Vintage V4-B weirdness...?

[mattvon]

Hey all, just got a mid-to-late 70s V4-B and dropped a matched quad of JJ 7027As in it...

It sounds fantastic, with a single exception: I'm getting a weird "ka-thump" sound repeating every second or so...it sounds for all the world like a flat tire?!?

LOL...obviously this amp is going to get a trip to the amp doctor, but for the sake of curiosity--has anyone ever heard of such a thing? It's not super loud, but it's very audible and strangely rhythmic.

Any insight would be greatly appreciated!

 

[Standalone]

Hey all, just got a mid-to-late 70s V4-B and dropped a matched quad of JJ 7027As in it...

It sounds fantastic, with a single exception: I'm getting a weird "ka-thump" sound repeating every second or so...it sounds for all the world like a flat tire?!?

LOL...obviously this amp is going to get a trip to the amp doctor, but for the sake of curiosity--has anyone ever heard of such a thing? It's not super loud, but it's very audible and strangely rhythmic.

Any insight would be greatly appreciated!

I have a similar amp, the VT-22 (it's the guit combo version of the V-4.) But that has nothing to do with my very non-technologically detailed answer.

Some guesses and rambling to eat up some bandwitdth:

There are doo dads in circuits that can do this kind of thing if something along the line is not working properly. What happens is that some kind of current or charge or whatever builds up in the bit and then releases, in a repeating pattern. Sometimes it makes things go "whup-whup-whup" or I guess even "ka thump ka thump."

I'm not the one to tell you if it's a transistor or a capacitor or a resistor or a tube. My guess is that our ampegs have a solid state rectifier, but I'm not sure about that.

Every time someone mentions the V-4B, I keep reminding mysellf that I have to drag my combo over to my eden set up and run it thru my xlt.

Good luck.

 

[Chris Breese]

My old SVT was doing something similar. It turned out to be a capacitor.

Did you have the amp's bias reset when you dropped the tubes in?

 

[mattvon]

Problem solved! It turned out to be my PBDDI?!? Just as I decided to pack the V4-B up for a trip to Savage Audio, I flipped my SVT-II on to practice a bit...same bizarre "flat tire" sound. Unplugged the SansAmp, problem solved.

Weird, but I'm glad to have saved myself the bench charge and gas to get the V4-B to the shop.

Now to noodle out the issue with my PBDDI...

 

[Chris Breese]

Yes, but...

Did you have the amp's bias reset when you dropped the tubes in?