered my JJ's from eurotubes and those are whats in my AC30 right now.

The case is a camera case I got at Walmart for $5 - $10 or so.
I had a bunch of foam rubber around so I cut it to fit, then cut out holes to size to fit the various tubes.

I no longer have the GZ34 (rectifier tube) though because I used it on a repair and the owner of the music store I did the repair for never got me a replacement The dirty Bastard (Other than that, he was a really nice guy when I was doing repairs for him, although there was one time he was a little unhappy about my $50 minimum charge for ANY repair. (I wasn't on staff, I was on call, lived about a 5 minute drive from the shop. He would call, I would be there in less than 10 minutes).
Last edited by CodeMonk at Oct 17, 2015,
That's a great idea CodeMonk.

I usually just keep a spare 12ax7 and PT in my gear case. Honestly though, I've never worried too much about it. I have road cases for my amps, and am just generally really careful when beating them around. I know a tube can wear out, but I change my out my PT every 1.5 years or so, depending on how many hours I've been putting on the amp.

9/10 it would be wayy too much of a hassle to swap out PT's, and sometimes preamp tubes in a timely fashion, depending on the amps layout of course. Sometimes you can't even get access to the tubes without pulling the chassic (my AC30CCH was that way). In a pinch tho, if you could easily throw a replacment PT in there, I wouldn't worry about having it matched, as it only has to get you through a pinch and can be swapped out later when you have more time.

Depending on the age of your amps, and what tubes are currently in them, I'd recommend just re-tubing them when you have the spare cash, that way you still have the old tunes sitting around as spares. If your tubes are newish then don't bother as it would kinda be a waste of money, unless you wanna change up the tubes anyhow.
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Well, in 40 years of gigs, I have experinced tube failure 3 times. Once was a pre tube in the middle of a gig with no backup. Bummer. Once was a 6v6 flameout that also took out the fuse in rehearsal. Once was in rehearsal and the power tubes were just fading noticeably. It doesn't happen often but having a backup pod or other multi-effects will allow the gig to go on. I do carry spares in the back of each amp now just in case but they never fail when you are fully prepared.

Cords seem to fail 10x more often than tubes.
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Last edited by Cajundaddy at Oct 17, 2015,
This is the reason I will buy a orange micro terror. It is small and I can keep it in my car, and would be fine mic'd.

I have lost two el84's and two kt88's in 10 or 12 years.
I always kept a full set of tubes lying around at shows just in the off chance one goes. Only had it happen once on my other guitarist's amp; grabbed the tools and had the singer vamp while I swapped them out real quick.

Honestly I've never had one fail in the 6 years I gigged, but it never hurts to be safe.

How do you say "I'm okay" to an answering machine?

ya the best is to have a single ended amp, a 15 watt amp and 30 watt amp. that way when your tubes blow you throw one away and put 2 in the other amp and 1 in the little amp.

Jumping on dat gear sig train.
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Quote by diabolical
I just have some amp modeling small pedal as backup to finish gig as backup.

that's a good idea !! , I have a 250 watt self powered speaker (small and light) that I could probably plug my guitar and pedal board straight into it to finish a gig in a pinch