imo1 wrote: ↑Tue Dec 26, 2023 8:17 pmJust to clarify, you are talking about the signal from the wiper, through the 470k, coming into pin 7? By "on the radial, do you mean that the grid wire would come in directly to the grid pin, as if on a radius?
Yes, and by on a radial I mean at a right angle to the socket, i.e. not tangent to it. Can you verify the frequency of the noise?
I apologize for my lack of tech skills but using the frequency in my fluke 179, every time I would touch the wiper of reverb and with black probe on ground, it would stop the oscillation. I was able to test frequency at the 220k mixing resistor and it was around 32hz. You can see it jump(and hear) to 37hz when I touch the probe
Fender designs definitely make it more difficult to go through filter nodes, but I will check tomorrow. I was pretty careful with labeling and checking each node, and the voltages sure line up with where they should be.
I talked to my old tech, who suggested swapping primary leads on the reverb driver. I haven’t done that yet either, but will try tomorrow as well
martin manning wrote: ↑Wed Dec 27, 2023 12:47 am
Ok so not line frequency. I think it might be power supply related. Can you put another cap across the "D" filter?
I'll see what I have. I should have something I can tack on there. Assuming something similar? 16U?
martin manning wrote: ↑Wed Dec 27, 2023 12:47 am
Ok so not line frequency. I think it might be power supply related. Can you put another cap across the "D" filter?
I tried a 16u strapped across the D node. No affect on oscillation. Wow! This is a journey..
thanks a lot for all the help gentlemen.
imo1 wrote: ↑Sun Dec 24, 2023 7:00 pm
No change with shorting reverb [pedal center pin to ground], but oscillation stops if I ground signal at 220k junction
imo1 wrote: ↑Sun Dec 24, 2023 7:10 pm
If I ground the signal from the .003 going into the rvb pot oscillation goes away. This is without the tank connected
imo1 wrote: ↑Tue Dec 26, 2023 11:38 pm...every time I would touch the wiper of reverb and with black probe on ground, it would stop the oscillation. I was able to test frequency at the 220k mixing resistor and it was around 32hz. You can see it jump (and hear) to 37hz when I touch the probe.
Post a photo showing full chassis depth front to back including reverb driver and recovery tube sockets and wiring.
So obvious now that I can see it. I had a .033 instead of a .0033 off the plate of the recovery valve. Makes total sense. It was letting way too much reverby low end through and the circuit couldn’t take it.
Thank you all for your very generous help and tips. I really appreciate the time and intention.
Now, back to tidy up all the havoc I wreaked in the last 4 days of trying to figure this out!
Still not sure exactly where the feedback loop is, but closing down the low frequency response out of the first recovery triode has apparently killed the oscillation. Are you keeping the original ground scheme or returning it to the modified one?
Re the 9-pin sockets, Belton Micalex have the gap close to one mounting screw, like the originals used by Fender. The Chinese ceramic ones seem to have it centered between the mounting screws.
So obvious now that I can see it. I had a .033 instead of a .0033 off the plate of the recovery valve. Makes total sense. It was letting way too much reverby low end through and the circuit
i saw that was your cap 0.0033 too big for the value
martin manning wrote: ↑Thu Dec 28, 2023 1:11 pm
Still not sure exactly where the feedback loop is
Fender reverb circuit has some feedback. If there's 20mV signal on the recovery tube grid and the reverb pot is fully turned up then about 1mV gets fed back to the reverb driver tube grid. Even small changes to the circuit can make it oscillate.
martin manning wrote: ↑Thu Dec 28, 2023 1:11 pm
Still not sure exactly where the feedback loop is, but closing down the low frequency response out of the first recovery triode has apparently killed the oscillation. Are you keeping the original ground scheme or returning it to the modified one?
Re the 9-pin sockets, Belton Micalex have the gap close to one mounting screw, like the originals used by Fender. The Chinese ceramic ones seem to have it centered between the mounting screws.
I am using your ground scheme. I bought my sockets from Tube Depot. As this point I’m going to keep them.
I feel like the “loop” was too much low end into a circuit that already, under stock circumstances, pushes the limits of the circuit. The fender blackface circuits with reverb on 10 sound like they are on the verge of feedback. I think my circuit is operating properly. It sounds exactly like any of my blackface Fenders that I have
