Strange high-freq noise, snubbers needed ?

[Ang3lus]

I've recorded the audible high-frequency i sometimes get when the amp is on, it happens with something plugged in/without.

I'm using UF5408 diodes for the circuit insted of the 1n4007, on the original schematic there are snubbers of 10Nf value, i've decided to go without, but is this the cause of the noise ? amp is working perfectly fine otherwise.

the other, low-freq hum is the mechanical noise coming from the PT, which is being amplified by the amp.

P.S the amp is VERY high gain

[strelok]

Sounds more like an oscillation to me, got any pics and/or schematic of the amp?

Edit: interesting to note that the HF oscillation is centered almost exactly on 1khz. Also how does adjusting the controls affect the problem?

[tubeswell]

Got a schematic of your amp handy?

[Ang3lus]

schematic attached

[roberto]

That's the Ubershall, it is well known to be prone to self-oscillations.
It only happens when you plug/unplug something?

[Ang3lus]

I just turned it on to see what happens when i play with the controls, I think it could be due to the flourescent light in my room that caused it, light is turned off now and there are no ossicillations.

weird thing is, the low-freq hum changes its character when i fiddle with the controls, only on the clean channel (bottom channel on the schem)

pulling V1 results in a dramatic reduction of the hum, tapping with a chopstick on R24/R25/C21 gives me a "thud" noise when i tap on them, does that mean they need to be replaced and that they are microphonic ?

could it be a low hum riding over the signal ? and not the mechanical hum of the PT ?

[strelok]

Have you figured out the issue yet? Would be helpful if we could see some pictures of the inside of the amp. Were you changing settings on the knobs in the recording you posted?

[Stevem]

When amps have two channels out of phase with each other like the classic black and silver face Fender amps with Reverb , you can adjust the volume controls on the unused channel to cut back on 120 hz hum, but this is not the case with your amp as both channels are in phase!

In regards to external noise pick up like what you found with the Florescent lamp, if you tape a sheet of Aluminum foil to a scrap of card board big enough to cover the chassis and clip lead the foil to the Chassis then you should find that issue gone!

since your already running the preamp heaters on D.C., what is the level of AC ripple you have on the first gain stage plate load resistor, the tube side of that resistor?

By the way, plastic / nylon shaft pots can cut down on hum levels also since a metal shaft I'd conected right to the gain stage on many pots and acts like a Antenna!
You just run the risk of snapping these off a bunch easier!

[gingertube]

THe B+5 power supply node supplies 3 cascaded gain stages - that makes it VERY prone to oscillation. This not the "normal" parasitic oscillation but a form of motorboating caused by feedback via the power supply node.
Had that happen on a Fender a friend brought to me for a cure. In that unit I added a 470nF/630V polypropylene additional power supply bypass cap to that node mounted right at the 3rd gain stage. That fixed the oscillation by making that power supply node super low impedance but this is treating the symptom and not the disease. Really it need an additional power supply node, you should NEVER have more than 2 cascaded gain stages on any power supply node.
Cheers,
Ian

[Ang3lus]

just an udpate, i did manage to solve the issue.

when i built the amp i used Carbon comp resistors (not NOS) from a banzai on some locations, almost all had drifted way beyond spec, one 100k was 118k, rest were a bit higher than the 10% i'm used to and the biggest culprit was a resistor going Megaohm on me (dead), that was what caused all the problems.

the FX loop wasn't working right as well, turned out a bad RG-174 shielded cable shorting in some angles, replaced that as well.

now all is well.

[Ang3lus]

THe B+5 power supply node supplies 3 cascaded gain stages - that makes it VERY prone to oscillation. This not the "normal" parasitic oscillation but a form of motorboating caused by feedback via the power supply node.
Had that happen on a Fender a friend brought to me for a cure. In that unit I added a 470nF/630V polypropylene additional power supply bypass cap to that node mounted right at the 3rd gain stage. That fixed the oscillation by making that power supply node super low impedance but this is treating the symptom and not the disease. Really it need an additional power supply node, you should NEVER have more than 2 cascaded gain stages on any power supply node.
Cheers,
Ian

that's very interesting Ian, I didn't know that