Ampeg SVT II => DIY Tube replacing?

[JimmyM]

Originally Posted by B-string

Jimmy if it didn't have gain it would not drive speakers . I guess you meant a "gain control"?

OK, thank you, Helpy Helperton.


Seriously, though, that is what I meant. But I still stand behind my statement that it's not a high gain amp. At least not compared to an amp like the Mesa Recto series.

[B-string]

No different goals for sure. The "high gain" stages are designed for "over-driven" sound, not clean or just compressed character.

[bri450r]

Awesome thread. Good info for me also.

[Krohurte]

Originally Posted by JimmyM

Tubes should be changed only when they're broken, and you should try to isolate it to the broken one and replace only it, unless it's a power tube that broke, and then you should only have to replace 3 at the most to keep them matching on each side. Having said that, I've popped in a single power tube on my SVT in an emergency and it worked just fine, but it can cause the other tubes to wear prematurely.

(NOTE: The above is only for SVT's and their power tubes. Other amps have different methods of wiring power tubes, but SVT's have them wired in triplet sets that bias separately.)

Yes, there are a lot of myths surrounding tubes, and almost all of them are because of a single book written by the former head of Groove Tubes, who was looking to sell more tubes.

Wow I'm seriously dazzled!

So I understand this:

Tubes generally do NOT wear down?

Unless there's a bad or unmatched one in there wearing the others down?

My problem here might be one broken tube but it could be anything whatever? I will take it to a pro then.

Also mine is a SVT-II which of course has small pre-amp tubes but no gain drive control knob as opposed to the SVT-II Pro.

gee almost 25 years down the road and still learning.

[Krohurte]

btw to isolate the problem tube (if it's a pre amp one), how would i do that? By taking out one tube after the other and then run the amp for each status?

[B-string]

Originally Posted by Krohurte

btw to isolate the problem tube (if it's a pre amp one), how would i do that? By taking out one tube after the other and then run the amp for each status?

Get good new tubes, one 12AX7 and one 12AU7 (NOS are still pretty cheap for AU7's) and swap tubes one at a time. Operate with new tube in the socket, if the trouble is still there put original tube back in and swap the next tube.
You can also try the chopstick test by gently tapping the tubes with a wooden chopstick, be very careful while doing this on a operating amp (dangerous voltages and burns from hot tubes). If one tube is very loud when tapped or you hear a jingling noise it is time to replace it.
You should hear no noise from the big output tubes so if any are noisy this way it is time to replace that one.

[Krohurte]

So as long as I only touch the tubes with the chopstick I'm safe from shocks? Only some of the metallic parts might strike me with voltage, right?

[JimmyM]

Wood doesn't conduct electricity, but all the same, you're best to only touch the tube glass with the chopsticks. That's all you need to do anyway.