new deluxe reverb style reverb oscillation

[imo1]

Hey everyone,

I'm finishing up a deluxe reverb style build for my older kid. I built it mostly stock but followed Martin Mannings's ground and bias mods on his build.
I initially thought the bias circuit was faulty because I was getting way too much current through my power tubes. It turned out that the extra current was on oscillation coming from the reverb circuit. If the reverb pot was at 5+, the amp would break into an oscillation. The reverb is really powerful and becomes almost unusable past about 1-2. Outside of the reverb oscillation, the amp behaves like a deluxe reverb should, and sounds good.

I tried swapping the AT7s as well as the recovery tube. All of the voltages are spot on. I tried a .0022 cap across the 220k. I have redone the ground circuit with no difference. I am grounding the reverb transformer to the RVB output RCA jack ground tab. 220k going from the RVB pedal to this ground point. This is just ground through the connection to the chassis, but seems pretty standard. input RCA gets it's ground through chassis as well.


I went through the amp with chopsticks, looking pretty closely through the reverb circuit, but didn't find anything with the lead dress. The tank is a standard 2 spring Mod tank. reverb transformer is new. The circuit oscillates whether the reverb tank is in circuit or not.

I've run out of my moderate troubleshooting skills, so am hopeful someone can chime in with some troubleshooting tips.

Happy holidays everyone!

Ian

[xtian]

Please post gut shots, Ian.

[martin manning]

The one I built hasn't had any issues such as you describe. Is your reverb driver tube wired exactly as shown in the Fender layout? That's often a cause of oscillation, but your issue seems to be in the recovery amp since its is affected by the reverb level pot.

[imo1]

Thanks ya'll for the quick replies! Here are some shots from the build. Martin, I used your ground design, so you should recognize the layout from that angle.
All the voltages are dead on in that area. The reverb driver is set up as Fender did. What would another option be? Splitting the cathodes?

Anyway, let me know if anything jumps out to you. I really appreciate the help!

IMG_1906.jpeg

IMG_1905.jpeg

IMG_1904.jpeg

IMG_1902.jpeg

[imo1]

The blue grid wires are not close to the plate wires(red). I noticed in the pic that it could look that way

[bepone]

what is the voltage on reverb driver tube cathode?

[bepone]

Btw, driver tube in fender with 12AT7 is failed design , it is by 2 triodes in parallel and if they are not matched (they are never) tube 12AT7 which is build FOR OSCILLATORS, not audio, can oscillate normally.
To avoid oscillating, 12AT7 must have separate cathode and g1 grid circuits, (2 cathode resistors double value from this now, 2 grid stoppers -10-20k) and only anodes can stay in parallel.
Ub+ capacitor supplying reverb recovery must be grouded close to this reverb return jack (here is chassis gnd i see, so somewhere close )

[martin manning]

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.

[imo1]

Thanks for the suggestions. My “shop” currently has a sleeping kid in it(home for Christmas) so it will be a few hours before I’m able to look.

The pre amp tubes are set slightly different than the fender style. There were two rotations possible with the pin holders I got, and this was the closest I could get to the original design.

I don’t have a reverb pedal but I can try grounding the reverb signal to see if that changes, as well as the 220k resistors.

I’ll try rerouting the shared plate wire. I do remember reading that people liked to split out the shared cathode, though regardless of design flaws, that was the norm in almost all fenders, so I’m hoping the plate wire rerouting could maybe solve this before I look to this as a next step. I’m assuming I would use a tag board for the additional grid and cathode resistor if needed.

[martin manning]

I do remember reading that people liked to split out the shared cathode, though regardless of design flaws, that was the norm in almost all fenders...

And on the reissues. I wouldn't''t worry about that.

[imo1]

Btw, driver tube in fender with 12AT7 is failed design , it is by 2 triodes in parallel and if they are not matched (they are never) tube 12AT7 which is build FOR OSCILLATORS, not audio, can oscillate normally.
To avoid oscillating, 12AT7 must have separate cathode and g1 grid circuits, (2 cathode resistors double value from this now, 2 grid stoppers -10-20k) and only anodes can stay in parallel.
Ub+ capacitor supplying reverb recovery must be grouded close to this reverb return jack (here is chassis gnd i see, so somewhere close )

I’m currently following Martin’s ground scheme from a similar style build, which grounds caps “b” and “c” to a tab by the output tubes.

Definitely not arguing that the split cathode/grid would be an improvement, but hoping/assuming that is not my fundamental issue as this is so common in almost all Fenders of the era, and I’ve never encountered it before.

[bepone]

I’m currently following Martin’s ground scheme from a similar style build, which grounds caps “b” and “c” to a tab by the output tubes.

Definitely not arguing that the split cathode/grid would be an improvement, but hoping/assuming that is not my fundamental issue as this is so common in almost all Fenders of the era, and I’ve never encountered it before.

People usually doesnt understand oscillations at all, how they are happening, why, cause, and so on, so you have no any source to read about.. i havent see any person here speaking about that other than me. And why oscillator tube is used in the worst possible way in fender circuit. And how is possible that tube oscillates. Strange

Also, in normal deluxe reverbs you can hear oscillations too, small but they are there.

So, nothing will explode if you change that in 5 min to all separate .
Voltage on cathode resistor in driver tube?

Also oscillograms will help on g1 grids output tubes, to see what is coming there.

[bepone]

The reverb is really powerful and becomes almost unusable past about 1-2.

data about reverb tank model , and reverb transformer type ? smells like doesnt quite fit..

[bepone]

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.

[imo1]

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