Mosquito, way up there!

[sonofmickel]

Express build.
I am getting a very, very, very high pitched mosquito noise when the volume is above "0".
It doesn't seem to get louder with volume, tone controls do not seem to effect it either.
It's almost like your ears are ringing after a loud concert.

[xtian]

That's parasitic oscillation. Is this a new build?

Most common causes are: 1. Positive feedback (instead of negative) due to polarity of OT primaries. (Though, this is unlikely because you'd hear it regardless of setting of Volume pot.) 2. Lead dress! The Express is on the edge of runaway, and will easily go into oscillation if wire routing isn't carefully done.

Am I telling you stuff you know already?

[sonofmickel]

I'll take all the information I can!
The high pitch is in the dog whistle range, waaay up there, it is not loud, the guitar drowns it out on the first note.
Just cracking the volume knob, I can hear the high pitch mosquito noise come in.

It is a new build, It had a very loud high pitched whistle that turned into the loud "braaaaaap" when I turned UP the presence pot. I unhooked the negative feedback and it went away. I flipped the OT primaries to fix that one.
Curious though, I had a "point to point" Express circuit in there at first, just to see if I liked it(everything soldered directly to the tube sockets). I liked it, so I put a board together only for the preamp-pi. Power tubes, basic power supply kept the same wiring. When the new build was done it oscillated and needed the OT primaries swapped? Am I crazy? Same circuit, It was not oscillating on the "point to point" test build.

Back to the current build: (I'll post a picture of my barbarism in the morning).
I have shielded wire on the first two grids with a 33k on the input and an 820R after the tone stack(I had radio interference, gone now). Shielded wire from the 470pf to the treble pot(not grounded at either end, but did make the amp more quiet from proximity, no longer got noisy when touching that wire). Shielded wire from the 8ohm tap to the 100k feedback resistor(didn't seem to do anything).
I also put 100pf ceramics bypassing the plates on the second stage and the cold clipper stage(I added the 100pf one by one, did not effect the dog whistle but did seem to calm down the upper frequencies, might keep them both, might remove the first one).

Want to add that the transformers are not even close to an original stock Express.
PT is an Antek Toroid AN-3T325, it humms something crazy when the guitar gets too close(toroids blast out EMF past 100ma current draw). Could be the PT inducing the noise? How to test?
OT is a ClassicTone Plexi 50 watt 3.4k(running 8 ohm cabinet into the 4 ohm tap).

I'll pull the preamp tubes tomorrow, see if that can pinpoint it further.

[sonofmickel]

My PT, Antek AN-3T325 power transformer does not appear to have the Purple ground wire that is on the newer AS-3T325.
This is a "high frequency" cancelling tap on the primary side.
Maybe that is the mosquito noise I am hearing(I don't have any mice or bugs in the house!).

AS-3T325.jpg

[sonofmickel]

As promised

20201112_100203.jpg

[xtian]

First thing that jumps out at me is the cathode wire from V1 pin 3 to the tag board (marked in red)--notice that you have it running almost parallel with the cathode and anode wires from V2--that's a good way to cause parasitic oscillation!

Have a look at the Express layout--you'll notice that, where the cathode wire runs to the tag board, he takes pains to make sure it crosses at a 90-degree angle. You can do even more--keep the cathode wire low to the chassis, and have the other wires higher, so nothing touches or is even close. I marked a suggested path in green.

And in general, remember that the three wires for each triode can be safely grouped closely together (pins 1,2,3 and pins 6,7,8), but should avoid getting near signal wires from other parts of the circuit.

Let me know if this lead dress issue affects your oscillation issue!

20201112_100203.jpg

[sonofmickel]

Nope, thanks for looking at the "thing".
I'm thinking something, something Power Transformer. This is the same PT that has such a violent proximity effect, when live guitar gets close to it it sounds like swapped OT primaries, scary ugly loud.

I am hopefully going to a friends house that has scopes and all the engineering cool guy stuff.
Or, is there a good beginner scope?
Recommend a Scope App? I've got a few android phones and a 17" Macbook Pro I wouldn't mind blowing up?

[sonofmickel]

Is this a clue?
With everything on 10, the ONLY place that makes noise/oscillates when touched is the 470pf treble cap and the wires from the 470pf on the bright switch that feed the volume pot.
But,
When I also touch the chassis and then touch those places the amp does not oscillate. I can even touch the grid resistors and they are mostly silent.

[sonofmickel]

Much less noise.
Rewired the bright caps/switch/turned the volume pot. I'll link to the genius here who may have done this first.
Amp sounds great even with all the wrong parts. I ended up with a 100pf plate bypass on the cold clipper and 68k on the voltage divider...for now.

IMG_20201112_201450.jpg

It was Martin Manning in post https://ampgarage.com/forum/viewtopic.p ... ps#p384504

Here is his image:

Bright_Caps.png