vibro champ vibrato circuit in a cathode biased DIY vibrolux reverb

[dinkotom]

I've built a one channel (vibrato/reverb) Vibrolux reverb amp but with cathode biased output. The vibrato circuit is from vibro champ amp, but instead of tapping from the inverter tube cathode, mine is tapped from cathode(s) at the last stage of reverb circuit, (where the 820 ohm resistor is). Is that OK, or should I move it to some earlier stage? And it does thump a lot with intenisty pot turned up...

[PRR]

Can you please draw a picture?

There's multiple Vibroluxes, I don't remember the Vibro Champ having an inverter, nor do I remember where it may have an 820r resistor.... can't picture it.

[sluckey]

The 820Ω is in the Vibrolux, not the champ. It's the shared cathode resistor for the reverb recovery and the reverb mixer/amp.

[dinkotom]

Ah, of course vibro champ doesn't have an inverter, was writing in a hurry  and it's vibrolux only because I use two 10" speakers, let's just call it a bf amp with reverb and vibrato from vibro champ...
my amp now looks as this:


light blue is the connection from vibrato circuit to the cathode with 820 Ohm resistor. So is there any better way of connecting this?

[sluckey]

I would begin by separating the shared cathodes of the reverb recovery and reverb mixer/amp. Use a separate 1500Ω/25µF on each cathode. Then send the blue line up the page to the cathode of the reverb mixer/amp triode ONLY. Then I'd play around with that.

I've never done this before so I'm just guessing. But it seems to me that injecting the trem signal at only one tube might be easier to debug. As shown, you're modulating the wet reverb signal and then also modulating the wet/dry mixed signal.

And it does thump a lot with intenisty pot turned up...

Do you have plenty of intensity on tap before it begins to thump? If so, simply adding a series resistor in the blue line may fix that. Or maybe hanging a 0.1µF cap on the blue line to ground???

[dinkotom]

Tried these, no luck: with separated cathodes, tremolo is more intense and so is the thumping; I don't have enough intensity withuout thumping - series resistor brings down the tump, but tremolo effect also; and .02 to ground didn't help...so I couldn't get a tremolo without the thump.
In the meantime I rewired the tremolo section to that from princeton reverb (with mid lug of intensity pot connected to two 220K resistors):
http://www.thevintagesound.com/ffg/schem/princeton_reverb_aa1164_schem.gif
 - but the tremolo is so weak, barely audible even with intensity pot full up. I tried a LED diode in place of 3.3K resistor, no change. I've read somewhere that for this type of tremolo, output tubes should be biased colder (if I understood that correctly), so instead of 270 Ohm,  I put a 470 Ohm resistor on output cathodes, no change.
any thoughts appreciated, before I dismantle everything, buy a new tranny with negative bias supply and make a usual "roach"  tremolo. 

[sluckey]

Connect a 1N4007 diode (anode to ground) between chassis and the blue line. Again, I'm just experimenting.

[dinkotom]

Will try. Can I also try a 1N4007 across intensity pot, like this:

[sluckey]

Can I also try a 1N4007 across intensity pot, like this:

Check your schematic and you'll see that what I suggested does put the diode across the intensity pot. But, put the anode to ground in your case, since you're modulating the cathode rather than the grid. The idea is to only allow the positive half cycle of the trem signal to reach the cathode. This should allow the trem signal to only reduce the gain of the tube that is being modulated. When you allow the negative half of the trem signal to reach the cathode, the gain will be increased during that negative half cycle and I think that's what is causing the thump. (The gain increase or reduction that I'm talking about occurs at the rate of the trem signal and changes the quiescent gain of the tube depending on the amplitude of the trem signal.)

I have not tried this so it's just an experiment. Easy to do and worth trying. Just use gator clip leads and you'll know in about a minute.

BTW, you don't need to change PTs just to get a negative bias supply if you should decide to go to fixed bias. Just tap into the PT HT for the AC source for the bias supply.

[dinkotom]

Steve thanks for explanation and having patience for a paint-by-numbers guy like me . I just thought, after all those maggies, the fender vibrato will be a breeze