Noisy reverb circuit

[Toxophilite]

On my Harmonilux reverb (AA964 vibrolux with harmonic vibrato circuit)
My reverb is really noisy when you crank it BUZZZZZZZZZZZZZZZZZZ
The reverb works
It's tolerable/usable (but still noisy) at the usual setting 0-3 but past that into effect/surf territory it gets pretty nasty
Any idears?
I tried unplugging the 12AT7 driver and it's still the same so maybe on the recovery side?
Poked around nothing seems to be loose or badly soldered
Swapped out tubes. no difference

[John]

2 things to try, make sure your reverb transformer wires aren't beside your grid wires. And try tacking  a 500pf silver mica between the plate and cathode of the driver and recovery valves.

[PRR]

The drive side has big signal and never buzzes.

The recovery side has very-very low signal (smaller than guitar) so really wants to buzz.

Wire routing. Grounding. Shielding. Treat the signal from tank to grid like baby-formula, susceptible to contamination.

We would add B+ filtering but your recovery amp gets the same B+ as the guitar preamps so can't be real nasty.

[Toxophilite]

excellent
Swapped in shielded Coax for the grids and the line to the foot switch, moved things around a bit and it's quieter now, could still be even quieter but a fair bit better
is it worthwhile to use coax for line running to  the reverb 100KL pot too? easy enough to do

[Willabe]

I would say yes because after the coupling cap that wire is now a grid wire. Run both wires to/from the verb pot as shielded. 

[Toxophilite]

THanks I used shielded coaxial cables for all the signal lines to and from the reverb recovery circuit (except for the 12AX7 plates) and it's a fair bit quieter. It's only significantly noisy if it's cranked


Out of curiousity is it okay to use shielded coaxial cable on plate wires?...might be a dumb question but they have a lot of juice running through them


THanks

[PRR]

Don't use coax on POWER plate wires. Can't stand the voltage. (It might, but you better look-up the specs for the exact stuff you have.)

On preamp plates... it may be safe but is almost never warranted.

[Toxophilite]

Thanks . That's kind of what I thought. I was just thinking preamp plates , definitely not power amp plates. but even for preamps it seemed wrong. Never even tried it.

[Willabe]

I was just thinking preamp plates.........

Still the same, most coax is not able/rated to handle the DCV.

And  it's not needed, because the plate wire has higher DCV, higher ACV signal and it's impedance is way less more immune to picking up stray noise then a grid wire. 

 

[Toxophilite]

Excellent! always good to know why
Thanks

[PRR]

> most coax is not able/rated to handle the DCV.

Skinny RG174 is rated 300V RMS (420V peak). This will cover the plate voltage of most small bottles.

(This is incidental to the main goal of low dielectric loss.)

Agree that it rarely makes sense.

[Fresh_Start]

Instead of shielding plate leads, try running them flat on the chassis "floor" and away from any small signal wires.

By the fourth Blackface reverb project, I used shielded wire to and from Reverb pot. Double bagged reverb tank. Tried different tubes for the recovery triode until I found a really quiet one. 

Chip