help with tremolo n blackface princeton reverb

[dansamp]

so I have a '66 blackface princeton reverb on my bench and the tremolo is not working
I have replaced the cathode caps and resistors and the amp sounds much better but still no tremolo
I have swapped out pre amp tubes also
and the oscillator caps

I can see the voltage pulsating on my DVOM and it changes when I turn the speed and intensity pots but it is extremely faint coming through the speaker
almost non existent
not sure what to check next
any input would be greatly appreciated

Thanks in advance

Dan...

[M Fowler]

Dan.

I haven't worked on tremolo's in a long time but wondering if the pedal jack is shorting to ground?

Mark

[Structo]

do you have the footswitch connected?

In looking at the schematic, the tremolo is switched on by shorting the two wires to the jack.

Are you getting the 260v+ on the trem tube plate?

[dartanion]

Is your intensity pot good? Maybe a bad solder joint there.

Also, if you replaced all the caps and resistors in the trem circuit, did you match values per the schematic? Are you also using a known good 12AX7 for the trem tube? A lower gain tube (12AU7, 12AY7) may not work as they would not provide enough forward gain to start and maintain oscillation. The caps and resistors are what set up the phase shifts, so if you went with a "close enough" mentality, it may not work as advertised either.

[dansamp]

yes voltages are good
and I have tried other tubes
as I said I can see the voltage shifting on my meter
and you can hear a very weak tremolo but it is vary faint

I don't think this tremolo requires the footswitch to be engaged
but I have tried it both ways

again
thanks

[M Fowler]

Did a wire under the eyelet board come off when soldering new components?

Mark

[dansamp]

OK,
problem solved !

since this is a bias varying tremolo circuit
it is necessary to have the power tubes biased properly
I was concentrating on the circuit board and preamp circuit
and I hadn't moved to the power section to bias it
when I checked the bias I found that it was way too high
so the tremolo was going into cut-off

I brought the bias down to where it should be and now the tremolo
works fine

one question regarding this type of PI
I have tried to sets of power tubes and I can't get the bias to match

I get 27 ma on one tube and 10 ma on the other

is this type of PI always going to be that far off ??

[M Fowler]

Sounds like you could use a matched set of tubes for that amp or find another one to replace the tube running at 10ma.

Mark

[dansamp]

I am getting the same readings
from two different sets of tubes
I wonder if the cathodyne PI is the cause of this ??

[Wayne]

Leaky coupling cap between the PI and the outputs?

W

[dansamp]

ok I found two closely matched tubes
so that's ok 26ma and 28ma at about 420 volts on the plates

the last thing is I had to adjust the bias resistor to 560k in order to bring down the bias voltage to about -34 volts

the stock value is 27k

not sure why the big difference

the resistor coming of the rectifier that brings the voltage down before the diode is measuring 115k and the schematic calls for 100k
so that's not too far off

not sure if I should worry about it

any ideas ??

[Structo]

What is your wall voltage?

[dansamp]

What is your wall voltage?

about 128 volts