Strange Noise on bass notes with Ceriatone Express

[Kborg]

I really love everything about this amp! I put some KT77s in it and 12AT7s in V1 and PI and got the response and tone to my taste but I have 1 problem??

Only when hitting bass notes, most noticeable with the neck pickup, I get a strange sort of raspy noise that sounds like a speaker giving out or being over powered. It isn't the speaker though because I get it with every cab I have tried, including 4 x 12s rated for 120 watts.

Does this describe something that any of you can direct me to a cure on?

I'm so close to perfection for this amp. I hope this makes sense and someone can help.

[rfgordon]

Without seeing a good gut shot of your amp, it's hard to say for sure, but your problem sounds (hey, a pun!) like the problem I had with this guy's 66 Pro Reverb. It turned out to be parasitic oscillation. This is when the physical layout creates a phantom circuit that isn't on the schematic, and one part of the signal path induces signal onto another part.

The cures involve proper lead dress and shielding. Did you:
1. Shield the jack-to grid wire?
2. make the runs from the board to the vol/tone pots and from board to tube sockets as short as possible?
3. If you have a bright cap, try rotating it 90 degrees in one direction or another.
4. Keep plate wires away from grid wires, if possible. If they need to cross, make the crossing 90 degrees, with grids as far away from plate wires.
5. Make sure OT output wires are as far away from grid wires as possible.
6. Check the lead dress for your NFB/presence wire.

Then again, it could be tube misbehavior.

[Jackie Treehorn]

I would try a grid resistor on v2 (second gain stage). You could try something large, like 100k and see if it solves the problem. If it does, you could go lower until you find the value that is just enough.

[bmcmusic]

I had a similar problem on another build and it turned out that the OT transformer leads to the tubes had been reversed. The low F# F and E notes would sound very strange. The effect would disappear when I removed the negative feedback. If that is the case try reversing the output leads from the OT to the power tubes and reconnect the neg feedback. If it squeals uncontrollably that wasn't the problem.

[Allynmey]

I would try a grid resistor on v2 (second gain stage). You could try something large, like 100k and see if it solves the problem. If it does, you could go lower until you find the value that is just enough.

Ken used 820 ohms there on a bunch of amps.

[Phil_S]

What brand of output tubes?

Is this on a particular note, like maybe the A?