Princeton Reverb B1270 - buzzing/crapping out

[wrongtube]

Hi all, this is my first post on ampgarage, though I've been lurking and reading as much as I can over the last year or so. My experience: I've built one tube amp, the 18 watt Superlite TMB. I started with the Mojotone 18 watt kit and adjusted the circuit with additional components etc. Amp works like a charm since day one!

But you don't learn anything until you need to troubleshoot the stuff that doesn't work. Which brings me here. I inherited this Princeton from my dad, who bought it in the 90s with a few mods already done (master volume, diode rectifier, some caps replaced, a couple of components strangely missing). When I received the amp it wasn't working at all.

To cut to the chase, my current issue is: The sound is immediately VERY overdriven, then as the guitar signal falls away the amplification drops suddenly. If I play lightly there is barely any amplification, dig in and it goes to this heavy/buzzy overdrive. Here is a link to an audio clip, volume/treble/bass all at 4 on the dial: https://drive.google.com/file/d/1yyo6Ok ... drive_link

Going back to the beginning, here's what I've done so far to get the amp from non-functional to its current state:

- replaced filter cap can with Mojotone 20/20/20/20
- tested all resistors and replaced any that had drifted more than 10% from the stated value
- installed missing cathode bias resistor on V1A and cathode bypass cap on V2 (reverb driver)
- replaced nonfunctional bias circuit with variable bias
- fresh tubes throughout

At this point, the amp was making sound but more like a synthesizer – wild oscillations all over the place. I chopsticked around and found the long lead from the .022 coupling cap into the phase inverter was one culprit. Another was the master volume circuit, which was moved to the front panel with the tremolo intensity pot criss-crossing over the board to the back panel. So:

- moved coupling cap directly to pin 7 of V4
- deleted master volume circuit

Better, but still getting an intermittent small oscillation that gets more intense when I turn up the reverb pot. Note that I haven't had the spring tank connected anyhow as the RCA send/return jacks are broken and need to be replaced. Also, some bleed from the tremolo even with the intensity zeroed. So:

- pulled V2 (reverb driver)
- removed plate and potentiometer leads from V3A (verb recovery)
- removed plate and potentiometer leads from V4A (trem oscillator)

OK, oscillation is fixed and out of the circuit, and I'll deal with the reverb/tremolo issues later. Finally, as a last ditch effort I thought maybe the voltages were too high. So:

- removed diodes and installed a GZ34

Which dropped around 10V but otherwise no audible change. This is where we're at currently, and I'm stumped. My first thought was that the cathode bypass electrolytics probably all need replacing (those Mallorys are 50 years old, right?) but before I go any further I wanted to get some more opinions.

Schematics and gut shots attached. One discrepancy is the A and B positions for V3 and V4 are swapped from the schematic. Disregard the circled areas on the schematic, these were part of the image when I sourced it online.

Thanks in advance for any help.

Voltages

Wall: 117 AC
HT: 350 AC
B+: 441

V1
1: 203
2: 0
3: 1.82
6: 198
7: 0
8: 1.76

V2 (no voltages, tube pulled)

V3
1: 0 (first triode disconnected)
2: 0
3: 0
6: 144
7: 1.23
8: 2.55

V4
1: 0 (first triode disconnected)
2: 0
3: 0
6: 327
7: 0
8: 7.52

V5
3: 438
4: 435
5: -41
8: 0

V5
3: 438
4: 435
5: -41
8: 0

Tubes are biased at around 20mA each (60-65% dissipation).

[Helmholtz]

The PI triode V4A is either bad or wired wrong. See schematic for pin voltages.

[angelodp]

If it were mine I would re-build it. Brad over at Brad's guitar garage has done a number of full re-builds of these style amps. Given the mods and lots of questionable soldering areas, this just might be fun to take it back to stock. Get rid of the spaghetti wire, put in cloth, refresh caps, check trannies, check all component values, etc.
Basically gut and rebuild to spec.

[lonote]

The PI triode V4A is either bad or wired wrong. See schematic for pin voltages.

This looks to be the case, I can see at least one error, maybe there are others.

The coupling cap that is presently going to pin 7 of V4 (right at the tube socket) is in the wrong position. It has been moved (physically & electrically) so it looks like you are feeding B+ from the previous stage to ground through the V4 cathode resistors.

Copied part of your pic, I will try to do a mark-up, but I am not fast...

[wrongtube]

[/quote]

The PI triode V4A is either bad or wired wrong. See schematic for pin voltages.

The coupling cap that is presently going to pin 7 of V4 (right at the tube socket) is in the wrong position. It has been moved (physically & electrically) so it looks like you are feeding B+ from the previous stage to ground through the V4 cathode resistors.

Good eye, both of you. After trying a couple different tubes and double-checking the resistors, I figured out what lonote saw. The 1 Meg resistor should be in parallel with the coupling cap to pin 7, and I had it wired in series.

Rewired according to spec, and the issue is improved. However, I'm still experiencing cutting out / farting out kind of sounds. I'm not good at describing sounds so here are some more audio clips.

This is with exact same pot settings as the previous clip (4 across the board). The volume starts to waver/cut out slightly as the note dies out:

https://drive.google.com/file/d/18_GlDT ... sp=sharing

This is with volume at 7, the effect is much more pronounced. Towards the end of the clip I turn up the bass to 7 as well, and it's even worse:

https://drive.google.com/file/d/1Y3kW8s ... sp=sharing

Also, the distortion generally sounds more buzzy than I remember. Granted, last time I heard this amp in action was about 20 years ago, so who knows.

Thanks again everyone!

[wrongtube]

If it were mine I would re-build it. Brad over at Brad's guitar garage has done a number of full re-builds of these style amps. Given the mods and lots of questionable soldering areas, this just might be fun to take it back to stock. Get rid of the spaghetti wire, put in cloth, refresh caps, check trannies, check all component values, etc.
Basically gut and rebuild to spec.

I hear ya. I'm really enjoying learning and troubleshooting though! And since the amp wasn't in pristine condition when I got it, I don't feel too bad if I make some mistakes along the way. Plus, it was my dad's, so there's some sentimental value there. I just think it'd be really cool if I could fix his amp, and I reckon wherever he is he's thinking the same thing.

[wrongtube]

Oh, here are the new voltages on V4A:

6: 231
7: 25
8: 71

Higher than the schematic, but right now everything is 20-40 volts higher.

[lonote]

Glad you are making headway.

This is what I saw, not sure how you have it at the moment.

PR V4 miswired coupling cap MARKUP.JPG

[wrongtube]

This is what I saw, not sure how you have it at the moment.

Yep, that's it. I moved that coupling cap directly to the grid because the long lead was causing oscillation, but neglected to get the arrangement right with the 1 meg resistor directly below the cap. I've since replaced that resistor and tied it directly onto the grid pin as well — I should be able to post a new picture tomorrow, but it's now correct with the schematic.

[Helmholtz]

Higher than the schematic, but right now everything is 20-40 volts higher.

No wonder when not all tubes are drawing current.

[lonote]

Listened to your sounds clips, are you rolling the volume up & down or is that what the amp is doing on it's own?

In any case, I would be inclined to just re-wire all the disconnected stuff back how it was originally. That is not a super-complicated amp, but having stuff disconnected & floating isn't an ideal starting point to try to guess what is making problems.

There are enough ancillary components hooked up to your main signal circuit that support the reverb & trem, that they are likely still affecting things, even though those circuits are "disconnected".

[wrongtube]

Listened to your sounds clips, are you rolling the volume up & down or is that what the amp is doing on it's own?

That's what the amp is doing on its own. As the volume and bass are turned up, the effect is more pronounced. Is this what is called "parasitic oscillation"? I've heard the term but am not sure what it sounds like. To my ears, it seems like as the amp delivers more gain, the circuit is impeded in some way, causing the volume to drop.

In any case, I would be inclined to just re-wire all the disconnected stuff back how it was originally. That is not a super-complicated amp, but having stuff disconnected & floating isn't an ideal starting point to try to guess what is making problems.

That's a fair point — I thought it might help with diagnosis to disconnect the verb/trem but it sounds like it just makes things more confusing!

I'll get it wired back up later today and report my findings.

[lonote]

That's what the amp is doing on its own. As the volume and bass are turned up, the effect is more pronounced.

To me, it sounds similar to tremolo, but not a consistent speed. The trem in that amp works by varying the power tube bias voltage, so I wonder if your bias is now funky with the trem circuit being half hooked up..?

[lonote]

After looking more, I would be inclined to do as @angelodp mentioned earlier & strip/rebuild the board.

Not sure if you are aware of the YT videos by Psionic & Brad's Guitar Garage (previously mentioned)..? They have any number of relevant videos, & this one especially fits the bill. https://www.youtube.com/watch?v=KYs0beb3l1w

The subject is a Twin, but of a similar age & condition to your amp


I have two Pro Reverbs from the era your Princeton was built. Similar issues with excessive spaghetti wiring but also wax applied to the board, crazy layout, etc. After chasing gremlins for too many hours, they are still works in progress, but are getting completely rebuilt.

[TUBEDUDE]

After looking more, I would be inclined to do as @angelodp mentioned earlier & strip/rebuild the board.

Not sure if you are aware of the YT videos by Psionic & Brad's Guitar Garage (previously mentioned)..? They have any number of relevant videos, & this one especially fits the bill. https://www.youtube.com/watch?v=KYs0beb3l1w

The subject is a Twin, but of a similar age & condition to your amp


I have two Pro Reverbs from the era your Princeton was built. Similar issues with excessive spaghetti wiring but also wax applied to the board, crazy layout, etc. After chasing gremlins for too many hours, they are still works in progress, but are getting completely.

Hopefully with a better substrate than the paper card Leo used.