new deluxe reverb style reverb oscillation

[imo1]

reverb circuit is small Single ended amplifier, and all theory about single ended construction should be respected, load matching, driving capabilities, current, voltage, bias, how to proper parallel tubes etc etc............. all data mising.

Also secondary of the reverb transformer MUST be grunded, otherwise " is in the air" and working like antenna - oscillations. So some reverb tanks are completely isolated on all jacks, care must be taken also on that point, giving some bets also here.

This was the area I was looking at. The Fender design grounds the reverb at the ground tab of the RCA jack. I ran mine to the output tab where the 220k also grounds. Im definitely seeing a nice ground to chassis on my meter

[martin manning]

Any chance you have the OT primaries reversed? Try disconnecting the NFB lead and see if that stops the oscillation.

[imo1]

My initial hookup had the squealing sound of reversed OT secondaries. Switched them and it went away. I’m assuming with that that they are now properly hooked up. I’m assuming with that being correct there is not need to switch OT. Also, the amp sounds good and like a typical Deluxe, as long as reverb is low. Normal channnel can be turned up to 10 without any oscillation, as can the vibrato channel(with reverb low)

[imo1]

I mean OT primaries— from output tubes

[imo1]

Just to clarify, as my knowledge is decent, but definitely has a glass ceiling—- since the amp is operating normally and sounds clean(outside of classic deluxe low end farting) , I should reasonably assume that the output stage is correct?

[martin manning]

My initial hookup had the squealing sound of reversed OT secondaries. Switched them and it went away. I’m assuming with that that they are now properly hooked up.

Ok. Sometimes there is a surprising tolerance, but you have explored it so that can be ruled out.

...the amp sounds good and like a typical Deluxe, as long as reverb is low. Normal channnel can be turned up to 10 without any oscillation, as can the vibrato channel(with reverb low)

I'm back to suspecting that it's a lead dress issue around the recovery tube.

[imo1]

I went ahead and redressed the plate wires from the reverb driver tube. They follow the fender style around the tube. No difference on oscillations.

I’m trying to see where I could be having a lead dress issue around the makeup. The grid is coming from the RCA Jack. Everything is clean there.
From the plate through the .003, then to high of RVB jack looks solid, and also doesn’t change at all if moved around while oscillating. Wiper back to the 470k is same. Looks clean, doesn’t change while moving around.

Reverb ground lug follows your ground pattern to volume ground lug and then ground a(preamp ground( all grounds are solid and well soldered)

I’m gonna try shorting reverb jack. I’m a bit confused on the 220k shorting. I would think I wouldn’t want to short my bias supply. Am I misunderstanding this?

[imo1]

The driver tube wiring I'm referring to is this:
Screenshot 2023-12-24 at 4.34.43 AM.png
Specifically the loop connecting pins 1 and 6, which should be as shown, and down on the chassis surface.

The circuit oscillates whether the reverb tank is in circuit or not.

If this is true then the oscillation is most likely in the recovery tube. Does the oscillation stop when you use the reverb pedal to turn the reverb off? Does it stop if you ground the PI input at the junction of the two 220k mixing resistors?

Your 9-pin sockets look odd. The pin 1-9 gap seems to be in the wrong place.

Gotcha on this. I was looking at schematic and didn’t read this right

[imo1]

No change with shorting reverb, but oscillation stops if I ground signal at 220k junction

[imo1]

what is the voltage on reverb driver tube cathode?

8.4v

[imo1]

If I ground the signal from the .003 going into the rvb pot oscillation goes away. This is without the tank connected

[imo1]

So, confirmed that it’s definitely somewhere in makeup portion.

[martin manning]

Yes and one or both of those triodes could be involved. I would shorten up the grid lead on pin 2, and run it direct from the reverb return jack along the chassis surface. edit: looking again I guess maybe it isn't that long.

[bepone]

Tanks is Mod® 4AB3C1B

https://www.tubesandmore.com/products/r ... y-2-spring

Driver is 125a-20b. https://www.mojotone.com/Mojotone-Reverb-Driver-125A20B

looks like reverb is grounded.. pls check all the connections to the gnd during all connected (secondary of the reverb transformer to be grounded, reverb chassis to be grounded, reverb recovery to be grounded)

here is one shot what can cure the problem, lets start with that

IMG_1908.jpeg

[bepone]

any grid stopper will be good, from 10-50 kohms