70's Ampeg SVT repair help needed

[randy1kenobi]

Posted this to alt.guitar.amps but don't think people actually talk about amps over there anymore so figured i'd try here too...

One day during practice my mid 70's SVT head started distorting and I
could see at least one glowing red tube, so I shut it off and put it
aside until I could find some time to fix it. That was 2 years ago.
Now i've decided I really want to get her going again so I cleared off
my workbench and opened it up. Found that one of the caps in the plate
supply for the 12DW7 had high ESR and the resistor following it was
toast. Unfortunately the cap was part of one of the multi section
cans. Some of the other filter caps were starting to bubble so I
decided its probably a good time to change all the filter caps.
Changed everything out. Replaced the toasted resistor. Quickly checked
through the other resistors in the power section and everything seemed
ok. Put it back together and brought the voltage up on the variac.
Everything looked good so I put the Svetlana 6550s back in. They are a
few years old but haven't seen heavy use. I set the bias and the amp
works but after a few minutes one of the 6550's starts to glow red
hot. I swapped the tube with one next to it and the red glow followed
the tube. I'm just a hobbist with enough knowledge to get me in
trouble and this is my first time working on an SVT. My two questions
are did I overlook anything obvious? And second, since I assume the
glowing red tube is toast, do I need to buy a whole new set of matched
6550's? Is there a way to get another Svetlana 6550 that would match
my set? Tossing these would be a shame since they are $$$ and these
haven't seen much use.

Thanks in advance for any help.

-Randy

[Enzo]

You can buy a single tube. They don't ship amps from the factories with matched tubes. We tend to prefer matched sets far various reasons, but mismatched ones work.

You will never match the tube that is in there now. even if you found the same vendor and found tubes matched on the same equipment, and those tubes had the same numbers as yours, yours have considerable time on them, even if you didn;t use it heavily, so they still won't be the same.

A bit of advice, when a powr tube glows red hot, the first thing to suspect is the tube. Tired filter caps might be ready to replace, and they might introduce more hum than you'd like, but they have no way to selectively affect one tube of six.

[Amp Kat]

If you know of a repair shop nearby take the tubes to them and let them test them on a tester if they have one. They will also show which ones are weaker than the others. Those old amps are awesome and nothing comes close IMO but my experience with them is usually 2 of the 6 tubes will wear out faster than the others but the key is to get them outta there before they cause damage. Those amps are so powerful that they eat tubes when pushed hard. My tube tester showed the weak ones from the strong ones and what's weird is they still had some life left for a guitar amp but wouldn't run good with the SVT at those high plate voltages.

[Enzo]

Just to offer analternative view, I don't tell people I have a tube tester. I might get it out once a year, but I would never test a customer's tubes. No, not because I want to sell him new ones. If I test a tube and it is bad, that's great. But if I tell a guy this tube is good and next week it blows up his amp, he will be mad at ME and blame me. Plus he will then tell everyone he knows how I blew up his amp.

Now I can explain to someone until I am blue in the face that testing good on a tester is no guarantee the tube will work forever, but all they hear is " tube is OK" and off they go.

[Amp Kat]

Very good point Enzo and trust me they will hold you up against that fire. Tubes are so sparatic in these amps because one minute they are good and the next gig they aren't. The sad thing is 6550's aren't cheap and neither are KT-88's but in the SVT I like to run 4- KT-88'S instead of 6- 6550's and you really don't loose any volume, not like you'll ever have any volume problems with a vintage SVT !

[mac1amps]

It's nice to hear that I'm not the only one that doesn't rely on what a tube tester tells me. Yes, they DO have their place, but there are so many things that a tester DOESN'T tell you. One example being Enzo's point of how a tube can be "on the brink" of failing (but it hasn't quite failed yet), and a tester will tell you that it's fine. A tester won't tell you if a tube is microphonic, actually, your amp itself is a much better "tester" for that. All the over-hype with respect to matching is also very mis-leading. Sure, you want the tubes to be "in the ball park" with eachother, whereas after a few hours of operation (especially in a guitar amp), the specs of the tubes change anyway. Whenever I order power tubes for my inventory, I never order them in "matched" sets, but when I test them (one of the few things that I WILL use my tester for), they're rarely "un-matched" to the degree that would make ANY difference (again......especially in a guitar amp). Keep in mind this is assuming that a "respected" grade/brand of tubes are used (JJ; Svetlana; Winged "C"..........no cheesey "Coke Bottle" Chinese tubes).