Modified Princeton trem problem

[wavley]

Ok, so I’m fixing up this Princeton that someone did a hack job of converting to 6L6s, they changed the power transformer and output, then someone else was in there trying to figure stuff out and did some silly stuff

Any way it goes, it’s a 79 so not exactly the neatest wired amp, I’ve cleaned up a lot of stuff, someone already disabled the pull boost (I had to finish the job though), it’s actually sounding really great now and super quiet.

BUT! I can’t get the trem to wiggle!  The LFO just won’t start.  I recently gutted a reissue Twin and rebuilt it and got 4 6L6s to wiggle. 

Someone had already replaced some of the trem caps and resistors, I finished the job.  He said the first guy disabled the trem because the reverb was too wet, which I can’t even wrap my head around how you would do that, but I see no evidence of that happening.  Somebody put a pot in the bias section so it was easy to try it at different bias points, but it’s not that anyway because I can’t get the LFO to start. 

Things I’ve done other than replacing the rest of the parts.  Added a kick start from the raw bias, replaced the cathode resistor/cap with an LED so I have a visual too.

Anyway, I’m getting 211 on the plate, 1.86 on the cathode, and 1.2mV on the grid

Thanks for the help folks

Edited to remove some unnecessary information/clarity

[PRR]

I know you can't list everything, but.... did you replace the trem osc tube? With a hot 12AX7?

[shooter]

does it have a foot switch? maybe verify the jacks/switch

[sluckey]

Post some pics of your trem mods.

Footswitch not needed for the stock PR trem. But who knows with all the mods you've done to it. If you properly "Added a kick start from the raw bias" like the AB763 trem then you will need a footswitch to enable the trem. I suggest you remove all trem mods and return the circuit to stock. It's a very simple proven circuit that is very reliable.

[wavley]

Thanks for all the help folks, I know it’s going to end up being something stupid that I’m going to be embarrassed about needing help to find.

Another fun fact about this amp is that the PI and OSC halves look like they were built reversed of normal from the factory.  I got tired of transposing tube halves when looking at voltages and figured I wouldn’t add to the confusion for any future techs and put it back to the normal factory layout.

I know you can't list everything, but.... did you replace the trem osc tube? With a hot 12AX7?

Hey Paul, nice to see you.
Yeah I keep bins of tubes within arm’s reach for this.  By the time I’ve tried half a dozen I’m of the opinion that even if I find the magic tube that works, it’s only a temp fix until this dude that reads too much TDPRI is going to start rolling tubes and it’s back on my bench, plus it’s not fair to kick the can down the road to future techs if that’s the case.

does it have a foot switch? maybe verify the jacks/switch

Princetons shouldn’t need one, but I’ve tried the switch from my amp just in case, but mostly I’m using a clip lead to simulate open and closed.

Post some pics of your trem mods.

Footswitch not needed for the stock PR trem. But who knows with all the mods you've done to it. If you properly "Added a kick start from the raw bias" like the AB763 trem then you will need a footswitch to enable the trem. I suggest you remove all trem mods and return the circuit to stock. It's a very simple proven circuit that is very reliable.

I should have added that the kickstart was temporary and I tried it with both a foot switch and clip lead.  The only mod that’s left is the LED on the cathode, but it didn’t work with new components of stock values either, the LED is still there because it’s a quick visual and the trem is going to need help wiggling those 6L6s.  I did replace what the other guy didn’t replace with new parts, but I consider that maintenance rather than a mod, everything he put in measured properly.

The only mods I did are irrelevant to the trem OSC, but I used the hole drilled for a poorly implemented mid control with a vox style cut and repurposed the ground switch to be a three way negative feedback which I’m leaving in the stock position but flip around when testing.  Undoing a bunch of bad mods is mostly what I’ve done in this amp. And at the customer’s request I replaced the new coupling caps with Jupiters. I poured over threads and tried basically everything you had suggested to other folks.

The dude bought it this way and the trem never worked.  Someone used the right parts modding, they just did a bad job of it, they replaced the PT with one of those taller TAD ones for 6L6s and it’s got a bigger OT, which they moved and is almost touching V3 and V4 and now that I think of it, you don’t think the field from that transformer is what’s messing me up do you?

I’ve attached a couple pics, please excuse the mess, it’s part I’ve been poking around and will clean up a bit and part you should have seen it when I got in there.

[wavley]

As an update, I figured it was easy enough to try, I unmounted the OT to see if being too close to the tubes was a problem, no luck, but it looks like right where he put one of the screws is exactly where the trem cancel switch wire goes into the eyelet, there’s a good chance it shorted to the eyelet.  The OT is currently sitting on a block and the screw isn’t in there anymore, but now I’m really suspicious that there’s either debris still shorting that eyelet or some other fiberboard damage. 

[sluckey]

Not all LEDs will work. Green LED gives wrong voltage for proper tremolo oscillation. Replace with red or yellow. Be sure cathode (flat side) connects to ground.

That board is a nasty mess. It's possible that it may be conductive. If that's the case you may never get this thing working, short of replacing the board.

[wavley]

Not all LEDs will work. Green LED gives wrong voltage for proper tremolo oscillation. Replace with red or yellow. Be sure cathode (flat side) connects to ground.

That board is a nasty mess. It's possible that it may be conductive. If that's the case you may never get this thing working, short of replacing the board.

Yep, cathode to ground, tried red and yellow too, even tried two reds in series just as an experiment.  The 3.3k and cap that was in it was closer to the 2.4v on the schem, the red was 1.6, green is 1.8, and the two reds in series was 3.3, I don’t remember what the yellow was, it was first and I changed it because I’ve always had good results with red in the past.

This amp is a mess all around, albeit less of a mess than when I started, sooo many bad solders.

I’m at the fiberboard is messed up too at this point.

Just some background on me so you know I’m not just an Internet dude who is being dangerous.  I started learning amps in the mid 90s, spent a few years as an amp tech in Tampa where I had great tutors, I moved back to VA where I went back to school at the same time spent a dozen years doing R&D for cryogenic low noise amplifiers, reflectionless filters, and local oscillators for the National Radio Astronomy Observatory and I also subcontracted for NASA’s DSN before getting fed up with my coworkers and started my own amp shop (four of those years overlapped, I started doing this full time again two years ago) and I’m an authorized service center for a few manufacturers.  I might not be as good as you or Paul, but I’m competent and I know when to ask for a second set of eyes or advice and I always take advice when it’s from a knowledgeable person, which i do for a lot of things because nobody knows everything.

[shooter]

sounds like you're more than qualified to stab around the board looking for ohm

[wavley]

sounds like you're more than qualified to stab around the board looking for ohm

Ha! I just like amps a lot.  I realized this thread was my first post ever on this forum and nobody here knows if I know what I’m doing or not and I know I also don’t care for encouraging folks to poke around in things they don’t understand when it can kill them, just wanted to put everybody’s minds at ease.  I already know PRR from DIYstompboxes, I was happy to see responses from you guys that know what you’re doing.  I read the forum a lot, but it’s mostly old threads when I’m looking for something so I realized I’ve never actually posted here, sure have seen Sluckey come up a lot when reading about trems.