Princeton Reverb Trem issues

[ratgon]

Hi all. I just finished building this Princeton Reverb from Doug's parts and board. And thank you Sluckey for the layout and schematic you provided for it!

It sounds great with and without the reverb but, as per usual with me and my tremolo curse, I'm getting no tremolo. I've attached a photo of the build and its trem stage. The only thing that seems to affect it is my movement of the yellow grid lead. As it gets nearer any other wire I can hear the oscillation tick like a metronome but now effect on the signal. I've encountered this before on an 18watt marshall kit made. The grid wire seemed to be very sensitive to its proximity to other leads in the circuit.

I'll thank you in advance (and after as well) for any help you might have with this.

So, thanks

[HotBluePlates]

There are two 220kΩ resistors from the bias supply to the output tube grids. Measure d.c. volts from the junction of those resistors (closest to the middle of the board) to ground.

What do you get with the Intensity turned to zero and turned up?

[ratgon]

Thanks HBP.

So here are the voltages:

Zero Intensity = -27.4

Full Intensity = -22.0

[HotBluePlates]

At full intensity, did the d.c. bobble around or was it steady? I'm asking if there is a big a.c. present at that point in the circuit, though with some meters it's easier to see as unsteady d.c.

Now measure voltage at V4 pin 1, the plate of the tremolo oscillator. Do you have any a.c./unsteady d.c. here?

Assuming you say "steady; no a.c."... On the board, there is a 0.022µF and two 0.01µF caps sandwiched between a pair of 1MΩ resistors in the trem oscillator section. Measure the resistance to ground of both legs of each of those 0.01µF caps. All should be over 1MΩ; are they?

[ratgon]

Hi. So the voltage at the plate is 391. no fluctuation.

The first .01 cap measured a bit over 1meg ohms but the second..the one going to the speed pot only measured about 480k.

[ratgon]

Ok..so I replaced that cap and now i DO have SOME trem acting on the signal. The rate seems to be responding fine but the depth is very, very slight.

But I'm getting there thanks to you!

Thanks.

[HotBluePlates]

The first .01 cap measured a bit over 1meg ohms but the second..the one going to the speed pot only measured about 480k.

This may be normal, depending on the position of the speed pot.

Ok..so I replaced that cap and now i DO have SOME trem acting on the signal. The rate seems to be responding fine but the depth is very, very slight.

Since the cap itself may have been fine (being new), this may indicate the need for soldering touch-ups.

Your photo didn't show how the wires connect to the trem speed/intensity pots. I just had to assume they went to the proper places.

So here are the voltages:

Zero Intensity = -27.4
Full Intensity = -22.0

For everyone else: Can someone verify with their Princeton that when the Intensity pot is adjusted, there is no change of bias voltage at the 6V6 grids?

Looking at the schematic, I don't see a d.c. path where the Intensity pot setting should change the d.c. bias voltage. So that leads me to suspect a wiring error or solder bridge. But I'd like confirmation from an actual, working circuit first.

[sluckey]

The negative dc bias voltage should not change. I don't have a PR but I do have an amp with the 6G16 trem circuit, which is coupled to the output tube grids exactly the same as the PR. It does not change.

I would expect the grid bias voltage to be approx. -35vdc for a PR. I think his amp may be running too hot for the trem to be very effective. I'd crank the bias voltage up to -35 on the 6V6 grids and see if the trem gets stronger.

[ratgon]

Man. I really appreciate the help guys. I'll be home soon and check the bias situation although it did occur to me that the bias might be involved. Fiddling with the bias pot earlier didn't seem to have any effect. And although I did double check all the connections. I'll recheck or just redo all the solder joints. I'll check that replaced cap just for kicks

One question: does the type of cap I use make much of a difference. I don't have a handy mica to replace the original one with so I'm using a Mallory white one. I wouldn't think so but the circuit seems so sensitive to simply moving the leafs around I don't know

Thanks again

I gotta say that it sounds great otherwise and I've been deciding if I like the NFB or not. I may pop a switch in for it.

[HotBluePlates]

... One question: does the type of cap I use make much of a difference. I don't have a handy mica to replace the original one with so I'm using a Mallory white one. ...

Cap for what?

The only white Mallory caps I've seen are late-60's-to-70's electrolytics, where mica is usually a very much smaller value. The only mica cap I'd expect to see in your amp is over in the tone stack.

[ratgon]

Sorry, not mica...I was referring to the brown polypropylene cap in the oscillator. Anyhow, i redid all the connections and it's still just weakly affecting the signal while popping rhythmically depending on the movement of the lead to the grid.

I appreciate all your patience. Truly. I'll keep at it.

[HotBluePlates]

Okay, those are mylar caps. They all looked like new Xicons. I'd be surprised if you had a batch of bad caps.

So list to check:
-  Bias is a bit hot, needs to be lowered (more negative voltage to output tube grids).
-  Something not right with Intensity pot and/or something at the tremolo side of the Intensity pot; Bias voltage @ 220kΩ's should be unchanged with Intensity changes.
-  You mentioned hearing at least some trem; verify you now have a.c. volts or "bobbling d.c." at the oscillator plate (V4 pin 1 on the layout, assuming you didn't swap the triodes).