Princeton Problem

[larhinds]

This is an interesting one. When you first power the amp it sounds fine. After about 5 minutes it starts to lose some power and sounds distorted. Voltages on the plates and screens seem reasonable. -32 volts bias. Primary of OP tranny seems equal within acceptable limits. Any ideas?

[acorkos]

power tubes

change 'em

[larhinds]

I will give that a try. Thanks

[larhinds]

Switched out the power tubes. Still the same behavior.

[Jazz P Bass]

Try monitoring the bias voltage.
Set your meter to read volts ac.
If that ac voltage (which is the phase inverter signal) drops when it acts up, then the problem is before the power tubes.

[52 Bill]

Just to clarify things, what sort of Princeton, tweed, reverb?

I'm with JPB, but I'll add; when it starts to distort and loose power do the output tubes start to get hot or redplate?

[larhinds]

Thanks for the reply Jazz P Bass. This morning I checked the current draw [bias] for the output tubes. It's drawing 29 millivolts with a plate voltage of 391. According to my charts it should be more like 19 millivolts. Since it is a fixed bias amp with no variable adjustment I'm 1st going to install a trim pot, set the bias and see how that behaves. I have a feeling it might be the under biased and running hot.

[larhinds]

Hi 52 Bill. Thanks for chiming in.
It's a Silverface Princeton Reverb. No, I don't perceive any redplating. Interesting faint blue light pulsation in sinc with the input [striking guitar chords]. As I indicated in a previous reply the output is somewhat over biased according to the charts I have respective to that tube [6V6] at a voltage of 391.

[MWJB]

What's your wall voltage? What's the AC at pins 4 & 6 of the rectifier?

How are the electrolytics? It's a little odd that your negative bias voltage is lower than it should be...can you give us measurements of AC before the diode & dc after the diode, measured at the junction of the 220Ks.

29mA @ 391 is not a problem for JJ or good NOS tubes, if you have Saratov 6V6s (EH) keep them in the early/mid 20's. 19mA would see the plate voltage rise but at 391vdc gives you 7.5W dissipation...cold for guitar. Nevertheless a trim pot is a good idea & a good time to change the bias cap if original, check 100K supply resistor for drift too.

[larhinds]

MWJB thanks for chiming in. Wall voltage 120. Pin 4 and 6 of 5AU4 is 355. Ac before the diode 200. Dc after -35.3 volts. Bias supply resistor 103K. The Filter cap doesn't show any signs of physical leakage and the amp doesn't exhibit hum. Anything seem out of wack here?

Thanks

[acorkos]

Switched out the power tubes. Still the same behavior.

change out all the tubes before you start on anything else...they are the easiest part of the circuit to swap, and usually the source of our problems

[Bruce / Mission Amps]

Also, a common failure mode on these amps is that 1 watt 1K resistor on the eyelet board next to the main filter cap can.
I usually replace them with a 1K@2 watt Mtl Ox resistor as a matter of course when ever one comes in for any work.

[MWJB]

The filter caps fail in terms of performance long before you hear any hum, or see any obvious signs of damage. Your B+ voltage is low. Your 5U4 rectifier should be turning that 355VAC in to more than 391vdc. I suspect your rectifier itself, or the filter caps in the cap can are pulling down the B+.

AC before the diode, but after the 100K resistor is 200VAC? -35.3vdc sounds fine though. 103K is fine too.