Standalone Preamp-Reverb-Vibrato Project

[alathIN]

I got this thing built but haven't gotten it sorted out yet; looking for help.

I'm attaching copies of the layout and schematic.

I did make a big printout of the layout and went through it with a highlighter and found a few errors, but still didn't solve all the problems. I took it to a tube amp guru here in town and he got some of the issues solved but not all of them.

Problems where I'm stumped:
1) The reverb has a wicked buzz in it. It goes louder and softer with the reverb level, and gets harsher when you turn the reverb tone knob towards bright. I've tried different cables, a different tank, and moving the tank around - no dice. When you hit the reverb-off footswitch it stops.
2) The tremolo depth pot acts fairly normal on very low depth settings, but once you turn the knob past 2 or 3 it gets scratchy and a thump-thump-thump noise in time with the tremolo rate kicks in - almost acts like a bad pot, or something is getting overloaded at higher depth settings.

There are some more minor tweaky things - like the reverb tone knob needs a different cap or different pot, because all settings are too bright. That kind of stuff I can figure out. But the two problems above are beyond me.

[PRR]

Surely socket V7 is not wired as shown?

[alathIN]

Surely socket V7 is not wired as shown?

I thought I had copied V7 directly from the standard Fender AB763 et al schematic and layout.
I'll double check and repost later today.

I hope there is something badly wrong with it - and hopefully that would explain the harsh buzz?

[st]

Are the two grids connected?

[pdf64]

The layout shows the V7 grids connected both the the dwell control and kinda to the tank output circuit?

[st]

Ah in the layout... Good catch!

[alathIN]

I think you're on to something.
I've attached Fender's original circuit to see if I can keep this straight in my head.

The grids of V7 are supposed to hook up to the dwell control (it's just a 1M pot instead of Fender's 1M grid leak resistor)
Nothing else is supposed to connect to the grids of V7, so that orange wire coming off of V7P2 doesn't belong.
In fact, I don't think that orange wire needs to be there at all. That 220k resistor should connect to the reverb pedal and the reverb transformer on one side, and ground on the other - there's no third wire going anywhere. I realize on Fender's circuit he doesn't have the 220pF cap there. That's a mod I believe I got from RobRob that's supposed to brighten the tone of the reverb.
The plates of V7 are supposed to connect to the blue lead of the reverb transformer so I think that's right.

My reverb recovery circuit doesn't look like Fender's because I wanted to use two triodes (amp up the wet signal instead of put a 3meg resistor on the dry signal).
When I look at the schematic and the layout of my reverb recovery, I don't find any errors.

Could it be that this problem is as easy as clipping a wire out that doesn't need to be there?

Thanks for the sharp eyes gentlemen! (apologies if one of y'all eagle eyes is a lady)

[shooter]

orange wire coming off of V7P2 doesn't belong.

I didn't go look for a wire, the fender clip shows BOTH grids tied together, jumper pin 2 to 7, walla

[pdf64]

That 220k//220pF pair should connect to the reverb TANK output (and reverb footswitch pedal etc); nothing to do with the reverb TRANSFORMER circuit.

[sluckey]

I have a concern that will probably come into focus once you have the reverb issues resolved. You have three gain stages in front of the vibrato modulator V6. Fender only had two and the Revibe only has one. I'm thinking your extra gain is gonna overdrive the finicky modulator and give you a harsh/distorted sound. I'm basing that on my own Revibe circuit. Refer to this schematic...

     http://sluckeyamps.com/revibe/revibe.pdf

I built my unit exactly IAW Hoffman's circuit and everything was fine with my Strat. Later I was helping some other member troubleshoot his distorted vibrato sound. We soon found out the modulator was being overdriven but why? Turns out he had high output active pickups in his guitar! So, the solution was to lower the gain of V1B (my circuit) and add a "Level" pot before the modulator.

IMO the vibrato is meant for a clean sound. I realize that with three preamp gain stages you are after a grittier sound. But you may have issues with the modulator. A level pot may be useful to you. Or you may not have issues at all.

Anyhow, this is just a heads up if you have problems with the vibrato.

[alathIN]

I have a concern that will probably come into focus once you have the reverb issues resolved. You have three gain stages in front of the vibrato modulator V6. Fender only had two and the Revibe only has one. I'm thinking your extra gain is gonna overdrive the finicky modulator and give you a harsh/distorted sound. I'm basing that on my own Revibe circuit. Refer to this schematic...

     http://sluckeyamps.com/revibe/revibe.pdf

I built my unit exactly IAW Hoffman's circuit and everything was fine with my Strat. Later I was helping some other member troubleshoot his distorted vibrato sound. We soon found out the modulator was being overdriven but why? Turns out he had high output active pickups in his guitar! So, the solution was to lower the gain of V1B (my circuit) and add a "Level" pot before the modulator.

IMO the vibrato is meant for a clean sound. I realize that with three preamp gain stages you are after a grittier sound. But you may have issues with the modulator. A level pot may be useful to you. Or you may not have issues at all.

Anyhow, this is just a heads up if you have problems with the vibrato.

I think you're right.
Actually, this project's built in preamp is so gainy it doesn't really tolerate much reverb, either.

However, this project can also take its input from the effects loop send of my regular amp  -  a Vox-derived Trinity TC15. When you use the loop input, this thing's preamp gain stage is completely bypassed.
I imagine I will use the TC15's preamp 99% of the time.

The one thing my TC15 doesn't do well is ear melting high gain. The preamp I put in this thing is for those rare occasions when I want that.

[alathIN]

That 220k//220pF pair should connect to the reverb TANK output (and reverb footswitch pedal etc); nothing to do with the reverb TRANSFORMER circuit.

Yes. I think if I take out the spurious wire, the connections you describe are all that will be left.

Now my only wish is that connecting the grid of the reverb driver to the reverb tank return (as I spuriously did) creates a vicious buzz.
Because if I can get rid of the buzz with nothing more involved than a wire cutter, I'm going to call that a win ;-)

I have no earthly idea why I did the layout that way. It's not that way on my schematic.
I do find my eyes get crossed when I am going back and forth from a schematic to a layout.

[shooter]

going back and forth

That will change with experience.  once you have both drawn, re-drawn, double checked by other eyes, THEN all you need is a giant layout as you build.  The schematic comes in handy later when it breaks 

[pdf64]

Another issue may be that the phase shift filters at the signal input of the vib section were designed to be fed from a known, fixed source impedance of ~10k, so we may assume that the filter turnover freqs were calculated using that; whereas you're feeding it from a 1M vol control (itself fed by a high impedance tone stack). So your source impedance will vary from 0 up to ~250k, depending on the MV control setting, and the turnover freq of those phase shift filters will get moved all over the shop.
It may all workout fine, these things have a habit of sounding fine whatever, but it's not ideal.
Some very insightful explanation of these tube vib circuits from Roly here https://www.guitargear.net.au/discussion/index.php?topic=30731.0

[alathIN]

Argh!
The spurious wire is gone. The buzz remains.

I'll go back through comparing the build to the layout to the schematic and see if I can find anything else.

[Williamblake]

I guess you already chopsticked the hell out of those soldering joints?

[shooter]

The buzz remains.

with NO input;
pull V2, gone?
pull V3, gone?
pull V4, gone?

[alathIN]

The buzz remains.

with NO input;
pull V2, gone?
pull V3, gone?
pull V4, gone?

Didn't have a chance to mess with this for a while, now I'm back.

The only valve that you can pull that makes the buzz go away is V8 - reverb recovery.
I've traced the wiring everywhere it goes from that tube and can't find any problems.
Of course two valves (both halves of a 12awhatever) for reverb recovery is different than Fender's original.

[shooter]

the buzz go away is V8

so is the hum there when the reverb is OFF using the FS?

[alathIN]

the buzz go away is V8

so is the hum there when the reverb is OFF using the FS?

Nope.
If you hit the footswitch, the buzz goes away.

(there's still a very low-level hum that's always there, I could live with it and I'm convinced its a separate problem).

But there are only two ways to silence the harsh buzz:
1) turn off the reverb with the footswitch
2) remove V8

[shooter]

If you hit the footswitch, the buzz goes away.

then your issue is probably tank, cable, jack, related, by shorting the grid of V8 no INPUT signal gets to the tube

[Willabe]

If you hit the footswitch, the buzz goes away.

then your issue is probably tank, cable, jack, related, by shorting the grid of V8 no INPUT signal gets to the tube.

Or the grounding of the verb recovery circuit.

[alathIN]

If you hit the footswitch, the buzz goes away.

then your issue is probably tank, cable, jack, related, by shorting the grid of V8 no INPUT signal gets to the tube.

Or the grounding of the verb recovery circuit.

Not a lot of progress here. Stuff I've tried:
1) checking continuity from all the reverb circuits to ground - all should be good
2) different reverb tank
3) different reverb tank wires (shielded RCA)
None of this helped.
There is still a harsh buzz.

One thing I discovered - the buzz goes away when the reverb tone control is set to full dark. This could simply be a filtering effect - the tone knob suppressing the frequency where the buzz is most prominent. Or maybe something wrong with my schematic?
- remember - the reverb recovery stage with two triodes in series and a tone knob is the only circuit on this entire project that isn't copied from someone else's design. So maybe it's just a bad design? I had originally considered using two parallel triodes in the reverb recovery stage (not as much amplification I realize).

There's also a deeper, lower level, less annoying hum that I thought might be a ground loop. This is not affected by reverb or vibrato on/off.
I made a patch cable with an inline 1:1 isolating transformer, which gives about 130 ohms resistance, to see if I could keep this unit's ground from looping with the amplifier's ground. To test the patch cable, I plugged in and used like a guitar cable - it worked, but it was a bit dark - "tone suck" - and so I abandoned that line of experimentation. But now I'm thinking that I have an active buffered effects loop, which I believe the point of is to put a return signal into the phase inverter that's the same impedance as the send signal from the preamp. So maybe it would compensate for the resistance of the isolating transformer? In any case, a robust preamp'd signal might be less susceptible to tone suck than a weak instrument signal. I think I will revive this line of experimentation.

[Williamblake]

Or your tone pod is broken. Considering the low-level hum, have you tried changing reverb tank orientation?

[alathIN]

Or your tone pod is broken. Considering the low-level hum, have you tried changing reverb tank orientation?

Yes. Right now it's just a chassis sitting on the bench, so I can orient the reverb tank just about any way I want. Doesn't help.

Bad pot sure would be an easy fix. Is there a way to test that? Or just replace and see?