Motor Boating Princeton Reverb

[terrapinflyer]

New to the forum. Just finished a princeton reverb build and Im having an issue. It's making a low frequency thump (motor boating?). The noise speeds up when I turn the vibrato speed pot. Some voltages are waaaaaaaaaay high. Also I noticed while testing voltages that when I touch the multimeter probe to the output coupling cap connected to pin 8 of the PI tube THE NOISE GOES AWAY COMPLETELY!!! PLEASE HELP!!! WHERE DO I START?

[soundguruman]

Well, it's normal to have the thump in a princeton because the vibrato does not shut off completely until you plug in the footswitch and turn it off.
You will always hear the pulse even with the knobs turned all the way down. So try shorting the footswitch jack to ground and see if the pulse stops.

Its not really motorboating, you are hearing the vibrato circuit.

Well if the voltages are high, bias the amp hotter, it will draw more current, and the voltages will drop. However, if you are using aftermarket parts, you can expect that the voltage will be different from the schematic.
Make a list of all the voltages and post it here. We cannot tell what "waaaaaaay" too high is. Or where it is.

[mort]

New to the forum. Just finished a princeton reverb build and Im having an issue. It's making a low frequency thump (motor boating?). The noise speeds up when I turn the vibrato speed pot. Some voltages are waaaaaaaaaay high. Also I noticed while testing voltages that when I touch the multimeter probe to the output coupling cap connected to pin 8 of the PI tube THE NOISE GOES AWAY COMPLETELY!!! PLEASE HELP!!! WHERE DO I START?

I'm a no0b too so I can't help, but I like your username

[terrapinflyer]

Thanks soundguruman!!! I shorted the vibrato jack to ground and the thump stopped!!! but.........does this mean I cant Turn it back on without the thump? I heard if i put a cap from the left lug of depth pot to ground this will help??? what value??? Also......I will record voltages after work today. Also........the thump stopped but the signal is now thin and distorted (not the good kind). When i Touch the multimeter probe to pin 8 on the phase inverter (side of) tube it sounds AWESOME!!! what's this? THANK YOU!!!!

[Jazz P Bass]

This 'Princeton build', is it the same as the Fender Princeton/Reverb.
A schematic would help.
Pin 8 is where the feedback loop from the output transformer comes in.
Did you use the feedback circuit?

[terrapinflyer]

yes this is a princeton reverb. yes i did use the negative feedback. but unless I'm missing something, pin 8 of the PI goes to an output coupling cap, not the negative feedback. ??? right???

[terrapinflyer]

ok I have voltages. one thing I should mention.......I've got the PI hooked to the 3rd filter stage instead of the 4th.......and I have 50uF for the first stage instead of 20uF (i'm also using a gz34 and Allen trannies). I'm using 10k resistor between 3rd and 4th stage, 4.7k between 2nd and 3rd (may be a suspect?), and a choke between 1st and 2nd if that helps figure out the power supply voltages.

Power Supply:
1st node-420v
2nd node-420v
3rd node-130v(not normal?)
4th node-353v(not normal?)

Power Tubes:
pin 3-420v
pin 4-420v

V4:
pin 1 150v-300v (fluctuates with pulsing)
pin 3 2.5v
pin 6 395v (not normal)
pin 8 385v (not normal)

V3:
pin 1 235v (not normal)
pin 3 1.8v
pin 6 238v (not normal)
pin 8 1.8v

V2:
pin 1 418v
pin 3 9v

V1:
pin 1 247v (not normal)
pin 3 1.7v
pin 6 239v (not normal)
pin 8 1.8v

HOPEFULLY THIS HELPS...

[terrapinflyer]

ooops.........3rd node of PS is 377v

[Jazz P Bass]

yes this is a princeton reverb. yes i did use the negative feedback. but unless I'm missing something, pin 8 of the PI goes to an output coupling cap, not the negative feedback. ??? right???

I looked a little to quickly at the schematic.
The feedback path is to V3.
Anyway, the PI is V4.
Yes, pin 8 is the output to the grid, through a coupling cap.
Touching V4/ pin 8 'makes it work' is strange.
Are you sure you have a path to chassis ground from pin 8?
Schematic indicates 1 K & 56K in series to ground.
The indicated voltages are 30 volts at pin 8 & 48 volts at the junction of the 1K & 56K.

[g1]

Are you sure you have a path to chassis ground from pin 8?
Schematic indicates 1 K & 56K in series to ground.
The indicated voltages are 30 volts at pin 8 & 48 volts at the junction of the 1K & 56K.

I was wondering the same about cathode voltage.

Terapinflyer: Please provide V4 voltages with vibrato off (jack shorted to ground).

[terrapinflyer]

HAHA!!!!!!!!! JAZZ P BASS you got it!!!!!!!! I did not have the cathode resistor grounded...........I emailed Dave Allen from Allen Amps today and he diagnosed the problem in 5 minutes thru email!!!!! If you guys don't already know, this guy is awesome. He has answered many of my stupid questions before.......and I definitely will be buying my next set of transformers from him. I'm really not sure how I missed that. I triple checked EVERYTHING. My voltages are all fine now, the thump is gone, and I was in blackface bliss for hours!!!!!!! I love my hot rodded prince!!!!! NOW............on to my next build

[terrapinflyer]

Thanks guys for responding. This is my first post (I've been lurking in the dark here for a while). Feels good to know there is tech support here.