12AU7/12AT7/12AX7 preamp schmatic needed

[Sal Brisindi]

Hi all,
I am currently rebuilding a Magnavox console 6BQ5 Push-Pull stereo amplifier. I am plugging my mp3 player into the RCA jacks but I don't have enough gain to use the amp to its full potential.

The chassis just so happens to have an extra hole drilled next to the 6EU7 tubes for which I installed a 9 pin socket. I want to build 1 extra stage of preamp connected into each channel.

Right now I am thinking using a 6N1P tube and copying the line stage amp that is based on Mikael Abdellah's SE KT88 amp. One triode of the 6N1P preamp will go to the left channel, and the other to the right.

I have many 12at7/u7and x7 tubes and I would like to use them as the filament current is 300ma vs 600ma for the 6N1P.

The B+ in the amp is approx 350vdc.

Anyone have a schematic up their sleeve? I am posting Mikael's schematic to give you an idea what I am thinking of using. Doing a search on the net, Mikael preamp stage is pretty generic as far as preamp stages go. I am breadboarding the preamp now to see if I can come up with something using a 12a(t.u.x)7 tube .

Thanks,
Sal

[Eli Duttman]

Sal,

What does the OEM circuitry look like? The 6EU7 contains the same triode as the 12AX7. If you CCS load those triodes, you should get a stage gain of nearly 100X. 12AU7s make acceptable "concertina" phase splitters. For a variety of reasons, the 'X7 triode is a poor choice for splitter duty.

BTW, how was Xmas? Happy New Year!

[roggom]

I just built one using a 6n2p (instead of the 6EU7) and it is great. It works best if you build the feedback circuit as well. Full credit to the author whomever it is.

[Sal Brisindi]

Eli,
My Christmas and New Years were excellent, thanks.

Attached is the schematic of the Magnavox amp I am rebuilding. I just need a little more gain and I would like to stick with passive components in this application.

Thanks,
Sal

[Eli Duttman]

I just need a little more gain and I would like to stick with passive components in this application.

Thanks to a competent power trafo, this is fairly easy. The bottle count will not increase. Switch to 6U8s, as used by Scott, for the small signal circuitry. Scott/Fisher/Dyna style circuitry of a voltage gain block DC coupled to a "concertina" phase splitter works WELL, with "12" W. O/P tube types. As properly matched pairs of EL84s are easier to acquire than good quads, use a separate RC bias network in each channel.

Simplify, simplify, simplify! Use a red LED for pentode voltage gain block bias. Peak HF gain by loading the voltage gain block with an inductive wire wound resistor. A 10 Ohm resistor connects the LED to ground. A 100 Ohm resistor connects the "hot" speaker terminal to the LED/10 Ohm resistor junction. Approx. 9% of the O/P voltage is fed back. Short the NFB loop out at ultrasonic frequencies by connecting the "hot" speaker terminal to ground via a 15 nF. capacitor.

Protect the O/P trafo cores against saturation by rolling infrasonic noise off at the amp's I/Ps. 100 KOhms as the I/P grid leak resistor is quite ample. Parallel a 68 nF. cap. and a 15 nF. cap. and use the pair (instead of wire) to connect a RCA jack's center pin to the 100 KOhm grid leak part. F₃ for the setup is roughly 19.2 Hz.

You can increase the B+ rail voltage by inserting a 5AR4 as the rectifier. Don't forget to move the B+ take off point to socket pin 8.

[Sal Brisindi]

Thanks for the info Eli, I knew you would keep it simple... lol
You probably forgot today more than I will ever know about the design aspect of tube amps...

Well, I prototyped a 12AT7 preamp on a breadboard.. took about 45 minutes to wire it up.

Here it is:


I must say it sounds pretty gosh darn good if I do say so myself!

B+ and filament power coming from the Magnavox amp.

Here is the whole temporary setup while I test it. Disregard the tube behind the RCA jacks on the amp, its not connected yet, that is where I want to put the 12AT7 tube.



Sal

[pboser]

Sal,
Do you have a schematic or verbal description of the circuit for your 12AT7 stage?
Thanks,
Pete