BF Princeton Reverb/Tremolo Problems

[RightLurker]

I serviced a '67 Princeton Reverb a year or two ago - electrolytic caps, new power tubes, 3 prong power cord, installed bias adjustment circuit, set bias, etc. It worked fine for a year or two, but it's back with a strange (to me) problem. The amp worked fine until you turned up the reverb. which produced a loud hum - the higher the knob setting, the louder the hum. In addition to the hum, you could hear the tremolo ticking. The higher the reverb setting, the louder the tremolo ticking, right along with the hum. I put all new preamp tubes in it and replaced the female RCA reverb output jack on the chassis. Some slight improvement, but not much. Next I found a bad reverb cable, and a new set has been ordered.

In the meantime I have the amp on the bench with the reverb tank completely disconnected. The amp is dead quiet until you turn up the reverb, and then there is a fairly loud hiss, and the tremolo ticking is there along with the hiss. Turning up the trem speed control increases the speed of the ticking. Turning up the trem depth control increases the depth of the ticking. Turn the reverb knob to zero and the amp is dead quiet.

I'm thinking about replacing the reverb driver transformer and the .02/.01/.01 caps in the trem circuit, but that's just throwing parts at the problem. Has anyone experienced this problem or have an idea what the problem might be?

Thanks.

[martin manning]

Regarding the hiss when you turn up the reverb, have you replaced the plate load resistor on the reverb recover stage?

I agree with you, hold off on the reverb driver transformer, but see if you have any DC at the top of the speed pot, after the first of the three phase shift capacitors. If you do, replace the caps. The oscillator is working, obviously, but new caps might help with the ticking.

I've read that shielding the lead from the junction of the 0.01u cap and 1M to to the grid (pin 2) of the LFO triode can cure the ticking in PR. Ground the shield at the socket end only. I don't know if this is a universal fix, but odds are you have some kind of unwanted coupling.

Does this amp have old fiberboard? You might try a hair dryer on it for a minute or so and see if anything changes.

[RightLurker]

Thanks, Martin. I haven't checked the plate load resistors or for the DC. It will be the first order of business. The amp does have the old fiberboard material, which I will hit with a hair dryer. I've read about the shielded cable thing as being the "ultimate PR ticking reverb silencer," so I'll try that, too. I'll try and be patient enough to do it one step at a time and check after each step, so maybe I can report back exactly what the problem was. Thanks again.

[Phil_S]

Maybe one of these will help.
http://timeelect.com/jbl-twin.htm

[Leo_Gnardo]

I'm thinking about replacing the reverb driver transformer and the .02/.01/.01 caps in the trem circuit, but that's just throwing parts at the problem. Has anyone experienced this problem or have an idea what the problem might be?

Replacing the trem timer caps would be the first order of business. Use some good film caps and you may find that sorts everything out. Why the reverb transformer would be involved I have no idea. Sometimes the tube can be the "ticking" problem too. Since the a Princeton doesn't use the neon lamp "bug" you can't blame that. (For those who do have Deluxe-on-up Fenders with the "bug", sometimes the neon lamp is source of the problem. And usually a 0.022/600 or 630V film cap across either the 10 Meg or from Neon/10M junction to ground will turn an annoying tick into a much less audible bump noise.)

I haven't found shielded cable to be much help. But there may be others who have.

[Milkmansound]

sounds like an oscillation to me - try poking around with a wooden chopstick

there are a few wires that can be a pain in that kind of tremolo circuit that it may help to shied. The wire going to the footswitch is one, the one going to pin 2 of V4 may be another

another option is that the reverb pot is not grounded properly, or has something bad happening at its max rotation.

make sure the trem depth wires are not near the pots or reverb control, and also make sure that the two circuits do not ground anywhere near each other. Never a fun one to track down!

[hans-jörg]

Hello,

I read, that tremolo shortens tube live a lot. Swap tube and try a longplate one.
As an add.: shielding gets a + !

Best
Hans-Jörg

[RightLurker]

Many thanks for all the pointers. I changed out the reverb cables and that took care of most of the problems. Adjusting the locations of some of the wiring seems to have gotten rid of the tremolo ticking.

But a new problem has raised its head, and I can't locate the source. The amp sounds okay at low volume. Turn it up to about 3 and hit a note (particularly a low note) or a chord hard, and the amp makes the most God-awful distortion right on the beginning of the note. It sounds like crackling or frying bacon that sort of rides on top of the note, if you know what I mean. All of the resistors measure within tolerances. All the voltages are where they should be. The power tubes are a pair of JJ 6V6s with very little time on them. Unfortunately I don't have a spare set of 6V6s to swap in. I don't want to start yanking original coupling caps and other parts out of the amp in a shotgun approach.

I should mention I've tried different speakers, speaker cords, etc., so I know the trouble is in the amp. I've also done a thorough chop stick test and nothing seems amiss.

Does this sound like maybe a power tube problem or a maybe coupling cap problem, or something else? Any suggestions would really be appreciated. Thanks in advance.

[xtian]

You're experiencing an intermittent short caused by vibration. You can confirm this simply by powering an external speaker, instead of the one in the combo. Do you still have the issue when using an external speaker?

Possibly bad tube (you can usually tap on them to expose issues) but more likely cold or broken solder joint.

[RightLurker]

Thanks, xtian. I hooked the chassis up to a Fender Bassman cabinet that was about five or six feet away from the chassis and got the same problem. I knocked on the tubes and nothing untoward happened. I also retensioned the power tube sockets, which appear to be in fine shape, and gave them a thorough cleaning with Deoxit. A bad or loose ground connection seems likely, but I'll be darned if I can find it. I'll just keep worrying at it until the problem reveals itself.

[martin manning]

A likely place for a sketchy ground is where the ground is made through a pot bushing. You could try jumping the the pots that are grounded that way to a good circuit ground and see if you can find a bad one.

Is there any effect on the problem as the position of the trem intensity pot is moved from full-on to full-off?

[RightLurker]

I checked the pot grounds as Martin suggested and everything seems fine. I've checked all the other grounds in the amp, too - no apparent problems.

I turned the reverb to zero, and the amp still made that wretched buzzing distortion right at the beginning of a loudly played note or chord - mostly lower notes. I then pulled the reverb driver tube (Phillips JAN 12AT7 just bought from a reputable supplier) - and the distortion disappeared. But, the tremolo ticking was again audible. This has me baffled, but I think something has to be up with the reverb circuit. A leaking coupling cap somewhere? The reverb transformer? I just dunno.

[martin manning]

Did you try it with the trem intensity pot at its extremes?

[RightLurker]

Oops. No, Martin, I failed to check it with the intensity pot turned up. I'll try it ASAP, but I have a meeting tonight. Maybe tomorrow night. Thanks.