Tremolo pedal switch- THUMP!

[Sea Chief]

Hi- Ive successfully p-to-p rebuilt my 65' RI Deluxe Reverb, new heyboer PT/ new TAD gz34/ sprague & OD caps/ cloth wire blah blah. All major tested at a pro builder's.. tho the thump didnt show up (maybe ftsw pedal just not pressed off during tests).

All good & pedal works, but for a very loud nasty trem pedal thump. Pedal itself (a effects/ rev peavey Bandit65 pedal) worked fine with the RI before rebuilt. Ive kept the RI's stereo ring/tip/shield Footswitch jack to save alot of hassle & hopefully wired it right (jack in place of the 2 rca's Trem pedal/ Rev pedal on the Ab763).

Ive added 2 rca's for Rev in & out like AB763. So on back: Stereo jack, rca (rev OUT), rca (rev IN).

I used fender AB763 layout, simply reworking the orig LHS rca (Trem pedal) tip's single wire from the board / now to my stereo jack tip. Then linked my new rev OUT rca's tip (also of course to V4 pin 2) to the stereo jack's ring, plus a 220k from here to gnd.

Pro builder chap didnt see any probs, but we prob didnt click the pedal I assume or would've shown up.

Any ideas? thx SC.

[R.G.]

Thumps are always a sudden shift in DC conditions. Something about your rebuild is not wired exactly right and is causing a sudden DC shift when it's activated.

I'm not familiar enough with the schematics of all the Fender amps to say just what off hand, but if you want to post the schem or a link to it I'll go look for likely suspects.

[Sea Chief]

Hi R.G. I've used the fender orig AB763 for this small area of the circuit right at btm (TAD layout for the build/ same same).

I dont know how to add a pdf, but googling AB763 layout > this 1st entry down shows a clone of the orig fender layout..

[PDF]"DELUXE REVERB-AMP AB763" LAYOUT - Mojotone.com

[Sea Chief]

Vintage Fender Schematics & Layout Diagrams | Vintage Fender Amp Repair

This is a Super Reverb AB763 (mines a Dlx Reverb, same circuit AB763) info direct link: the 4 rca's are identical.

Just to reiiterate: I have one stereo 1/4" jack socket in place of the 1st and 2nd rca's along L-to-R.

[R.G.]

Get out your meter and measure the terminals on the new stereo jack socket. All of the terminals should read 0.000000V in all conditions of the switch.

[Sea Chief]

Good idea R.G. I'll do this in due course/ when Ive time & report back.

My pro builder friend says its 'lead dress usually' & little that can be done. Urgh!

thanks SC

[Sea Chief]

Just had a thought.. is there anything detrimental having the tremolo swtiched permenantly on? IE do the switch ON before amp turn on, then just turn INT to min? is that bad for the circuit, affect the tone generally, ad/or wear out all the trem circuit caps or something?

[Dave H]

Have you tried replacing the trem oscillator 12AX7?

[R.G.]

Good idea R.G. I'll do this in due course/ when Ive time & report back.

My pro builder friend says its 'lead dress usually' & little that can be done. Urgh!

Just to be sure we're talking about the same thing - when you say "thump" is this (a) a single "thump" when you switch it in/out, or (b) a continuous thumping in time with the speed control? I realized that my internal definition of "thump" may be too restrictive. I think of it as (a), not (b).

Single thumps are from sudden DC shifts as a side effect of the switching. Continuous thumping is from the LFO bleeding into the signal path.

It is unusual for a continuous thumping beat to couple into a circuit by lead layout problems. Lead layout can cause very small capacitance between wires as you move them close together. However, only the high frequencies can couple through, so it's more of a ticking than a thump.

To get the low frequencies associated with a "thump" sound, you nearly always have to have direct connection through wires. This can be through the grounding or power supply. Or it can be a bypass cap that's not doing its job.

There is no problem with the tremo switched on permanently. In fact, it always is in that circuit. What the footswitch does is to disable it oscillating, so it stays "on" but runs at zero frequency. When there is no footswitch plugged in, it runs all the time, but the intensity is just dialed down so it can't affect the signal.

So - which kinda thump you got??

[Sea Chief]

Have you tried replacing the trem oscillator 12AX7?

Hi Dave H, happy new year. No must say i didnt think of this- the tubes are all pretty low hours. Will do.

[Sea Chief]

My thump is just the pedal switch, just the LHS tremolo. On and off. No nasty motorboating or any oscillations- tremolo is good once sudden thump is done with.

Great: if the trem on is permenantly w'out issue/ fine.. then thats the best for me/ Im happy just to dial it in on the amp when I feel all 70's and groovy.

Tbh I think, and as my pro builder aluded to (I do some work for him but dont hassle him too much for answers you see he's damned busy- hence asking on here) it might be a very tricky one to actually resolve. So Im fine/ sorted.

Thanks for your help R.G.

[R.G.]

Do go and try the metering and see if you find something before you give up. It may be simple once you find anything that corresponds to DC changing with the switch changing.

[loudthud]

Get out your meter and measure the terminals on the new stereo jack socket. All of the terminals should read 0.000000V in all conditions of the switch.

The version of the AB763 schematic I'm looking at applies the full negative bias Voltage to the Tremolo Footswitch jack through a 2.2Meg resistor. Other versions connect a lower Voltage there, usually from the oscillator cathode. This Voltage acts like a kick starter to the Tremolo oscillator so that it will start immediately when you close the footswitch. Is this wired correctly?

If the 2.2 Meg resistor (double check the value) is missing, the footswitch will kill the bias Voltage. (Does blowing the fuse count as a thump?) With no footswitch installed you should see the bias Voltage on the footswitch jack terminal. Your meter might not give an accurate reading but you should get something between -25V to -50V. Is this negative Voltage coupling into the Reverb footswitch? Turn the Reverb control all the way down to test.

[R.G.]

Dang, you're right. I didn't trace that line back to the bias supply.

And that raises another issue - the schemo I'm looking at does not show the "ring" of the tremo pedal as being grounded, although I think it has to be, otherwise the tremo tube would never oscillate.

A likely source of the thump would then be the 2.2M being either missing or the wrong value, substantially lower, and whopping the output stage bias when the tremo terminal is grounded.

See, I told you I don't know much about all the older Fender schemos. I have to go learn them with each question.