Fender Silverface Twin Hums after PT upgrade.

[Amp Kat]

RG's page is an excellent troubleshooting guide so stay focused on it. Motorboating is usually a sign of an ungrounded high voltage source so make sure the caps under the hood are properly connected and grounded. Connection problem is looking pretty high about right now.

[Jag]

AC on the plates, motorboating and loud hum...

I'd be suspecting filters.

Just to be clear, with all preamp and the PI tube out and power tubes in you don't get the hum, but with the preamp tubes out and the PI and power tubes in you do get the hum?

[AndrewM]

So things have improved... I changed the suspect cap and both channels appear hum free.
The normal channel is appears ok now..

The vibrato channel has an obscene buzz even with the reverb off. With the reverb on its worse... With the pan unplugged and the 12at7 pulled its still there..... AAARGH!

[Amp Kat]

Did you change all of the E-caps in that amp ? You may try that as they are getting dried.