Ampeg V4.com

That isnt right.

if you pull the power tubes,  and check the blue and brown wire,  do you still measure 260v?

just to be sure,  pull the OT center tap and measure DC resistance between it and the blue and brown wires.

The amp should have no B+ when in standby.   I would go over your recap work and make sure that nothing got mis-wired. 
thats a circuit board amp... so I can't imagine its too weird. 

Also,  check the AC voltage on the Center tap... maybe your reservoir cap isn't working or isn't hooked up right?

260 is odd... but its more strange that you would have 500volts down the line.   it doesn't make sense that you would have a lower voltage at power supply point A than the rest of the amp.... because the rest of the power supply points are taken from power supply point A. 

the only possibility i could think of off hand,  is if the cap isn't working there.. and you're getting a pulsating DC voltage.  in which case your dc measurement wouldn't tell you the whole story.

does your amp have flyback diodes?

No flyback diodes. I don't think Ampeg ever used them in this model. I'm missing something obvious so I decided to start over. I pulled the output tubes and the 5AR4 rectifier. Measuring from the center tap, the PT secondary reads 400ac-0-400ac. It goes to the correct pins of the 5AR4. the 5vac winding also goes to the correct pins of the 5AR4. With the tube out the 5v reads 5vac. I put the rectifier back in and measure the ponit "a" DC voltage while still in standby. The voltage is initially zero but as the tube warms up the voltage rises to roughly 260vdc. When I flip to "on" the voltage goes to 535dc but returns to 260 if I go back to standby. Still no output tubes. All three OT leads follow the "a" voltage. Could there be a problem with the PT?

Yes I do.

Hangman, I dug out the O-scope and discovered a massive (hundreds of volts p-p) high frequency oscillation on the plates. I've chased it down to the trebel control. When the treble control is full CCW the oscillation goes away and the voltages return to normal. Once I go about a third CW the oscillation returns. I'm thinking the 470pf cap in the tone module must be bad. I'm looking through my parts to find a spare cap. I'll post the results soon. Thanks for the help. You were definitely correct.

I did the tone circuit upgrade and the problem still persists. If I take the signal side of tone stack completely out of the circuit by moving the input wire of the tone stack to the input grid of the phase inverter, the amp plays killer with lots of power and fullness distorion free. But if I just touch the output wire from the tone stack to the grid while the amp is working well, the amp immediately loses volume and becomes buzzy. As a precaution, I removed the sheilded external amp wire from the tone stack to be sure it didn't have a problem and there was no effect. I'm getting cross eyed on this one. Ampeg had two variations of hum balance on this model. The schmatic shows the hum wiper going to the neg bias voltage. My control has no effect on hum like that. If I put the wiper to ground then the hum control works. Any thoughts?