Tube FX loop. Questions.

[Mostro]

Hi people, I recently finished my first tube amp, it has a passive FX loop, just a send and return jack between the last preamp gain stage and the phase inverter. As it is now it´s pretty useless, so I tried to make an external JFET-based active loop, but that never worked well (not enough headroom). So I decided to make a tube one, it´s the Dumblelator. I had to adapt it to what I can make, but since I don´t have that much experience with tubes, I have some questions for you.

Here´s my schem:

[IMG644]http://i68.photobucket.com/albums/i5/Mo ... rev-10.jpg[/img]

Questions:

1) I´m gonna power the tube heaters with 12V DC.It should actually be 12.6V, but with a 7812 regulator I´ll get 12V. Is that OK, or will it be bad for the tube?

2) Regarding supply voltages: Using xformers back to back like shown, I calculated that I´ll get, at the 2nd xformer primary (220V side) around 293V. After the rectifier bridge, I should get 293 x 1,414= 414,69V DC minus the diodes drop (all this is theory, haven´t tried it yet). Now, the part of the schem from the standby switch to the A and B points I´m not that sure... The point A is supposed to get 368V, and the point B 256V. I´m not that sure of how to get those values... I guess R1 and R2 could be 22K, and C7 / C8 could be 22uF? Also, are C2/3/4/5 too much filtering? could I do with just two 47 uF caps? That part of the schem is a gray area to me...

3) About the fuse F1, I´m not that sure of the value. Would 1A slo blo be OK? Or less?

4) anything else you see that wouldn´t work?

Thanks for reading! Any help will be useful.

[Andy Le Blanc]

That kind of power supply works pretty well. I'd use tranny's with a larger
current rating, I'd try the heater supply AC without the rectifier or regulator, unless your dead set on using DC.
You wont need the standby. Now a loop isn't supposed to color the signal, I understand the feedback on the recovery,
but the bright switches might be overkill.

[ampdoc1]

I built a Dumbleator into a Dumble/Trainwreck hodge-podge amp 2-3 years ago, on an old Fender chassis. The wiring was only average neat, but the dumbleator was silent with just the standard 6.3VAC tube heaters. The amp has gain to burn, and there is no problem with heater hum.

Your idea is neat, but I don't think you're going to gain anything.

a'doc

[Mostro]

Guys, thanks for the answers.
I fear hum if I power the heaters with AC... if I use DC I guarantee (at least I think so!) that I won´t get hum from there. And it´s only 3 more parts... besides, if the power line goes down in voltage (It´s nominally 220V but it never reaches quite that here in Argentina) I´ll have my 12V stable despite variations in line voltage. Maybe I´m just too cautious...