Squeal and noise from tone circuit

[vivpio]

Help! I finished the Two Stroke Build last month. It has worked well until the last week. The amp began to squeal and make noise from the volume and tone pots. I replaced the Volume pot and the problem is possibly worse. The ground solders look good on both pots and the connections seem good for the other wires. I replaced the preamp tubes and power tubes just in case. I rechecked solder connections for other components on the board.
Any suggestions? I know that heating the pot case to set the ground creates a lot of heat. I checked on the pots on a digital meter and the readings change smoothly as you rotate the pots.
Mike

[Andy]

Hey Mike,

What do you mean when you say it squeals from the pots? Can you be a bit more specific? Is it squealing at high volumes, hi tone levels? Squealing isn’t typically a pot problem, could be a voltage, gain, or wiring problem more than likely. Can you give us some more information about what it is doing/not doing?

[Robin]

Possibly parasitic oscillation. Do you have any photos of the chassis you can post? Be careful with squeal, it could possible damage the speaker and if its around 10k, your hearing. Try the chop stick method of trouble shooting: Take a chop stick (or similar) and poke about the circuit while the amp is exhibiting the problem. Move the wires a bit and see if it changes the sound. Parasitic oscillation can cause squeals, motor-boating and even no sound at all (even though the amp is cranking).

Be sure to keep the leads as short as possible in the amp.
Sometimes removing only a short length of wire will fix the issue.
Parasitics are not usually a problem with the Two Stroke if built close to DH’s original chassis layout but its possible. Confirm that the cap polarities are correct.

[vivpio]

When I turn the volume knob, the “squeal” (high pitched or fuzzy tones) starts. The sound isn’t like a scratchy, dirty pot. Like I said, the amp worked well for several weeks (quiet) with good tone. The grid wires to the premp are shielded and grounded on one side. I have tried the chopstick method and no bad connections. In fact, I resoldered the connections to rule out cold solders. The leads are pretty much at minimum length to make connection.

As for the pots, when I turn the volume pot, I get the sounds at low or higher volumes. When I turn the volume all the way down, the amp volume goes down until I get to zero. Then the volume comes back on at full volume. Rolling the tone completely to zero cuts the noise. I replaced the volume pot only and it got worse. I resoldered and rechecked and it is better (but not right). The caps are corrects. I am assuming you are referring to the electrolytic caps.

I might try to replace the two pots and resolder it again to rule out. Thanks for your help. If picture shows, the grid wires (input to 2/vol. pot to pin 7) with Lindy Frailin (?) shielded pickup wire. I might try to change these grounds on these wires to grounding the shields to the plate (Gerald Weber’s book mentions this to reduce any parasitic oscillations.)
[attachment=68]20121124_165630.jpg[/attachment]

[Author]

Posts