Fender Bassman AB165 Bias noise?

[edthill]

I've been working on a friends AB165 bassman. I converted it to blackface specs and also discovered that a few of the diodes coming off the PT were completely fried. Once I replaced the diodes and made the bias circuit the adjustable kind that you could actually bias tubes with, I started to notice that a noise was being fed into the output stage. It sounds like white noise. I am 99% positive this noise is happening at the output stage. I have replaced all the resistors/Caps near the PI, I am wondering if it could be the bias pot or something in the bias circuit. It has new bias diode, cap, range resistor. The only thing unchanged is the stock bias pot. The pot is maintaining proper voltage but could it be shot and injecting noise into my tubes? I should say the noise starts off at a lower volume then gets progressively worse which makes me think it's a resistor or Pot. Could this be the source of my noise? Has anyone encountered a problem like this with an older BF fender amp?
Thanks in advance,

Ed

[shooter]

injecting noise into my tubes

Measure *any* AC on the DC bias?

the noise starts off at a lower volume then gets progressively worse

pull all the tubes before the PI, still noise?

[Blind Lemon]

What do the 1.5k resistors on the power tube sockets look like? Carbon comp? Dark/discolored? Screen resisters are they the original carbon comp resistors? Those tube sockets get hot and I have had those 1.5k half watt be broke in two and it be hard to tell.


Just a though if there is still noise after you pull the preamp tubes.


BL

[edthill]

injecting noise into my tubes

Measure *any* AC on the DC bias?

the noise starts off at a lower volume then gets progressively worse

pull all the tubes before the PI, still noise?

Pulled the tubes before the PI.

Still have that noise so I know it's not something in the preamp.
I will measure for AC.
Im doing that on the middle lug of the bias pot? or coming off the bias cap?

Thank you,

Ed

[edthill]

What do the 1.5k resistors on the power tube sockets look like? Carbon comp? Dark/discolored? Screen resisters are they the original carbon comp resistors? Those tube sockets get hot and I have had those 1.5k half watt be broke in two and it be hard to tell.


Just a though if there is still noise after you pull the preamp tubes.


BL

Replaced the screens and grid stoppers, still get that funky white noise.

Thank you,

Ed

[shooter]

I will measure for AC.

Where ever you measure your bias volts in DC, do that, then flip meter to AC and measure.  This probably isn't your problem, but you asked, so lets get it outta the way.  "white noise" is probably something "creeping in", unshielded signal, flaky R or leaky C, bad solder connection.

[HotBluePlates]

Pull the phase inverter tube itself.

If you still have noise, the problem is either in the bias supply or in the B+ supply.  Arcing (power) tube sockets or an arcing choke/PT can throw weird noises that might be heard even with the phase inverter pulled.