Interesting interaction : OT primaries and heater wires

[dave g]

I just finished up my latest express build and was running into some oscillations - the trademark squeal you get when you turn the treble and presence up too far. After some chopsticking, I found that the culprit was one of the OT primary wires. Rather than just move the wire until the amp stopped squealing and then glue it down, I decided to keep poking around and find out what other wire(s) were interacting with the OT lead. It ended up being the heater wire I had going to one lug of the hum balance pot; as I moved that wire closer and further away from the OT lead while leaving the OT lead stationary, I could induce or quell the oscillation.

So it looks like the OT lead was broadcasting noise that was being picked up by the heaters and then reintroduced into the preamp via the 12AX7 filaments, creating an odd sort of positive feedback loop...weird.

Just thought I'd share the experience because it opened my eyes a little. The heaters are one of the last places I would suspect of actually picking up stray noise, but I guess with a wreck you never know...

[drhulsey]

Thanks for sharing

[rooster]

OK, what trannies are you using?

[paulruby]

Is your filament winding ground-referenced by a low-resistance pot? What is the resistance of your hum-balance pot? The only way I can see this being an issue is if the filament winding is weakly ground referenced such that the OT primary lines can yank the whole winding up and down. In other words, the harder the lines are tied to ground (or any solid DC value) the less likely for them to provide a feedback path.

[dave g]

I'm using the Toneslut transformers.

The heaters are tied to a 100 ohm PEC hum balance pot. The wiper of the pot is raised to 25 VDC.

[paulruby]

What circuit exactly is used to create the 25V DC?

[skyboltone]

I'm using the Toneslut transformers.

The heaters are tied to a 100 ohm PEC hum balance pot. The wiper of the pot is raised to 25 VDC.

I use 50 volts derived from the screen supply here, and use a 100uf cap right at the center of the voltage divider. Has worked flawlessly so far.

[dave g]

What circuit exactly is used to create the 25V DC?

470k/68k divider coming off of the B+5. And having just looked at the amp again, I see that I forgot to throw in a filter cap

That'll do it...

[paulruby]

Yep...

[rooster]

OK, glad that was tracked down....