• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Please,a) cause of high pitched hum, b)DC wires twisted or not

frank2395

Member
2005-04-13 2:35 pm
AUD
thank you,
a)Mono amp, when I touch the input + (while testing) instead of the normal hum I get a high pitched squeal, amp works ok and silent without touching.

WHY SQUEAL AND NOT HUM

(Not related)

b) on internal wiring AC should be twisted, DC not(advised)

IS THIS CORECT RE DC AND WHY OR NO DIFFERANCE
 
a) have you used grid stoppers on the first or so tubes? have you watched the polarity of the NFB wire coming off the OT? try reverse its polarity... I assume that your input is grounded when there isn't anything connected. Btw what amp is? Guitar amp?
Check the layout to avoid that early stages wires doesn't cross later stages...

b) There is no point of twisting DC heaters. But remember that most of the time DC heaters are useless, especially to the output tubes (of course if it isn't directly heated). I would twist them so I can return to AC if I want to.

Hope this helps!
 
That sounds like a classic symptom of RF around....Got a phase controlled washing machine running at the same time ?The squeal and whine will vary with motor speed..

A striplight on the bench is also wicked source of AM.....I don't get this on my amps.....although the new flourescent save bulbs also emit copious amounts of interference.

Make sure than any sensitive tube wiring is well awy from the HV rectifier/choke circuitry. An unearthed heater supply will also create conditions.

richj
 
Unstable circuit, probably wired wrong or something. I'd go over it with a scope and function generator with zobels and grid stoppers in hand.

Wouldn't doubt it if it's oscillating right now, just that touching the input detunes it and either drops the frequency or switches it to another mode, which is in any case audible.

Heater dress isn't going to have much to do with this, that's more of a hum issue. Heaters can carry unwanted signals however, it's best to AC ground them (tieing the DC potenential to an elevated source usually).

Tim