Preamp squeaks - basic understanding

[txstrat]

Hi folks,

I'm still working on my new amp and added a distortion channel to it. It's based on the Deluxe Reverb II schematic. Channel 1 (clean) is fine so far. But when I turn up the amps gain pot in channel 2 it starts to squeak (starts at around 10 on the dial) in a very high pitch that increases with turning up the pot. When I move the wires leading to the pot the squeaking changes but never disappears until I turn down the pot again.
For my basic understanding. Which wires should preferably not to be crossed?

The relais' are for channel switching (posted another question bout this in the forum "Theory and Design"). I haven't put in the relais' yet and it already makes this noise.

While checking the schematic I stumbled ober a 500pF cap between the anode and the cathode on the second tube stage that I haven't installed yet. I suppose this one could be stopping the oscillating and prevent the tube stage from squeaking. Am I right?

Any other suggestions about the layout?
The layout pic is not 100% accurate as I changed some things here and there over the building process.

thanks

Matt

[cbarrow7625]

Are the wires going to and from the gain pot shielded? If not, start with that. It can work wonders. The 500pF cap from anode to cathode should help as well but that is not my preferred "fix" for this sort of issue.

Be careful how your cathode resistors / wires are routed with respect to your other grid & plate components. Something a lot of people overlook on higher gain channels is th routing of the cathodes. If you get a cathode reisitor next to an in-polarity anode resistor you can get positive feedback coupling between the two. Same thing with your grid to anode routing (this is more obvious and we tend to pay attention to it).

Just a couple of thoughts. Let us know what you find.

Chris

[txstrat]

I thought of using shielded wires too, but wanted to ask for other opinions first since it's a lot of more work to swap the cables than putting in a single cap. Probably it's best to do both. I'll let you know what happens.

[cbarrow7625]

If you want to try shielding just one of the wires, shield the return wire first (the one connected to the wiper going to the following grid).

It's really best (for tone) to kill the oscillation at the source (which is just signal routing a lot of times) rather than kill it by adding capacitors everywhere to just netralize it IMHO. It is more time consuming but hopefully you will end up with a more "open" tone rather than a constricted tone missing in many high harmonic overtones from the over-use of caps.

[txstrat]

I added the 500pF to the circuit just to complete the schematic but that brought no improvement.
I will now go to buy more shielded wire (I ran out of it) but there are only one or two stores that have it in stock in this area. Gotta go there when I find the time.
Meanwhile I'm gonna check the routing again and maybe place the wires in a different/better way.
Will let you know what happens.

[txstrat]

Well, it was the routing.
I looked at the schematic layout carefully and found that the gain pot was originally connected with shielded wire. I put that in and it got better (no loss of treble), but still the squeak was there, when I turned up the pots a little more. When I moved the wires it changed to the better again. Finally I found the right position for the wires. And the amp sounds amazing. Although, I still have to put in the relays. Will see what this might cause.
Thank you all for your help. (so far)

Matt