Twin Reverb Build background noise

[Rock_Glenn]

I've just finished a twin reverb build and cannot figure out the source of this noise.

It sounds like a rapid square wave tremolo. The guitar sounds great, but always this rapid heartbeat in the background.

The speed doesn't change.

All knobs at zero, it's still there.

Pulling ANY preamp tube does not get rid of it. Including Reverb driver/recovery and tremolo tube. Disconnecting reverb input/output does not help

I rolled all new tubes and it went away for a day. But came back.

I re-routed my preamp grounds, because touching the chassis quieted it down. After rerouting grounds, now even touching the chassis doesn't help.

Any ideas what's going on here?

 

[Snfoilhat]

I've just finished a twin reverb build and cannot figure out the source of this noise.

It sounds like a rapid square wave tremolo. The guitar sounds great, but always this rapid heartbeat in the background.

The speed doesn't change.

All knobs at zero, it's still there.

Pulling ANY preamp tube does not get rid of it. Including Reverb driver/recovery and tremolo tube. Disconnecting reverb input/output does not help

This makes me think of a power supply problem called motorboating

I rolled all new tubes and it went away for a day. But came back.

I re-routed my preamp grounds, because touching the chassis quieted it down. After rerouting grounds, now even touching the chassis doesn't help.

Any ideas what's going on here?

This makes me think of a lead dress problem showing up as a high frequency oscillation. I've worked on an amp where a small change in gain or lead dress would momentarily quiet the amp, but then the noise would grow back to its original level. I believe it's a positive feedback loop being disrupted but then ramping back up. I was ultimately solved with lead dress / removing a ground loop

 

[King Fan]

Good^^^, I was hoping someone smart could analyze your helpful description of the symptoms.. The usual requests at this point are things like complete voltages, detailed pics, and a sound sample. You've got a lot of amp there, so the sound sample might be the easiest, and might add to the kind of good insight you're getting from @Snfoilhat .

 

[Rock_Glenn]

Thank you, I was hoping this was a common enough problem it could be easily identified.

I'll poke around with a chop stick and see what happens. I already moved the cathode ground on the tremolo directly to the star ground. I've shortened a few leads.

Is it worthwhile to shield the signal wire that runs to the tremolo switch on the back of the rate pot? Or is it worth it to shield any of the wires in that area?

Also the rate pot is very near the rectifier, could this cause a problem? I had to orient the pot with the terminals near the diodes cathode or else the switch terminal would short.

 

[YellowBoots]

This is an interesting case and I'm watching with interest. The fact that this thing makes the "rapid square wave tremolo" noise even after pulling every preamp tube (one at a time?) makes me think it's got to be in the power amp. Verify your power supply resistors are good and of correct value. Verify all your power supply caps are grounded properly. Can you post a schematic of your build?

 

[Rock_Glenn]

I've verified power supply caps and resistors. Originally I had the preamp ground separated, but combined them trying to fix this. Now all grounds goes back to star via a wire wrapped around AB & C.

The only weird thing there is if I read ground to BC or D I get a reading of 60uF. As if the caps are all paralleled. The resistors test fine though and are properly placed and valued.

I used cheap Chinese 100uFs for A because I don't stock 100s usually, they could be the problem, but they test fine.

 

[YellowBoots]

I've verified power supply caps and resistors. Originally I had the preamp ground separated, but combined them trying to fix this. Now all grounds goes back to star via a wire wrapped around AB & C.

The only weird thing there is if I read ground to BC or D I get a reading of 60uF. As if the caps are all paralleled. The resistors test fine though and are properly placed and valued.

I used cheap Chinese 100uFs for A because I don't stock 100s usually, they could be the problem, but they test fine.

Hmmm... with standby OFF (play mode) I would think you should get 95uF 110uF (with 2x 100uF). But I could be wrong because I don't really know how capacitance meters work.

It could just be a coincidence.

 

[Rock_Glenn]

The 2x100 are in series and read 50uf as they should. Three 3x20 give me 60, even with the dropping resistors between them. I don't believe I've seen this before.

I re-wired a bunch of leads, shortened the reverb primary red connection. To no avail. I think I need to get those cheap caps out of there. I'll check back after I do that

 

[Mr Ridesglide]

which Twin Reverb schematic are you building to? Does it have a Master Volume? And for sure a choke?

 

[schmee]

Most builds have an issue to solve. It's often something routed to the wrong spot or similar to that. ven though triple checked.

Does it work when played through even though it has the noise?
Did you follow the normal Fender grounding scheme or improvise?

 

[dan40]

This has happened to me when the NFB phase was flipped. You can test this by unsoldering the NFB wire that connects to the output jack. Simply unsolder the end at the jack, and then try the amp again. If this stops the motorboating, you can reattach the NFB wire and then swap the two OT wires that connect to the power tube sockets.

 

[Rock_Glenn]

Built using Weber 6A80 layout. I used a Hammond Choke.

The amp works great, good sounding guitar, working reverb, working tremolo. All knobs work as they should. Just this constant ticking in the background.

I swapped the Ot wires first hoping that was the problem, it was not... That made it much much worse .

 

[sds1]

I swapped the Ot wires first hoping that was the problem, it was not... That made it much much worse .

When you say it made it worse -- you mean this made the same heartbeat noise get louder or?

I get ticking crap on my bench sometimes, not exactly sure which device it's from but something in my electrical environment (modem, etc) is to blame.

 

[Rock_Glenn]

No no, it made the amp squeal very loudly, as is my experience with positive feedback. I quickly changed them back. I can say with confidence they are oriented correctly.

 

[Rock_Glenn]

I've noticed a difference in the Weber layout and Rob Robinette's. On the bias pot Weber has lugs two and three jumpered. Rob does not. Could this be the freaking problem??

 

[Lowerleftcoast]

On the bias pot Weber has lugs two and three jumpered. Rob does not. Could this be the freaking problem?

No.

 

[Lowerleftcoast]

I am beginning to think this may be motor boating due to the cheap filter caps or maybe a bad solder joint.

In the meantime, try clipping in a 100pF here (green on layout below). This will help quell oscillations in this area. A 47pF cap is standard on a 5F6A Bassman. I almost always use a cap here for insurance against ultra high frequency oscillation.

 

[Lowerleftcoast]

Oh and swap the 6L6 around (Hail Mary).

 

[Rock_Glenn]

Thank you, I will try the mica cap tomorrow. Also I ordered a pair of quality 100uf caps.

 

[Lowerleftcoast]

When you change the 100uF reservoir caps, check that the two balance resistors that parallel those caps measure the same, or very close to the same, resistances. Iow, a matched pair of resistors for this location.