Help me fix my Vibro-King!

[EarlSlick]

The power tubes recently blew on my mid-90s Fender Vibro-King. I replaced the the power tubes, and also notices that the "screen-grid" resistor, across two pins of one of the power tubes had blown (There was carbonization on it). I replaced that as well.

Now when a turn it on, I can barely nudge the volume knob up, without getting an loud and high-pitched squealing noise.

I did notice, that if I took out the rightmost tube (a 12ax7) then I would get something that sounds almost normal, but not quite loud enough when I really crank it, and it seems too noisy at high levels.

Any ideas?

[Scodiddly]

There's something else wrong, obviously.

Usually if the screen grid resistor burns up then there's a short in one or more of the power tubes.

Hmmm... nothing else is coming to mind yet. Do you have access to a tube tester?

Also, are any of the big capacitors looking funny or maybe leaking?

[themagicmanmdt]

high pitched squealing sounds like oscillation....

anything else fried or looking questionable?

here's something i do - and now, you need to be very very careful -

get yr amp chassis out and on the bench, speaker plugged in, tubes in, turn on. get something NON CONDUCTIVE - say, a wooden stick or something - and start tapping around on components. it's a great way to find out intermittant connections, fried stuff, etc. when you hit something and it makes a bunch of noise - hey, something's up there. but you've gotta be careful.

really, what i would do first, is measure the plate voltages on all the tubes and compare with schematics. that, and the 'tap test' above, as well as a bigger inspection to components that might be fried (plate resistors especially).

mt

[EarlSlick]

Thanks for the replies, guys.

I gave the board a good inspection, to make sure that there were no leaky caps, or anything else visibly amiss. I tried swapping out each preamp tube with a known good tube.

This week I'll try magicman's advice, and check the plate voltages across both power tubes, and check back. I'll test for other shorts the way he described, too (using a pencil or chopstick of course).

In the mean time, if anyone has any more advice, I am eager to follow it.

Thanks!

[muttonhead]

Check the diodes on the plates of the output tubes CR7,8. The squealing made me think of an output transformer with the secondary leads reversed, but the VK has no negative feedback, so it wouldn't matter if you did. Do you know that plugging into the return jack bypasses the reverb and is pretty much the same as plugging into a regular Fender amp? Good luck with the repair!

[EarlSlick]

Do you know that plugging into the return jack bypasses the reverb and is pretty much the same as plugging into a regular Fender amp?

I did not try this, but did just remove the tubes for the reverb. I'll give it a go. I'll also check those diodes.

Been a bit busy lately, so I haven't gotten to open it up again in the last week. Thanks to everyone for keeping the thread going!

I'll check in again, soon.