Princeton Reverb clone tremolo intensity help

[MWaldorf]

I've got a peculiar problem with my Princeton Reverb clone. I hadn't used it in a few months, and when I fired it up, the tremolo barely functioned. I removed the footswitch and tried swapped tubes but that didn't help.

A few notes and observations:
I can hear the tremolo pulsing the white noise, but it barely affects the guitar signal.
The intensity and speed knobs both control the pulsing. The speed range is normal, but the intensity is very very low.
One odd thing - when I turn off the power or standby the tremolo pulses with normal intensity. This seems like it's a big hint, but of what I don't know.

The amp has the Paul C. and Stokes mods, and the tremolo was working just fine for months after I did the mods.

Any ideas on next steps for troubleshooting?

Thanks,
Mel

[Chuck H]

Since you can hear and control the speed of the pulsing I would think that the oscillator is functioning properly. So it's not the tube. This is a bias vary trem. My first concern would be the actual bias voltage getting to the tubes since that connection is through a potentiometer center tap. The trem pot or a cold solder joint are the most likely suspects and either could cause a dangerous bias fault (see note below).

The fact that the amp sat for awhile before the problem manifested might be a clue. When amps sit and moisture comes and goes things oxidize. This exacerbates cold solder joints and pot contacts. First thing to check is bias voltage. Hopefully you have it. Then clean the pot. If that doesn't fix it then re melt the trem and bias circuit solder joints with a little fresh flux or flux core solder. I have a hunch one of these two operations will fix your problem.

You mentioned a standby switch but the Princeton Reverb schems I looked at don't include one. I also don't know that your clone is using an identical power supply to the original. So, not knowing your circuit, I can't say for sure what it means that the trem manifests when the power or standby are flipped. My guess is that as the amp loses bias voltage after a switch is flipped, the oscillator stays active longer and takes over the power tube grid bias. I think you may have your bias supply on the wrong side of the standby switch. The grid bias should be on full time regardless of the standby setting.

NOTE: On the bias vary trem I built I didn't like having the bias voltage dependent on a cheesy pot wiper so I included a 2.2M fixed resistor from the bias supply to the center tap. This will affect trem function very little and still provide grid bias for the power tubes if the pot wiper fails.

[MWaldorf]

Thanks Chuck. I know the stock AA1164 circuit doesn't have a standby, but this particular clone has one added to the circuit. I'll look for cold solder joints; the most suspect joints would be anything I touched when I did the Paul C and Stokes mods.

[Chuck H]

Suspect the pot first. Just blow it out with compressed air of a keyboard cleaner and then spray some pot cleaner/lube into it and give it some twisting. If still, then.... Resolder the trem and bias circuit eyelets. I think the Stokes mod does get into the bias circuit so maybe... Nothing about the Paul C mod should effect your problem, so you can skip that area.

[MWaldorf]

Thanks again. I hit the intensity pot with compressed air and electronic cleaner and I remelted all the solder connections I'd made. Still having the same issue. One thing I did notice - the voltages on the amp seem low. Pin 3 on the 6V6s read 335V DC instead of 410V on the schematic. On the cap can I read 260V and 216V on either side of the 18K resistor, rather than 320V and 240V as on the diagram. Is this a sign that the rectifier tube is dying? I assume low voltage would mean low current so maybe there's not enough current for the tremolo to oscillate?

[Chuck H]

I mentioned:

My first concern would be the actual bias voltage getting to the tubes...

and

First thing to check is bias voltage. Hopefully you have it.

Remove the power tube. Don't fire up the amp again with the power tube in before checking for bias voltage. The low voltage reading isn't the fault of the rectifier tube. They tend to either work or not. They don't usually just provide low voltages. The low voltage reading is probably due to an improper bias condition. That may also explain the poor trem function. The amp is biased so hot that the oscillator can't fluctuate the bias current below the tubes full audible output!!! YIKES!

You should have already turned off the oscillator and checked pin 5 on the 6V6 socket for bias voltage. I mentioned it twice in the first response. Please take out the power tube and don't put it back in before you see to it that you have bias voltage (you probably don't) before continuing.

[MWaldorf]

Chuck,

I'm a novice and I didn't understand what you meant. This evening I did remedial study on bias measurement on youtube - three great videos by the same guy who explains things from the basics and also measures bias with just a multimeter instead of one of those socket bias meters. For anyone who comes along in the future, the link is: https://www.youtube.com/watch?v=L3rRk3eSTnA

Anyhow, following your advice, I pulled the power tubes and checked pin 5 and there was voltage. Then, following the instructions in the video, I measured the resistance and voltage from pin 3 on each power tube to pin 8 on the rectifier, used that to calculate plate current (R/V, again for anyone who finds this thread going forward), measured the plate voltage (power tube pin 3 to ground) and calculated the plate dissipation (PC)(PV). The plate dissipation was way too high, reading 13.8 W on one tube and 19 W on the other!

This particular amp has a bias pot added to the circuit, and using that I was able to increase the grid resistance to get the plate dissipation down under 12W. Once I did that, my vibrato came back with a wonderful warbling vengence.

So, my guess is that when I used the amp last time, I had put a bunch of stuff in the back and maybe, somehow the bias pot got turned? I'll have to put some duct tape over it so it can't get moved

Anyway, thank you again for your help and starting me on my way here.

Best, Mel

[Chuck H]

Novice is fine! That's a lot of what we do here

I didn't watch the vid, but I will say that a lot of your description doesn't fit neatly into proper nerd speak on the matter. Unimportant since it sounds like you twiddled the bias adjustment and got the dissipation down. I will say that if you're running 6V6's you should probably be under 10W idle current per tube. And even that is a little high since the PR runs somewhat high voltages on it's 6V6's. It also seems that your tubes are poorly matched. Also not a problem UNLESS this mismatch in current is caused by a circuit error rather than just unmatched tubes.

I'd be interested to know what your bias voltage measurement was then and is now. Also, what is your actual current on each tube now? And one more thing... Swap the power tubes and see of any current difference between them follows the tube or stays with the socket.

[g1]

That video was about biasing single ended cathode biased amps. I believe the measurement of pin8 of the rectifier was for finding the plate voltage.
I don't see how you could come up with a plate current from that method with this amp.
Does your amp have a cathode resistor? (I doubt it because you said you had a voltage at the grids).
At the end of the video he mentioned he might later do another video that covered amps with 2 power tubes and grid bias. He also mentioned it would be more complicated.
I'd suggest you watch the measurement portion of the video again, especially about the cathode resistor.

[xtian]

I'm late to this thread, but +1 on Chuck's mention of power tube bias. You need to bias the PR a little cold in order for the tremolo to work nicely.

[g1]

You can ignore my previous response, I think you linked to the wrong video, maybe you meant his other one titled "biasing double ended amplifier tubes"?

Anyway, it sounds like you were measuring the voltage drop across the OT primary resistances, and calculating the current from that, which is a valid method.

[Alex R]

I'm late to this thread, but +1 on Chuck's mention of power tube bias. You need to bias the PR a little cold in order for the tremolo to work nicely.

+ another 1. Trem is very sensitive to bias adjustment on these amps, and you often end up with bias on the cool side to get the trem right, and also stop the noise it can cause.