Getting to a Micro-phonic preamp tube

[nsgelocos]

I've found a culprit! Can't get the master volume in channel 3 above 9:00 without squealing from the amp. I've isolated it that it is not the guitar pickups causing it. I'm using it in 90 watts Mark IV on channel 3. I'd figure it must be a microphonic pre-amp tube. With the head, I would have to remove the power tubes to get to the pre-amp tubes. This creates a difficulty working back there. Any strategies on getting to the pre-amp tubes and removing the metal coverings. I have not figured out which tube is the culprit, but i know it's there.

 

[SteveO]

If you remove the reverb tank it's a straight shot to the preamp tubes through the bottom of the head.

 

[nsgelocos]

If you remove the reverb tank it's a straight shot to the preamp tubes through the bottom of the head.

I assume that you mean the metal inclosure at the bottom of the head supported by rubber grommets?

That's one way i guess... I would figure that you remove the power tubes and remove the coverings on the preamp tubes then replace power tubes and poke-test the preamp tubes till the rotten tomato exposes itself.

But the reverb tank is worth a try. 8)

 

[nsgelocos]

Actually... Which preamp valve do you all think might be the culprit in this microphonic problem? :?:

 

[svbassist]

Actually... Which preamp valve do you all think might be the culprit in this microphonic problem? :?:

Mine was doing the exact thing, i changed out V1, problem was solved. There is still a little bit of harsh high end, so I'm still playing around with the other preamp's but i'd say start there.

 

[nsgelocos]

I forgot which one it was exactly but I did find it. It was the V6 i think. I'd have to check.