Really need help with Twin build

[Gaz]

After being a Marshall guy I finally decided to do a BF Twin Reverb, but reverb channel only, and no vibrato. My only additions was a pre-PI master. It's very much like schematic attached below.

I wired it up without the reverb and everything sounded grand, but when I wired up the reverb and cranked it up I noticed a torn speaker static-y ripping fuzzy sound. It's with the note then fizzles off. Again, it only happens with the gain up.

After tearing the reverb section apart over the past few nights, I think I've been able to isolate the noise to the driver stage, including the reverb tranny. The noise will still occur with the tank unplugged, but ceases with the 12AT7 driver tube pulled. It will also stop when I llift the 500p cap going into the driver.

I have tried replacing the 500p cap, 3.3M/10p mixing combo, driver stage cathode cap/resistor, driver tube and socket, and have reflowed the joints.

The strange part to me is that with tank/recovery disconnected there is not a clear path for the fuzzy sound to get through, but just connecting the driver to the 'dry' circuit will cause it.

I'm really struggling here, so any advice is much appreciated!

Thanks!

[pdf64]

That sounds like oscillation, may be caused by lead dress around the reverb driver & transformer.
Does the treble / volume control settings affect it?
There's a lot of gain / phase shift between the grids of V1 and reverb transformer.
Try a grid stopper on the reverb driver, eg 22k, and use a chopstick to nudge the wires around.
Also try only partially bypassing the reverb driver cathode, eg an additional resistor (~470 ohms) in series with the 25uF cap. That may help with excessive reverb levels when using the master volume.
And/or a dwell pot, replace reverb driver 1M grid leak resistor with a pot.
Pete

[Firestorm]

Where have you got the reverb driver grounded? It ought to be with the screen supply cap. As pdf64 says, lead dress is also very important with this circuit.

[Gaz]

Thanks a lot for the ideas guys. I've been on a wild goose chase all day, and am nearly at my wits end.

Yes, I'm thinking it's some sort of unwanted coupling/feedback/oscialltion as well. The noise is affected by gain, which is why lowering the gain to the driver in any way reduces the noise. I was able to 'cure' some of the noise by adding a tube shield on the reverb driver tube, but turning up the treble control brings it right back.

I did try a grid stop (all different values) on the driver, but it didn't help. The noise does sound like bad blocking distortion, only quieter. I attached a sound sample below.

My layout is a bit different, and I do have all the stages decoupled with their own cap and ground point ala Kevin O'Connor.