Bright channel squeal oscillation in final testing

[Crwilsn]

I took the plunge and bought a chinese kit. Built to metro specs with exception of a MV in the tone stack (using tube depo diagram) since no instructions are included (knew that going in).

Went through start up and had to trouble shoot a number of issues and have the amp half working now. Normal channel is working great but bright channel is still giving me issues. Since it only effects the bright channel, I am assuming it is somewhere between the jacks and the input to v2a.

Here are the symptoms
- echo like squeal at any volume with guitar plugged in
- scratchy squeal with bright channel volume up with nothing plugged in.
- if I tap v1 socket, I can hear it. Swapped tube with another channel and problem did not follow. Can a socket be microphonic?

My biggest complaint about this kit are the 9 pin sockets. Very small lugs and appear very cheap. Will likely change out someday.

Here is a pick for reference.


I have some known good tubes in another amp and may swap them in.

[Crwilsn]

I changed the bright channel to a shielded cable and the problem went away. Still get is when I bridge the channels but I think it is coming into the normal channel at high volume so I will change that to shielded as well.

[neikeel]

I changed the bright channel to a shielded cable and the problem went away. Still get is when I bridge the channels but I think it is coming into the normal channel at high volume so I will change that to shielded as well.

You are getting parascitic oscillation. Often more a feature of high gain amps or amps with poor lead dress.

Sorry your pic is not coming up to view.

The JTM45 in normal form is not high gain so often it is a case of checking the lead dress (length of wires, which wires cross and in which direction, pushing pre-amp plate wires down against the chassis and slightly liftin the grid wires, make sure the OT primaries and scondaries are not too close and that the NFB wire is away from other wires.
If you have George's build instructions all those points are made. Another thing that can cause problems if using KT66s is that the output tubes benefit from a grid stopper on pin 5. Typically a 4k7 or 5k6 0.5w carbon filmmounted directly on the pin to which you solder the (normally orange and green) grid wires that come from the 220k resistors on the board.

Which kit did you use? The tube depot layout is not really a JTM45, more a JMP50 with split cathode and switchable rectifier. Those long leads across the front panel to the MV running past the input and input grids will be a recipe for noise of the sort you describe without shilelded wire