Current driven reverb unit

[Bieworm]

Hey guys..
Built another reverb unit , using the Lamington reverb schematic.  I built this one a few years ago , but it kept humming. I didn't have the knowledge then to solve it completely.. so I used it through the fx loop. Then it was OK... but I don't have it anymore because somebody wanted it in a guitar trade deal.
I built another unit  using my current knowledge. A scrap build...
It's wired up beautifully and it works without hum this time. But there is a problem. It sounds fine for a few seconds and then it loses body and volume, resulting in a nasal bright reverb. And then the mix pot doesn't work like it should.. it keeps giving too much nasal wet signal even at zero on the pot.
Tried a lot of stuff.. but I was wondering it it's due to the wrong tank. I have 2 old Gibbs tanks that read 170 ohm on the in and output. The tank I had in the earlier unit was an 8EB2C1B, which has 800 ohm in and 2575 ohm out. Could this be the culprit? Could this explain what I'm experiencing? I don't have the impression there is something wrong with the circuit.
Oh, and I didn't use an electrolytic cap  at C4 (1uf 350V) but a poly cap 1uf 630V. Is this wrong?
Thanks!!

[tubeswell]

It sounds fine for a few seconds and then it loses body and volume, resulting in a nasal bright reverb. And then the mix pot doesn't work like it should.. it keeps giving too much nasal wet signal even at zero on the pot.

No grid leak path on the reverb recovery stage. Try 220k

[PRR]

> No grid leak path on the reverb recovery stage.

The tank's pickup coil is ample grid-leak.

True, the few-seconds timing does suggest a lost or lame grid-leak.

[tubeswell]

Hmmm, okay then, try a tube swap, cable swap and/or check that C5 or C6 isn't leaking DC (or that you didn't omit C6)

[Williamblake]

Doesn't the Lamington reverb feed back by the way the mixer is designed? Maybe i did it wrong when i tried it but the Lamington mixer sounded like driving the wrong tank when a different mixer worked fine.

[Bieworm]

Hi guys

Thx for the replies. I will try to add a 220k grid ref resistor on the recovery. I think i did that automatically on my previous build. I modified schematic the way I built it (sans 220k grid ref). I added a tone control per 6G15 schematic. The issue has nothing to do with it, because even when it's disconnected the problem remains the same. I think it only loses some gain in a non dramatic way.
You'll notice I added an extra filter cap on the B+ line, due to the hum I had on my former build (advice from Sluckey)
I also added some 10k grid blocking resistors on V1A and V2.
Maybe it's better to add a grid stopper at V1B too?

Talking about the feedback from the mix pot... on the 6G15 there's a .1uf cap in that connection. Is it beneficial I add it here too?

[Bieworm]

The grid leak resistor did the trick.  Thanks.
Have to tune it in for tone some, but I'm getting there👍

[Bieworm]

I think something is off with the sound of it. Probably the wrong tank. I was given some 1122 tanks (1 accutronics and 1 gibbs)
From what I can find on the net they resemble the current 4FB series. The schematic calls for an 8EB2C1B.. I'll order one and see if that improves the sound.

[tubeswell]

Besides the coil impedance, different pan types can have differently-oriented transducer coil laminations. Some orientations work better pan-flat, others pan-sideways.


(Edit: it’s to do with the way the spring mechanically oscillates w.r.t. the EM flux density of the transducer. The coil picks up the spring movement better if the spring is moving ‘along’ the lamination gap -instead of ‘across’ the gap. Different pans have the transducer installed different ways so you can choose to have a vertical mount pan - like in a 6G15, or a horizontal mount pan like a BF/SF amp, with optimal spring movement relative to the transducer)

[Williamblake]

Maybe phase of the reverb return is important in this circuit?

[Bieworm]

Besides the coil impedance, different pan types can have differently-oriented transducer coil laminations. Some orientations work better pan-flat, others pan-sideways.


(Edit: it’s to do with the way the spring mechanically oscillates w.r.t. the EM flux density of the transducer. The coil picks up the spring movement better if the spring is moving ‘along’ the lamination gap -instead of ‘across’ the gap. Different pans have the transducer installed different ways so you can choose to have a vertical mount pan - like in a 6G15, or a horizontal mount pan like a BF/SF amp, with optimal spring movement relative to the transducer)

Thx. I'll have to experiment a bit with that.. since there's not much info on the 1122 tanks available.  Resistance is 170 ohms on both in and output.. but that tells me little. Starting from the fact they are compared to the 4FB series they have the same pitput impedance,  but nearly twice the input impedance I need.
All in all it sounds ok, but I think it's a little hollow, thin from what I expected. I can't be sure until I put an 8EB2C1B on it. Will check 3D orientation too, thanks!!👍

[Bieworm]

What can I expect when I would put a .1uf cap between the cathode of the recovery tube and the mix pot, like the 6G15 has? Now it's just a wire...

[sluckey]

What can I expect when I would put a .1uf cap between the cathode of the recovery tube and the mix pot, like the 6G15 has? Now it's just a wire...

You can expect to mess it up. The 6G15 doesn't have a cap between the recovery tube and the mix pot. The cap you are thinking about on the 6G15 is connected to the cathode of the dry signal ***CATHODE FOLLOWER***. It's needed to block dcv from the mix pot.

[Bieworm]

What can I expect when I would put a .1uf cap between the cathode of the recovery tube and the mix pot, like the 6G15 has? Now it's just a wire...

You can expect to mess it up. The 6G15 doesn't have a cap between the recovery tube and the mix pot. The cap you are thinking about on the 6G15 is connected to the cathode of the dry signal ***CATHODE FOLLOWER***. It's needed to block dcv from the mix pot.

Crappp.. I meant the driver tube cathode. This CF is the dry signal on the Lamington too. But I guess that DC blocking is taken care of after the wiper of the mix pot? Same deal?

[sluckey]

I guess that DC blocking is taken care of after the wiper of the mix pot? Same deal?

Correct. C6 keeps the dcv inside the lame circuit. That .1µF keeps the dcv inside the 6G15 circuit.

[Williamblake]

I have some reverb tanks with 100 to 200 Ohm input dc resistance and as of yet i have tried a lot but never got these driven decently.