Old GE tube mic-phono preamp

[Twangmaster]

Hey all, I found this neat old 1940's mic-phono preamp that has been modded (not by me) some time ago. It has this transformer in a can at the output. Pic's attached. I also redrew the schematic to reflect how it is wired now. It does work and is very quiet with no volume and provides mucho tweed crunch when playing my strat through it into a 5C1 champ type amp that is usually very clean. Check it out and see what you think. Maybe it would make a good boost pedal.

[Twangmaster]

 :think1:I tried a 6SN7 and I think the lower gain tube sounds way better.The 6SL7 gain made it way over the top in a bad way.

[Cups]

That's really neat. Think of all the mayhem you can cause with switchable cathode bypass caps.  Of course the first triode is run without a cathode resistor (so run wide open?) It would be neat to experiment with cathode resistance too.
Not super high voltage either so a good candidate for a pedal type boost (of course octal pre tubes are never anybodies first choice because of size right?)

[Twangmaster]

Cups,

I think the input (1st triode) is grid leak or contact bias. Very touch sensitive dynamic and responsive when used with my volume knob on the guitar. I used it at a gig Saturday and loved it. I'm sure there are some tweaks that could make it better like the switchable cathode bypass. Open for more suggestions guys....

[HotBluePlates]

Well, it appears the original circuit used a 6SC7, which would preclude the two different bias methods for the two triodes (that tube uses a shared cathode for both halves).

Grid-leak bias seems to work well for all but the hottest pickups, unless you want distortion. Not good for a mic preamp, but should work well for your use.

The primary impedance of the UTC transformer is likely a lot higher than advertised because you don't have a 500Ω load you're driving.

[Twangmaster]

The primary impedance of the UTC transformer is likely a lot higher than advertised because you don't have a 500Ω load you're driving.

What would the load be? I do have a 200 and a 50 ohm tap on the UTC transformer. 

Thanks HBP!

[HotBluePlates]

Well, the load will be the input impedance of whatever you connect to the output of this preamp. So if it's a guitar amp input, it will probably be 1M unless you use the low input of common amps, where the load might be 128k.

The primary impedance will then be the impedance ratio, times the secondary load. However, the 2nd gain stage will see something close to the value of R12 (47k) because the reflected primary impedance is likely to be much higher.