EVMode wrote: ↑Wed Sep 30, 2020 11:32 pmWould manually dampening with a chopstick work to locate the problem leads? I was poking around inside of the amp today with a loop running and I couldn't find anything that I was able to manually dampen.
I'm just not positive if a chopstick would work for oscillations that high.
You're not trying to solve mechanical vibration. The parasitic oscillation that Martin proposes is caused by signal-bearing wires being too close to each other, or especially, running closely in parallel. A chopstick can be helpful here, because you use it to push the wires around, in an attempt to improve the situation.
Got it, thanks! The amp builder suggested to try separating the shield wires going to V1.
I did notice sometimes when moving the twisted pair going to the reverb mix pot, this occasionally reduced the ring but not consistently/dramatically enough for me to suspect any issues here.
pdf64 wrote: ↑Thu Oct 01, 2020 7:48 am
Crikey, it’s hard to do amp tech stuff in the absence of test kit
Does the issue manifest if the amp is operated with the reverb driver tube in V2 removed (and reverb control turned to minimum)?
With the reverb mix to 0 and/or v2 removed the problem almost goes away completely. The very strange thing is I can use a digital reverb effect pedal to duplicate the ring. I think the reverb is allowing the transient to sustain longer inside of the amp which allows the oscillation to occur. To my ears, I can hear the ring in my first video even when reverb is set to 0. It just does not sustain so you have to really listen for the note separation to hear it is not one of the guitar strings making the ringing.
Stevem wrote: ↑Thu Oct 01, 2020 11:29 am
Read up on how to install your coupling caps the right way, as this must be right first before doing any further trouble shooting if getting this right does not cure your issue on its own.
In general in terms of good signal flow wire layout, grid wires should be very short, as short as possible since they act like antennas and you can even make plate wires longer without harm many times to get to this end!
Moving tone stack caps and resistors off the board and onto the pots is many times a great way to achieve shorter grid wires.
Another thing you can do to locate this oscillating problem is to one at a time solder a long enough lenght of wire to each preamp tubes plate that can then be rapped around that gain stages grid wire a 2 or 3 turns.
If this makes the issue worse, or makes a new issue then that stage is not the culprit, if it makes it better then you have atleast narrowed it down to wiring / parts layout issues in and around that gain stage.
Be sure to insulate then bare end of that wire you solder to the plate(s) for this test.
Hmmm, do you see coupling caps which are installed wrong? I did not build this amp as I am not an amp builder. I purchased this amp.
In this tech section you posted in scroll down more from your post here and read " cap foil tester"
Most times with new manufacture signal caps the down stream or output side of the cap is to the right side of the lettering when its facing you, but it pays to confirm this!
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For what it’s worth, I had a similar problem with transient ringing while playing on an 18 Watt Marshall build and it disappeared when I replaced my V1A cathode bypass capacitor. Certainly a shot in the dark...
Read about this "enhance cap" and if it seems like a reasonable consideration, you can do this experiment very easily with a 220p cap and two insulated alligator clipped wires.
EVMode wrote: ↑Thu Oct 01, 2020 2:16 pm
With the reverb mix to 0 and/or v2 removed the problem almost goes away completely. The very strange thing is I can use a digital reverb effect pedal to duplicate the ring. I think the reverb is allowing the transient to sustain longer inside of the amp which allows the oscillation to occur. To my ears, I can hear the ring in my first video even when reverb is set to 0. It just does not sustain so you have to really listen for the note separation to hear it is not one of the guitar strings making the ringing.
this is telling you that the problem can be in the reverb tube. i see that reverb recovery tube is without grid stopper (i don't know why fender didn't install this there) so it is easy to put 33k 1/2W resistor in grid circuit od 12AX7 reverb recovery tube.. this can fix this problem
