new deluxe reverb style reverb oscillation

[imo1]

I tried the 4.7 resistor with clipping the shared cathode. i feel like i can rule out the at7 being the culprit. At this point, all signs are pointing to the next stage.
i went ahead and rebuilt all my grounds, just to
make sure. I also went into the doghouse and checked all of those connections but everything was solid.

a little perplexed and not sure of a logical next step. i need to look at the schematic some more
and come up with a plan

[bepone]

ok, will be good to see what kinsd of oscillation we have, on oscilloscope on 6V6 g1 pins.

so problem is (after) reverb pot? if the potentiometer on 1 or 2 there is no problem? wires from/to reverb pot?

i have check the wiring on the potentiometers.. usually wires to the pots are separated not bunched, and pressed to the chassis.. maybe this wiring is making positive feedback.

lets put coax wires to the reverb pot , to max and viper, to isolate reverb pot from the rest and see if will be better

[imo1]

Thanks!
I initially had coax to and from the reverb pot, but am using Canare cable and replaced it. I don’t remember if I did this before discovering the oscillation. Will try again.

The oscillation is a low end version. It was around a C, below a low E string, which I think is 32H. I haven’t looked on the scope but can try that. My scope chops are limited. I mostly use it for glorified signal tracing

[imo1]

I am trying to conceptualize what could be causing the issue. I will try the shielded cable, but I think I had this issue before I replaced it with wire. I am going to try subbing in a different reverb tank to see. It seems that the extreme level of reverb is the issue. I am used to Fender reverbs and how you can't get too high on the reverb knob, but this is extreme, which makes me think something is mismatched more than there is an issue with the lead dress. When the reverb oscillates, it is as high or higher than a normal Fender amp with reverb on 10. The standard Fender circuit is close to oscillation as it is with the reverb opened up, so this makes me think that somewhere there is an impedance mismatch sending too much signal. I don't think it's the tubes because I have subbed in different tubes, and the voltages are all spot on. I thought it was the 10p over the 3M3, but I tried a 2M2(without cap) there to see if that helped attenuate the signal, and the oscillation was still there. The Reverb pot measures correctly at 100k.

The grounds are pretty simple. The reverb pot is grounded through vibrato volume to the preamp ground spot, which is also where the cathodes and filter cap for that sections ground. I've re-examined, and re-flowed that section, and everything seems solid.

The only other things I can think of is a component out of spec(I was meticulous in checking on the initial build) or one of the underboard wires somehow picking up some type of inductive hum.

[imo1]

I'm also curious if the pitch leads you to any thoughts. I am assuming at that low of a frequency that I could rule out 60 cycle or 120hz hum.

[bepone]

I am trying to conceptualize what could be causing the issue. I will try the shielded cable, but I think I had this issue before I replaced it with wire. I am going to try subbing in a different reverb tank to see. It seems that the extreme level of reverb is the issue. I am used to Fender reverbs and how you can't get too high on the reverb knob, but this is extreme, which makes me think something is mismatched more than there is an issue with the lead dress. When the reverb oscillates, it is as high or higher than a normal Fender amp with reverb on 10. The standard Fender circuit is close to oscillation as it is with the reverb opened up, so this makes me think that somewhere there is an impedance mismatch sending too much signal.

me too, it is impossible to have extreme reverb... to reduce level and check you can remove bypass capacitor on cathodes on driver and recovery tube, but also you can limit input signal to driver tube , this is easieast way.. or there is a mistake in wiring - feedeng the driver tube.

[imo1]

It is possible that there is a wiring mistake, but it would have to be something greatly increasing the gain going into the drive circuit(or in the makeup circuit), which I don’t see a lot of places for that to happen. If measured components, looked for anything to pop out and am not seeing anything.

I’ve replaced the cable to and from reverb pot with shielded cable. No change

[Stevem]

There is not much filtering in these amps .

Your type of issue is more commonly called motor boating and it’s source most times is a bad filter AND or the grounding layout.

If you have not done so yet I would run two separate ground wires, for the last preamp filter and also the filter before that.

Solder or machine screw bolt them down with a soldered on crimp lug near where the main grounds of the PT are landed.

You should also know that to power supply uncouple gain stages properly as needed many of us who build will not power any more the 2 gain stages off of one filter node, and all too many Fender amps power 4 gain stages off of 1 filter node.

Doing such is a invite to having issues if you do not get the layout right.

[martin manning]

The oscillation is a low end version. It was around a C, below a low E string, which I think is 32H.

The C below the low E on a 6-string guitar is 65 Hz, Not far from line frequency. That's much lower than the typical parasitic oscillation, and higher than the typical putt-putt of power supply motor boating. Find some way to measure the frequency of what you are hearing, or maybe record it and post. Maybe it's 60 Hz hum from the AC line that appears when you put high resistance between the reverb pot wiper and ground?

[imo1]

There is not much filtering in these amps .

Your type of issue is more commonly called motor boating and it’s source most times is a bad filter AND or the grounding layout.

If you have not done so yet I would run two separate ground wires, for the last preamp filter and also the filter before that.

Solder or machine screw bolt them down with a soldered on crimp lug near where the main grounds of the PT are landed.

You should also know that to power supply uncouple gain stages properly as needed many of us who build will not power any more the 2 gain stages off of one filter node, and all too many Fender amps power 4 gain stages off of 1 filter node.

Doing such is a invite to having issues if you do not get the layout right.

I’m definitely aware of the shared nodes on fender power supplies. My goal was to build the amp close to stock, with a couple of small changes. I am really fond of how Deluxes sound and respond. I’m hoping to figure out the core issue before changing too much in the standard Deluxe circuit. I’ve built a number of Dumble style amps, and though I appreciate a lot of his design implementation, I’ve also learned that I like elements that exist in the core Fender designs.
The interesting thing with the “motorboating” is that it is only through the reverb, only when turned up around 6-7, and it is in line with an excessive amount of reverb for what the circuit should be providing.

[imo1]

The oscillation is a low end version. It was around a C, below a low E string, which I think is 32H.

The C below the low E on a 6-string guitar is 65 Hz, Not far from line frequency. That's much lower than the typical parasitic oscillation, and higher than the typical putt-putt of power supply motor boating. Find some way to measure the frequency of what you are hearing, or maybe record it and post. Maybe it's 60 Hz hum from the AC line that appears when you put high resistance between the reverb pot wiper and ground?

You are right. I found the note and then went to the piano and looked up the corresponding frequency, which I misread as 32hz. If this is the case, and it is picking up AC hum in the reverb line, where/how would I look to correct this? Could it be coming through the heater wiring? Not seeing anything in terms of proximity between grids and heater wires. Possibly a leaking filter cap? I didn’t see any noticeable ripple.
Im using relatively small wire for my ground wires(22g). The grounds are following the design you laid out, and are well soldered and connected to chassis. There are a couple of longer runs(particularly the BC-).

[martin manning]

If it is line frequency entering the signal path, it is most likely coming from the heater leads. Measure the frequency of the noise with your meter or scope. If it is exactly 60 Hz, then that is more than likely the source. Perhaps the reverb pot wiper lead is wrapped too closely around the recover tube socket? The last few mm should always approach the sockets on a radial.

[imo1]

If it is line frequency entering the signal path, it is most likely coming from the heater leads. Measure the frequency of the noise with your meter or scope. If it is exactly 60 Hz, then that is more than likely the source. Perhaps the reverb pot wiper lead is wrapped too closely around the recover tube socket? The last few mm should always approach the sockets on a radial.

Just 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?

I'm thinking of re-routing the pin 6 grid wire to the right of the socket, and then pulling that grid wire closer to the chassis once it clears the plate wires coming off the board

[martin manning]

Just 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?

[bepone]

You are right. I found the note and then went to the piano and looked up the corresponding frequency, which I misread as 32hz. If this is the case, and it is picking up AC hum in the reverb line, where/how would I look to correct this? Could it be coming through the heater wiring?

touch the reverb tank with hand and report if the hum increases