gl
P

also don’t confuse DB gain with ‘amplification factor’

they are not the same thing.

good luck

c.

I can’t help you with this man sorry. I am not an EE. Run it thru SPICE if you like. in the 1566 there are only three positive gain stages (the I/T, stage 1 and 2). The output stage is approx. unity gain and the O/T is about 8db loss. Don’t get hung up on the termination issue. I really can’t offer any more help. Good luck. c.

hi Chris,

thank you for your hints! I have the very same UTC O-1 as you wich is 1:15 as I recall and Altec 15095 wich is 5:1 on 600ohms. It’s sill a mistery to me how could 3 lower gain triode stages deliver the same amount of gain with same type of xformers, in simmilar circuit…because 12AX7’s gain is 100, 6SL7 has 70, and 76 has only 13(!). Or maybe the 1566’s given max.65db gain is with 600ohm termination load? If so, on modern high impedance equipment means no load and give much more gain. You mentioned you got around 60db on your 76/6SL7 preamp. Is it on 600ohm or without load? Thank you for your kind help!

P

hi pete. I don’t have a unified schematic for this thing. I work from the RCA receiving tube manuals. I’d be happy to make a schematic for you but there would be some cost involved, since i don’t have/need one personally. BTW, One 76 and one 6SL7 will give you around 65 db of gain including the 1:10 input trans and accounting for losses in a 5:1 output trans. You def do not need two. that would be way too much gain. Good luck, c.

This project is really interresting to me, with theese 76s.
Maybe you could help me with sharing the schematics of yours?
I’ve already built the Altec 1566 and it sounds amazing to me on vocals. Now i’m want to build something like it (no NFB loop) with a 76 on the first stage and two 6sl7s on the second and third stages to compensate the old tubes lower mu and somehow get the same gain as the 1566 (cca. 65db). I am curious if you do something simmilar on this project? If you could help me some resistor/cap values and voltages I mutch appriciate it.

Pete

c.

Cured. I had stupidly long leads to the first grid that seemed to be picking up NASA signals.

Hey john. The problem u r experiencing is called parasitic oscillation. Assuming yr device is wired correctly, this is caused by improper layout and or lead-length. You can research this online. Or just use a 12ay7 in the first stage, that might cure it too.

Thanks for the advice. Using the headphone trick I found if the Edcor was on its side the problem went away! I’ve fabricated an L-shaped bracket and I’m back in business.

Just have a weird oscillation/gain drop out thing going on when the knob is at 3/4, but hey ho…

Thanks again Chris