Pro Reverb with Intensity switch mod

[sunnydaze]

Hello everyone,

I recently acquired a 68 Pro Reverb, Silverface, but has the AA165 circuit. I've done a couple of mods, one is replacing the 50K intensity pot with a switched 50K pot that allows the tremolo to be disconnected from the circuit. The other mod was a fix to a ticking trem from a Fender tech note. Both mods are attached along with the schematic.

The problem, although I'm not really sure this is a problem, is that I can measure a little over 3 VDC at the mixer resistors feeding the PI when the tremolo is disengaged. This voltage dissappears if I switch the tremolo on, which basically puts the circuit back to stock. I am sure the DC is not coming from a leaky capacitor as I tested by lifting the caps feeding the resistors from both the normal and vibrato channel and also lifted the dowline capacitor feeding the PI after the resistors. I also checked the component board itself but I can't measure the DC on the board.

As I said, I'm not sure this is really a problem, as I *think* the DC is isolated from hitting the pots or grids of any tubes. The Vibrato channel sounds great with the tremolo disengaged has more gain and a much richer sound, but that's when I can meaure the DC. Would appreciate any comments. Also, there is a fairly loud thump when switching the tremolo back on, no thump when switching off. Is there a good way to eliminate the thump without changing the performance of the tremolo?

Thanks
Mike

[Structo]

Have you tried putting a small film cap (.01 or so) across the Vibrato footswitch jack to ground?

Or perhaps a 4M7 resistor across it?

[sunnydaze]

Have you tried putting a small film cap (.01 or so) across the Vibrato footswitch jack to ground?

Or perhaps a 4M7 resistor across it?

Hi Tom,

The thump is not from the foot switch. I replaced the 50K intensity pot with a 50K pot with that has a switch. Kind of like the old volume/power pots. The switch is on the back of the pot, at zero its off and when you turn towards 1 it clicks on.

I haven't tried anything yet. The switch is a pretty popular mod and is supposed to have the "thump". I haven't seen anything on getting rid of the thump. A big resistor across the switch seems like it should work, once the switch is on, the circuit will shunt the resistor as if its not there, and 4 megs or so should be big enough to keep signal from leaking out the trem circuit (purpose of the mod). A cap would probably take care of the thump, but would probably leak a little signal. Either way, its not a big deal, I rarely use the tremolo, and when I do, can avoid the thump by putting in standby and then engaging the trem. There is no thump when disengaging the trem.

It's getting late here in Taipei (sunday night now). I'll try a big resistor tomorrow evening after work if I have a chance, and let you know how it goes. Any ideas on why I am seeing the DC voltage when the trem is disengaged?

Thanks
Mike

[JazzGuitarGimp]

A 4M7 resistor in parallel with the switch will likely fix both the thump issue as well as the measured DV voltage at the mixer resistor node. Pretty sure both issues are related.

Cheers,
Lou

[sunnydaze]

Tom & Lou,

Thanks.

I put a 4M7 across the switch and it greatly reduced the thump to tolerable level, but didn't fully eliminate. It would be OK to hit the switch in a live situation. As expected the resistor had no perceptible impact on the tremolo performance.

Didn't have any impact on the DC at the mixer resistor node, though. Still can measure around three or four volts when the tremolo is disengaged and it dissappears with the trem is engaged. I don't really see how switching the tremolo out of the circuit can put DC at the mixer resistor node.

Hope someone can enlighten me.

Thanks
Mike

[JazzGuitarGimp]

When you don't have a GND reference on one side of a cap, it will build up DC (assuming there is DC on the other side). When you have the switch open, the mixer node has IIRC three caps that feed into it, but there is no GND reference there anymore because the switch is open - in other words, that node normally finds its GND reference through the pot. It's possible that 4M7 is just too high to make a difference. One thing I might experiment with is lowering that 4M7 gradually until you can hear a difference, then push it back up just a bit. I would think this will cause the thump to further decrease in magnitude (and likely bring the DC voltage at that node down - because I am pretty sure the thump is caused when the cap is suddenly discharged when its GND reference is restored by closing the switch).

[sunnydaze]

Thanks Lou,

You're memory is correct, the resistor node is fed (or shielded) by three (3) capacitors. One after the node feeding the PI and othe two are preceeding the node and coming from the normal and vibrato channels respectively.

I'll try reducing the resistor value and let you know what I learn.

Mike

[sunnydaze]

Lou,

I settled on a 470K resistor across the switch. Quite a move from the 4M7. Thump is there, but not much. The voltage at the mixer node is down to .2 - .4 VDC. The increased gain and richness from disengaging the tremolo is about the same with the 470K as with nothing, so that's good.

I got this as a chassis only not really knowing how much work it would take to get it going. Main problem was a bad rectifier tube and bad and mismatched filter caps. Someone had put five 100 UF nichicons under the dog house. I replaced the power and bypass caps and ended up replacing the vibrato channel volume pot. That pot may be OK now - I took it apart and cleaned it enough so that it seems to turn pretty smoothly. Initially, I had small amounts of DCV on the component board itself near B+ connections. I removed the under board and sanded both sides, baked in the oven and wrapped in cellophane. When I first removed the underboard a couple of solder blobs came out with the under board. Not sure if the sanding baking wrapping or removing the solder blobs did the trick maybe all helped, but that took care of the DC on the component board.

I'm currently running a 12AU7 as the reverb driver. I like that much better than the 12AT7's I have. Gives a bit more control. The best sounding was a 12AY7. Unfortunately that one died shortly after being put in the circuit. This circuit puts 430 volts on the plates, sadly killed a nice sounding 12AY7.

This one has the blue ajax tubular caps and I'm pleasantly surprised by how good this amp sounds, especially with a strat.

Thanks again.
Mike

[Milkmansound]

https://tubeamparchive.com/viewtopic.ph ... 89ed0ae588

[sunnydaze]

Thanks Milkman,

I read your post when you first put it up. Great information, but my issue had nothing to do with the footswitch. I replaced the standard 50K intensity pot with a 50K switched pot. Allows me to fully switch the tremolo out of the circuit. That's different than than the function performed by the footswitch. The tremolo leaks signal even when the footswitch is off. The switch pot allows you to take the tremolo totally out of the circuit. Problem with this approach is switching it back on made a fairly loud thump. Also, in the switched off position caused a small amount of DC voltage to appear at the mixer resistor node. Placing a 470K resistor in parallel with the switch mitigated the adverse side effects.

Check my original post for a pic of the mod and then change that with the 470K resistor across the switch, and it's a really nice mod.

Mike