Interesting interaction : OT primaries and heater wires
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Interesting interaction : OT primaries and heater wires
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...
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...
Re: Interesting interaction : OT primaries and heater wires
Thanks for sharing
Tim
In case the NSA is listening, KMA!
In case the NSA is listening, KMA!
Re: Interesting interaction : OT primaries and heater wires
OK, what trannies are you using?
Most people stall out when fixing a mistake that they've made. Why?
Re: Interesting interaction : OT primaries and heater wires
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.
Re: Interesting interaction : OT primaries and heater wires
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.
The heaters are tied to a 100 ohm PEC hum balance pot. The wiper of the pot is raised to 25 VDC.
Re: Interesting interaction : OT primaries and heater wires
What circuit exactly is used to create the 25V DC?
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Re: Interesting interaction : OT primaries and heater wires
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 wrote: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.
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Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
Re: Interesting interaction : OT primaries and heater wires
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 cappaulruby wrote:What circuit exactly is used to create the 25V DC?
That'll do it...
Re: Interesting interaction : OT primaries and heater wires
OK, glad that was tracked down....
Most people stall out when fixing a mistake that they've made. Why?