Need help debugging a PR trem

[Luthierwnc]

Hi All,

I'm building a Princeton Reverb from scratch. I can't get the trem to work at all. I've tripled checked the components, grounds, schematic and voltages and still get nothing. On the scope I can definitely see major pattern disruption at the 220k grid/intensity #2 lug junction but the only changes in sound with a guitar turning the intensity and speed knobs are minute gain differences.

Everything is bone stock except: a) I reduced the 1m resistor coming off the plate to 270k. Subbing the 1m back didn't make any difference and 2) I'm using an adjustable fixed bias circuit which just substitutes a 22k resistor and 10k pot for the bleed resistor on the schematic. Currently 27k to ground. Tubes are pulling 23/24 ma. I pulled the .02 bleed cap too just to see if I could find a pulse but nada.

I've subbed tubes already. The amp sounds fine except no trem. For most of the fiddling the trem footswitch has been removed and I get 1.03m resistance to ground at the .01 .01 junction.

Preamp plate voltages:

V1a 184, V1b 192, trem plate 233, PI 248, reverb 370,

A long history of working on amps assures me it is something simple that I've overlooked so many times I take it for granted.

Any ideas on trem troubleshooting would help. Thanks, sh

[Deric]

AA1164 ?

What's your plate voltage on the power tubes? Is the 23/24 ma per tube or total? With a bias vary trem the output tubes need to be biased on the cold side. I think stock is about 50%. I've found that biasing the oscillator with an LED helps keep it thumping at higher bias settings.

[tubeswell]

A socket pin wiring mistake? (and what Deric said about the 6V6 bias - you want 18-20mA to get a good trem intensity)

[Stevem]

If you pull the outputs out than what range of voltage shift do you see on there pin 5?

[Luthierwnc]

I have to get another PT. I had a Hammond from an AO series organ amp in the transformer box that tested 330-0-330 no-load but even with a diode rectifier it only put out 401VDC with no gain and turning the volume up half dropped that to 365. I'll probably get a DR transformer and drill some new bolt holes.

Even in this configuration I was getting 248 on the V4a plate. The cathode on the trem side was at 2.33 which is pretty close to the schematic. .001 on the grid.

I'll get back to this in a week when the new PT is installed.

Unrelated: I still trigger toxic alams with Chrome and Firefox visiting the site. IE seems to work. I'm also not getting emails for reply posts. FWIW, Skip

[Luthierwnc]

Got a new power transformer and am still getting no trem.

stevem: with the 6V6s out I get -39 VDC on the grids with the intensity off and -38 with the intensity dimed. At any setting the level is dead still. No AC showing either.

Attached are two JPGs. The first shows the PI 220k grid leak junction with the intensity at 9 and the speed at 7. With the intensity off that is a flat line. 1k sine, volume on 3, reverb off, footswitch unplugged. Voltage on the oscillator plate 248 VDC. Tubes biased at 22 ma.

The second shot is the signal at one of the power tube grids. It looks pretty clear. The other tube clips on the other side of the wave.

It is almost like the oscillation gets killed in the grid leak resistors. But trems aren't one of my strong suits.

Any more ideas will help. Other than that, the amp is ready to play.

Thanks, as always, sh

[tubeswell]

Can you upload a hi-res gutshot of your build?

[Luthierwnc]

here you go:

[tubeswell]

Questions attached

[Luthierwnc]

Q: 2 things linked up? A: yes .01 cap feeds the oscillator tube grid.
Q: .02 off oscillator plate to 3m speed pot #3 lug? A: Yes
Q: .1 to intensity pot #3 lug? A: Yes
Q: 3 things linked up? A: Yes, jumper between plate resistor, .02 coupler
and plate -- 253 VDC.
Q: 2 thinks linked? A: Yes, oscillator K goes to 3k3/25uf cap and 1m
resistor. 2.38 VDC cathode voltage.

All HV wires are red, cathodes are yellow and signal wires are purple on the preamp tubes. Except for being on garolite, the layout is identical to the original Fender on the left side of the board. The right side is the same except for an extra filtration node between V1 and V3 and a 1, trimmer for dwell. (upper right).

Attached is my working schematic including all mods. Red circles: I don't have the diode across the intensity pot but I did sub a 270k resistor for the 1m from the oscillator plate to the bias circuit. Tried it both ways with no effect. TMB tone stack with PAB switch. V!a and b are closer to 180 VDC than original. Fine by me.

Do you think the power tube grid stoppers could be killing the trem? Those are new fangled too.

[sluckey]

Post a scope pic of V4 pin 1. Should be a large low freq sine wave, approx. 3Hz to 10Hz.

[Deric]

On my builds it's usually the simple stuff...

I believe the footswitch on these turns the trem off when shorted. Maybe double check wiring of footswitch jack. ?? Is it possible the (orange?) footswitch wire is wired to the ring/ground of the jack instead of the tip?

[Luthierwnc]

Hi Deric -- been a while. Continuity at the jack tip is 1 meg. Fraid I ran out of stoopid stuff a week ago.

Slucky: attached is a picture of the scope screen. The wave form was actually much better live but I couldn't get a clean shot. I posted the one that showed the best pattern. Settings: 5 ms, 50MV boosted 5X -- althought the size jumped around quite a bit. Volume on 4 but no input signal.

Of note; fiddling with the speed knob would occasionally flat-line the sine wave. 20 years ago or more I remembered something similar with a bug tremolo. It took some percussive maintenance to wake it back up again. I don't know if there's a common thread but I'm grasping here.

In between then and now I also pulled some of the caps and checked for dc. They were all clean. Plus, the junction of the .02 and the speed knob gave me a sweep of 100k to 3.1 meg so nothing is leaving other than through the pot.

Thanks all, sh

[sluckey]

Well, an optimistic guess would be 5 vertical divisions. Multiply by .1V/DIV would be .5Vpp. X5 would be 2.5Vpp. That's way, way low! Now if you are also using a X10 probe you have 25Vpp. 25Vpp may not be enough. 50Vpp should wiggle the bias enough to hear the trem effect.That oscillator circuit should put out at least 50Vpp. Rotate a bunch of 12AX7s thru that socket.

Is your probe a X10? If not, you need to concentrate on the V4 circuit.

[Luthierwnc]

I apologize for not being more careful. The first readings I sent were from a cold amp. I let the amp warm-up for 10 minutes and got very different results. The attached pic (in the dark helps a lot) was at .5v, 5 ms using a 10X probe.

Again about 5 vertical divisions. Very consistent waveform. no input signal. Reverb off, intensity dimed. speed around 7.