Help? 36wt build squeals with anything but NOS Mullard in PI

[tele_jas]

This is in my Weber 36wt Plexi build... It's a Weber 36wt Marshall (EL84's) with a few upgrades. I had a buddy build it to these specs for me.

It's got a post phase inverter master volume and a footswitchable mid boost with a 5-way selectable "voicing" control.

I'm glad I got the 36wt version, I'm not sure 18wts would have worked for me needing a clean tone at levels I can hear them with my loud drummer.

I had some problems with the power tubes, but after swapping them out I've ran into another issue.......
I cannot put any other tube but a NOS Mullard in the phase inverter socket. All other tubes are noisy and squeal real bad, some worse than others.

I've tried 12 different preamp tubes - GT, JJ, Mesa and a Sovtek 12ax7 as well as a GT and JJ 12at7 and they all squeal.


Any reason you can think of that would make it do this? I might add that I'm not skilled or have much knowledge in building amps, so please
don't use big words with me


Here's some pics, that jack by the PIMV is the footswitch for the mid-boost:

http://img704.imageshack.us/img704/4645/pict0190d.jpg

http://img188.imageshack.us/img188/8778/pict0187u.jpg

http://img705.imageshack.us/img705/1900/pict0146a.jpg

http://img19.imageshack.us/img19/7220/pict0145ko.jpg

http://img189.imageshack.us/img189/3243/pict0144zc.jpg

http://img121.imageshack.us/img121/1743/pict0148g.jpg

[Plexi]

posted this on another forum

Like most 18watt type circuits or TMBs.. even though this is a 36 watter, mostly up to the PI is the same. But the PI is the second tube. In these builds using a TMB, its best to move the PI to the 3 spot like marshalls or fender that use a similar circuit.
The problems in the 18s using the TMB, "most" of the time is those long leads to the plates of the PI, if the PI is still at V2 spot..

Wire or lead dress ,things these wires cross or are next to,can cause the problems your talking about. might try twisting the wires to the plates on V2 PI tube. Run them along the chassis back along the front of the board.
Sometimes moving one plate wire to the PI will also cure it. The different tubes used,can have different gains,even though they are 12ax7s.. so some might work where others won't. Its when you can get one of the won't work tubes to work,by rerouting ot cleaning up or changing the lead dress, then the others will also work.

Might ask too, does both channels have the problem,or just the TMB channel? the TMB channel has more gain than the Normal channel. So..if you just have problems with the TMB channel,effecting the PI..try swapping the V3 tube..see if it helps in what tube will work in V2 {PI}
__________________
Richie