Static on Notes with Reverb

[jhaas]

Looking for some suggestions on what might be causing an issue I'm having with a new build.

I'm using a standard Fender style reverb circuit, between an active parallel FX Loop and a LTPI.

The issue is fizz or static on the signal, more noticeable at lower volumes. The problem goes away if I unplug the Send side of the reverb cable from the amp. If I unplug the return side of the reverb cable with the send side still connected, the problem remains - static on the notes.

I've swapped in a different reverb driver AT7, different pan and different cable, none of which has made a difference.

The amp is on the bench, so I've moved to pan as far away from the chassis as the 3 foot cable will allow, no difference.

I've attached a partial schematic circling in red, the connection I disconnect that makes the fizz go away.

Anyone ever run into this before?

-John

[Milkmansound]

Looks good on paper

Fender style reverb can be a real pain to lay out properly. Even what fender did only worked quietly half of the time.

My guess is that there's grounding or oscillation happening here and you'll need to refine your layout until it's silent

[xtian]

I had some similar trouble with my 18-watt build, using Hoffman's Stout Reverb plans. Same reverb circuit. I had to reduce gain in the preceding stages. I think I ended up with AT7 in V1 and AU7 as reverb driver. Although, if I remember, I was experiencing clipping or cutoff distortion when hitting the input hard (the distortion was coming back in from the reverb return).

Good luck!

[jhaas]

Thanks for the input, guys, it's helpful.

This is a very complex build, so I'm building/debugging in stages. The preamp isn't wired yet, "just" the PS, poweramp, fx loop reverb and trem.

For testing I'm feeding the effects return from a 18W Lite type preamp in another amp. I'll try the AU7 reverb driver and see what difference that makes. There is a long wire run to the reverb pot, so I'll try hard wiring a voltage divider on the boards so those wires are out of the circuit see if that helps.

Thanks again,

John

[pdf64]

I suggest adding grid stoppers to every grid, even just 10k for a 12AX7 and 2k2 for a 12AT7 can push the effect of inter wire capacitences out of the trouble zone.
Pete

[Gaz]

Ok, Pete's answer is correct is some cases, and adding a grid stop is definitely a welcome addition to the reverb driver, especially when it is being overdriven. I actually always add one from reading some of his old posts on the topic!

But since you said it was at low volume, I think it may be a lead dress/oscillation issue. First of all, do you mean low master volume setting, or low gain setting?

My suspicion is that it is the dress of the wire connecting the parallel anodes of the plate on the driver. Do you have it simply going across the socket? If so, Wrong. Needs to be dressed just like Fender did - a big loop around the socket away from the other wires, especially heaters.

I chased this problem forever in a Twin build, and read every drop on the subject online. Tore apart the reverb section many times. I could see on a scope where it was happening, but stubbornly ignored lead dress, since i had never an issue with it, even on 5 gain stage high gain stuff.

I really hope this solves it for you! Make that pin 1 to pin 6 wire long, and play with the dressing of it to see if it affects the static-y crud your're getting on top of the note.

[jhaas]

Thanks again guys, I was able to fix the problem by reducing the gain of the driver. I didn't swap it for an AU7, but instead removed the cathode bypass cap. This alone was enough to eliminate the fizz, but I did then add the grid stopper for good measure. I agree that is a good practice.

With Rk removed, the reverb sounds good and there is more on tap than I'll ever need.

Gaz, I am connecting the plates the "wrong" way, directly across the socket. I've never done it any other way, but this is a good nugget of info to store away for future reference, thanks!

-John

[Milkmansound]

you should think about moving that wire between pins 6 and 1 away from the top of the socket. This will save a big pop when you flip the amp off standby, and get rid of the parasitic stuff. On every Fender layout they specifically show it going around the long way - this is why!

[Gaz]

Milkman's right, and the "fix" you found is really just a hack solution (which may be perfectly fine in your case). Removing the cap simply reduces gain to the point where oscillation can't be triggered. It worked for me as well, but I wanted a stronger reverb. Adding a Dwell or divider before the driver, or using a lower gain tube will get the same result, but it's not actually solving the root problem, which is lead dress.

[jhaas]

I hear ya, I hear ya. I agree, you guys are right. I rerouted the wire as per the Fender layout as suggested. I'm in the middle of wiring up the preamp, so it's not in a state I can power it up at the moment.

thanks again for your input.

[Gaz]

Cool, haha, I just wanna see if it works

[jhaas]

Cool, haha, I just wanna see if it works

me too! When I do power it back up, I'll put Rk back in just to see if the lead dress was the root cause.