Princeton vibrato problem

[MC808]

Delta362 As I understand it DR amps have the default to off but this is a Princeton circuit and in the Princeton then the default is vib and reverb ON. The short Jack then switches off.
Thanks for advice on V4 that would seem logical that the short is causing a problem here. Can anyone explain exactly how the short interacts with the trem circuit and what could be going wrong with V4?

[Delta362]

MC808 Thanks for clearing that up for me! I now realize that 've only had the bug vibrato circuits come thru and mistakenly thought this operated under a similar principle.

[Chuck H]

Delta362 As I understand it DR amps have the default to off but this is a Princeton circuit and in the Princeton then the default is vib and reverb ON. The short Jack then switches off.
Thanks for advice on V4 that would seem logical that the short is causing a problem here. Can anyone explain exactly how the short interacts with the trem circuit and what could be going wrong with V4?

The short grounds the signal being fed back from the oscillator plate to the oscillator grid. No feedback means no oscillation, and no trem. Nothing about the method should cause the problem you're having. It may help to see how the jacks are wired.

[Chuck H]

The picture is not especially sharp and the capacitors are large so I can't see the eyelets, but...

[Helmholtz]

Can anyone explain exactly how the short interacts with the trem circuit and what could be going wrong with V4?

The LFO works by feedback from plate to grid.

Grounding the point between the 10nF caps shorts the feedback signal to ground.

Either the jack is connected to a wrong circuit point or the jack doesn't make a good ground contact.

Edit: Hadn't seen Chuck's post above.

[MC808]

Hey everyone, thanks so much for your thoughts! Here are some photos and I have a video also.

[MC808]

I can’t seem to post photos it says I’ve reached my limit? But here’s a link to a video of the circuit board. The blue cable is indeed attached to the vIbrato RCA port. Unfortunately it goes under the black circuit board in the middle so I can’t see exactly where it’s attached in the trem circuit.

https://youtu.be/IbRGGkAcAIs

I’ll try and get the photos uploaded…

[Helmholtz]

You can use your Ohmmeter to find where the wire connects. Look for zero Ohms between wire ends.
Also use your Ohmmeter to see if your shorting plug actually grounds the wire.
The outer (crown shaped) contacts of RCA plugs sometimes require some bending for good contact.

Otherwise you need to lift the board and use a little (dental) mirror.

[g1]

Further to what Chuck and Helmholtz stated above, red circled lead of 1M resistor in this picture should measure 0 ohms to center terminal of Vib.pedal jack.
Then with shorting plug inserted, red circled lead should measure 0 ohms to chassis.

[MC808]

@g1 Thanks! So both reading show 0. With the short RCA cable inserted the reading jumped around a bit but settled on 0. Is that normal?

Further to what Chuck and Helmholtz stated above, red circled lead of 1M resistor in this picture should measure 0 ohms to center terminal of Vib.pedal jack.
Then with shorting plug inserted, red circled lead should measure 0 ohms to chassis.

[TomCarlos]

I'm looking at your video. It looks like the Blue wire is going to the wrong place. I don't see the blue wire going under the board. I see it connected to the wrong spot. Look at Chuck's image and his note - "Because it should be here."

[g1]

I'm looking at your video. It looks like the Blue wire is going to the wrong place. I don't see the blue wire going under the board.

At 26-27 seconds into the video, you can barely see the blue going into the hole under the yellow wire. From there it goes under the board to the 1M resistor lead I pointed out in post #24. Resistance readings have confirmed it's going to the right place, so I'm moving on to what I'll call the 'caterpiller'.

MC808, did you build this amp? What is with the black wire wrapped around the others circled in this picture? It does not show up in the documentation and sometimes additions like this look nice but cause weird problems.

[MC808]

@g1 Good skills, seems to be the case. No this is not my build. The guy who built it had help from the owner of Olsson Amps, a builder here in Sweden, so I’m fairly confident it was well put together - this issue withstanding. The caterpillar section I guess is something that Olsson Amps might do on their amps?

one though I have: could it be an issue with V4? Is the ground being sent to the valve and amplified? Could it be a valve fault?? Worth replacing the valve?

Extra info: I do know of two mods…

1) input 2 has been turned into a tone circuit as the brownface amps had. This was done with Olsson Amps supervision.

2) A second mod has been done on the volume input to leak a bit of the signal as for a short time it was used by the harmonica player in a band (I know scandalous ) and this is apparently a common mod to reduce feedback from high gain mic input. I’ve spoken to the person who made the mod and he seems sure that that wouldn’t cause this problem which is only present when I insert the RCA short. From what I can understand that mod is reversible I just need to disconnect the wire.

That’s pretty much everything I know. Let me know if I should test the caterpillar section.

[Helmholtz]

Does the "vibrato" stop without making noise if you disconnect the blue wire from the RCA jack and connect it to the grounding point where the 25µ cathode cap is grounded?

[MC808]

Helmholtz I can try that. Any chance you can show me on the photo where the cathode gap is grounded?