Mojotone Princeton Reverb kit Tremolo squealing

[Frankie Chestnuts]

Just finishing up a Mojotone Princeton Reverb kit. Just at the start up stage. Reverb sounds fine. The tremolo begins to squeal or squeak (chirp?) as I turn up the intensity. I've poked around and the only connection that seems squirrely is pin 7 on the V3 12AX7. I've changed out that tube with no change. When I kick off the trem with foot switch, everything seems fine. No tremolo, but no noise.

My thought now is to change out that tube socket.

Lots of pics available for what they are worth. This is my second build. I have a Tweed Deluxe I put together a couple of years ago.

Please let me know your thoughts. Thanks all.

 

[dan40]

Try using a wooden chopstick or other insulated tool to move the wires a bit leading to the socket and pots. The amp must be powered up while doing this so be careful. Sometimes just a small wire adjustment can quiet an amp down. If that doesn't help, some pics and a full voltage chart of every socket will help us track down a problem.

 

[Frankie Chestnuts]

I did the poking thing. Pin 7 on V3. The solder seems tight. I don’t think this tube has anything to do with the trem circuit, does it? But it is the only connection that seems to pop when I prove it. I’ll resolder it.

Aside from this, pictures and voltage chart for all the pins. I’ve never done this, so just power up and measure voltage from each pin to ground. For all the Tubes? Thanks again

 

[Frankie Chestnuts]

Bump

 

[SnidelyWhiplash]

V3 is reverb recovery & mix. V4 is phase inverter & tremolo.

Pics would be nice...

 

[Frankie Chestnuts]

Should I measure pin to ground for each tube socket, with tubes inserted?

 

[Frankie Chestnuts]

 

[__HKGuns__]

It may help if you scroll down to the Princeton Reverb signal flow diagrams to help you better understand what components are involved.

The solder joint at 16 looks suspicous in your pictures, but it may be the light. Looks like perhaps a cold solder joint at 41.

V3 voltages to ground would probably help as well, assuming you have sound. If you don't have sound, start with B+ and work your way from the Power Supply.

Princeton Reverb Signal Flow

 

[schmee]

I sure wouldn't jump to replacing a tube socket. There's always some little thing starting up a new build.

 

[Snfoilhat]

High frequency oscillation getting beamed across air space in the chassis as signal level peaks with every cycle of the LFO?

Chopsticking and ultimately rerouting leads associated with the trem may be easier thanmaking changes around the v3 socket

Edit to add some detail to this idea: pin 7 is a grid, possibly sensitive to stray signal, but there’s the transmitter side of the loop too, and a change to either could remove it

 

[Frankie Chestnuts]

Thanks for all the suggestions. This is the kind of help that I need. I’ll work on it this evening.

 

[dan40]

Aside from this, pictures and voltage chart for all the pins. I’ve never done this, so just power up and measure voltage from each pin to ground. For all the Tubes? Thanks again

Yes. The amp must be powered up and a speaker plugged in. The green filament wires that run to each socket will be an AC voltage measurement. For this measurement you want to place your meter leads on both filament wires at the same time. For the power tubes, that will be pins 2 and 7. For the preamp tubes, that will be pins 4/5 (they are connected with a jumper) and pin 9. Take this measurement on each socket.

For the high voltage measurements, set your meter to DC voltage and use your red mead to touch the connection and the black lead to ground. Take a reading on pins 3,4 and 8 on the power tubes. For the preamp, the measurements will be on pins 1,3,6, and 8. Place only one hand inside of the chassis and make sure not to touch two pins at once. I like to place a small piece of heatshrink on the end of my leads, leaving only the very tip exposed to take the readings.

 

[dan40]

Another possibility is positive feedback being caused by the nfb circuit. It normally causes a squeal when you first power up the amp but it has been known to rear it's ugly face in other strange ways. A quick and easy test is to unsolder the wire at the output jack that runs from the 2.7k resistor on the board to the output jack. If the squeal goes away after unsoldering this wire, you will need to swap the OT wires connected to pin 3 of each power tube socket. There is a 50/50 chance of wiring this up right the first time during the build so it's one of the first things to check whenever the amp exhibits odd behavior like yours.

 

[Dacious]

Try moving the reverb transformer leads. Are you using shielded cable to the pan?

 

[Frankie Chestnuts]

"A quick and easy test is to unsolder the wire at the output jack that runs from the 2.7k resistor on the board to the output jack"

Are you talking about this one? Won't that remove all signal going to the speaker?

 

[Frankie Chestnuts]

	Filament Wires
	All tubes = 5 V AC
Preamp Tubes
Pin	Tube 1	Tube 2	Tube 3	Tube 4
1	342	352	158	214
3	1.2	6.9	0	0
6	149	352	153	178
8	1.3	6.9	1.3	52
Power Tubes
Pin	Tube 5	Tube 6
3	803	803
4	782	786
8	0	0

 

[Frankie Chestnuts]

SUCCESS!!!

Swapped the Output Transformer leads on pins 3 of the power tubes. I got nice quiet trem. And nice lush reverb. The amp seems a lot quieter now also. I got myself a Princeton Reverb!!!

Thanks to all, particularly @dan40.

 

[dan40]

Great news! Glad that it worked. It's a common thing even if you do follow the layout to a T. Many of us will leave those leads long until after the first power up to verify they are correct. Once you know they are good you can trim them up to length and make the final connection.

 

[Frankie Chestnuts]

It's funny... I double checked all the schematics I have. It should be the other way. I never would have found it myself.

 

[schmee]

SUCCESS!!!

Swapped the Output Transformer leads on pins 3 of the power tubes. I got nice quiet trem. And nice lush reverb. The amp seems a lot quieter now also. I got myself a Princeton Reverb!!!

Thanks to all, particularly @dan40.

I almost suggested that but usually it's a wild howl immediately when you turn on the amp! But there have been times when just a little occurs, and you found one! Good job.