Help With Squeal/Crosstalk...

[EETStudent]

I'm in the process of debugging my custom build, and Ive run into a few unexpected problems. Prior to starting this build, I did a TON of planning. I gathered information based off my previous experience, coupled with the resources found on the internet. I drew up a complete layout, with the emphasis on a quiet grounding scheme and clean lead dress. I have built quiet amps before that were far messier with no thought-out schemes. With this build, Im getting some annoying squeal/crosstalk. The squeal is not in the output stage, such as reversed transformer leads and the 7591 grid resistors are tied right at the pins as usual. The preamp is a simple two-stage design with basic tone controls in between, followed by a long tailed phase inverter. As I gradually increase the volume, the squeal becomes introduced leading me to believe that its something before the volume control. Also, if I grip the chassis while the squeal is present, it goes away. In an attempt to isolate the problem, I currently have the output of the tone controls going directly to the phase inverter. Im concerned that there may be an issue with the input jacks and/or grounding. I have always used Switchcraft jacks, but I went with the plastic-nosed Cliff Jacks for better isolation. I used the layout provided by triode electronics, in which the Hi and Lo jacks are reversed for a simpler hookup, plus my faceplate has the Hi and Lo inputs oriented this way (do a google search and you will see the reversed jacks). They seem to function properly, and I have the input grid resistors tied right at the socket pins, and Im using shielded cable for both inputs.

In addition, Im getting some crosstalk. When I had both gain stages connected, I disconnected the output to the PI to try and isolate some noise issues. In doing so, I noticed I was still able to get some low level signal to pass through. The low signal was controllable with the volume control. Im baffled by this because I am 100% confident in the lead dress being clean. Im also showing infinite resistance between the terminals on the strip. I am wondering if whatever is causing the crosstalk could also blamed for the squeal at the input. Maybe a grounding issue? Where do I begin to look?

[Enzo]

Hook it up to a dummy load instead of a speaker, scope the output. Still squeal? Turn it up as far as possible without squealing, now tap each tube. Are any microphonic?

When it squeals, do ANY of the controls affect the frequency of the squeal?

Scope the B+ through the small tube circuits, is ther any of the squeal signal riding on the B+ supply?

How many preamp triode stages share the same B+ filter node?

[Leo_Gnardo]

What Enzo said, plus a couple other things to consider.

If squeal goes away with amp driving a load resistor you may have to try different feedback loop values R & C to make the amp stable into a speaker.

Often there's a bit of crosstalk WITHIN a dual triode, 12AX7 f'rinstance, and that can play hob with isolating clean/OD channels. You might have to avoid assigning the same tube to handle both clean & OD signals.

You could try tacking an HF rolloff cap onto the plate resistors, one after another, to control squeal in super hi gain cascaded stages. 1000 pF will sure-fire quench just about any squeal, and when you've found the problem zone, try smaller caps until the squeal comes back, then back up a step & apply that value cap. Might have to do this in more than one stage of cascaded preamp triodes. Good luck!