Ticking in Princeton Reverb Tremolo...

[Bob M.]

I have a friend's early SF Princeton Reverb on my bench. It received a bias pot, a Stokes mod on the power supply and a couple of other minor tweaks. It's a very nice sounding amp, one of the best PRs I've ever heard but its got a ticking in the tremolo. It is apparent all the time but most noticeable with the volume down (or off), Speed on 10 and Intensity on zero (or off). I measured all tremolo components and replaced anything that measured +/- 10% off. I then paid attention to the tremolo lead dress and 'bundled' all wires away from any others. The ticking was a little reduced but still noticeable and not silent enough for recording. I knew that Fender issued a service update on optoisolator two-triode tremolos. Ticking was eliminated by adding a .022 cap in parallel with the 10 meg plate load resistor. Comparing the schematics, the one-triode PR tremolo has a 220K plate load resistor and I experimented with various cap values in parallel with this resistor. With the chassis hanging off the back of the cab, I was able to eliminate the tick but once the chosen cap value was soldered in place and the chassis properly mounted in the cab, the tick returned. Now I'm stuck.

Has anyone got a solution/cure for the trem tick in a Princeton Reverb?

Thanks in adv,

Bob M.

[txstrat]

Found this in another thread here in the forum:

The official Fender solution (Service Bulletin number 9):
******************************************************** *
"The ticking caused by the Vibrato is caused by improper lead dress.

It can almost be "cured" by connecting a .01 mfd 600 volt mylar capacitor
on the 10 meg ohm resistor in the vibrato circuit. this resistor is located
on the parts panel. Remove capacitor across the 10 meg ohm resistor (old
modification) if in place."

(IOW, Solder the cap from the junction of the 10 meg resistor and the
optoisolator to ground.)

"If this does not produce the desired results, then the leads should be
dressed as follows and excessive lengths shortened.

1. Dress the leads to the vibrato speed and intensity controls away from
the tone controls and filter leads.

2. "Bunch" the leads to the components on the parts panel which connect to
the tube socket of the 7025 (12AX7) vibrato tube."
******************************************************** **

[Bruce / Mission Amps]

Found this in another thread here in the forum:

The official Fender solution (Service Bulletin number 9):
******************************************************** .... **

Wrong amp... the PR uses a single triode of a 12AX7 as a low frequency oscillator to superimpose an AC pulse on the bias circuit of the power tubes, thus creating a momentary over bias and under bias condition of the power tubes to "alter" their out put.

Has the amp had fresh power supply, bypass and bias supply caps installed?
Check the idle current of the power tubes as messing with this is what the tremolo circuit is doing.
Also sometimes it helps to replace the 1K 1W resistor in the B+ rail with a new 1K 2W resistor and then use another 1K 2W resistor in place of that wire coming from the eyelet board over to the screen lug of the 6V6 socket.
This isn't really much of a problem with these amps so I hesitate to send you on a wild goose chase.

[Bob M.]

Trem Tick in PR...

Thanks, Bruce, for your reply.

Yes, this amp has a one-triode tremolo, as do all stock Princeton Reverbs. The service bulletin that txstrat posted was what I was referring to in my description and solutions taken concerning this Princeton Reverb tremolo ticking problem.

I paid considerable attention to lead dress and and bundling the tremolo wires away from any other wires. I also replaced all resistors in the filter/power supply section. I used a 1K/5 watt unit between B+ and the screen supply, as that's what I had on hand and I fitted the amp's power tubes with extra heavy duty screen grid protection so as to (hopefully) eliminate this as a future source of trouble. All the electrolytics in the amp were replaced. The bias mAs were set neither high nor low, about in the middle range for 2x6V6 at 395 Vdc plate voltage. The amp sounds absolutely great but there is a small tick that's not going away. I haven't encountered this (unsolvable tick) in a Princeton Reverb before. Of course, I could replace every tremolo part plus the tube and see what happens then!

Bob M.

[MWJB]

I'd be more worried about the 395v plate voltage than the ticking...unless you have zenered down the B+/use a 5Y3?

You can't usuall set the bias anything but "high" in a SF PR, what exactly are the mA's?

Changed the trem triode plate resistor?

[Bob M.]

Trem ticking...

MWJB,
Without taking this thread too far off point, I usually start with the Fender schematic which for model PR AA 1164 lists B+ and plate voltages at 420Vdc and 410Vdc respectively, using the stock 5U4GB rectifier tube. Do you think 'plus 400 volts' is too high for 6V6 tubes? That would make Jim Kelley look like Einstein! I've been running my Deluxe Reverbs at 420 plate voltage for about 30 years with really good success and very long-lived tube service. I think the current on the power tubes of the Princeton Reverb in question was about 26~27 mAs per tube. Although the tremolo increases and decreases the bias on the power tubes momentarily, as long as the bias isn't set too high, I've don't see it as a problem. That's why I added a bias (amount) pot to the circuit: to be able to set the median range and to also monitor bias swing when the tremelo is being used.

But how does all this affect the ticking problem?

[MWJB]

"Do you think 'plus 400 volts' is too high for 6V6 tubes?" No, NOS will take up to 425v (maybe certain brands will take more, check with your tube vendor), EH & JJ will take any voltage that a PR can throw at them (470vdc+ with a RI PR).

You should be monitoring the plate current with the trem switched off, not turned down, but switched off at the footswitch.

Unless you have VERY low wall AC, it's not usually possible to run Princetons & Deluxes at 420vdc AND the bias in the median range. 420v usually requires 30mA+ with the stock rectifier fitted (over 12.6W per plate, dissipation).

395vdc and a plate current of 26-27mA sound ideal for a 6V6 in fixed bias, BUT the low B+ is suspicious to me, as is the low current. At this point I'm not so concerned with the ticking, I'm more concerned with the low volts. Is the power transformer original? Has the amp ever had a cap job (B+ can drop with old filter caps)?

[melvin]

I would make sure the 'improper lead dress' issue was thoroughly addressed... How early of an SF? There are all kinds of lead dress problems the later they get...
For example, it may be that the trem is getting on your B+ supply to the rest of the preamp tubes, in which case I would check the grounds (location and quality) as well as the high-side connections. You might see if it's still there without the trem hooked up to the bias (but still on).
And bad lead dress elsewhere could cause cross-talk...and solving any dress problems with a cap is usually crap. The weirdest thing to me is that you got rid of it with the chassis hanging out the back. Perhaps you could just mount the chassis permanently that way? Tell the customer the amp needs to "breathe". Problem solved!

Another thing, have you checked the voltages with another 5U4 in place? And are you sure it's not a GZ34 model?

[dombrowskim]

Wow! I bias my '69 blackline at 19-20ma and 400VDC plate. It's right about 70% and it sounds great with my Tung Sol RI. 26-27ma seems a bit high-- am I missing something? That's about 90% dissipation on a 12W tube.

"Do you think 'plus 400 volts' is too high for 6V6 tubes?" No, NOS will take up to 425v (maybe certain brands will take more, check with your tube vendor), EH & JJ will take any voltage that a PR can throw at them (470vdc+ with a RI PR).

You should be monitoring the plate current with the trem switched off, not turned down, but switched off at the footswitch.

Unless you have VERY low wall AC, it's not usually possible to run Princetons & Deluxes at 420vdc AND the bias in the median range. 420v usually requires 30mA+ with the stock rectifier fitted (over 12.6W per plate, dissipation).

395vdc and a plate current of 26-27mA sound ideal for a 6V6 in fixed bias, BUT the low B+ is suspicious to me, as is the low current. At this point I'm not so concerned with the ticking, I'm more concerned with the low volts. Is the power transformer original? Has the amp ever had a cap job (B+ can drop with old filter caps)?

[MWJB]

A 6V6GT is a 14W tube, not 12W.

.027*395 = 10.7W...should be fine.

If you like 20mA @ 400v then that's great, it's 57% dissipation, but you bias for tone - it's not purely a maths excersise. Whatever you like the sound of, that isn't damaging the tubes or anything else - is right.