12AX7-as-Output Question

[neddyboy]

Hello all! I recently acquired a small Masco intercom. It runs (or ran) on one 12AT7 tube and is/was a widowmaker circuit of sorts -- no PT. I stripped it and was left with an output transformer for the 12AT7, which apparently supplied enough power to run a 2.5" speaker, plus an impedance matching transformer. The intercom uses the speaker as a mic, and the second transformer makes that possible. I have 2 questions. 1: Could I use that tiny OT on the output of the Hoffman tube pedal preamp to drive a tiny speaker? Did the original intercom circuit push the 12AT7 harder than the Hoffman? Any feedback would be appreciated. 2: What could I do with the impedance matching transformer? It's an 8ohm speaker -- does that mean the input impedance is 8ohms? Any idea what the secondary impedance might be, going into a 12AT7?

[neddyboy]

I think I answered my own question. The little OT sees 135V from the 12AT7. Holy moly, I hope I never see that much out of my Hoffman pedal! Follow on question: could I build a single-tube amp around this 12AT7 tube using this OT but safely, with a PT and a diode bridge?

[shooter]

Could I use that tiny OT on the output of the Hoffman tube pedal preamp

the easy answer is No, to get to Yes, you'll expend way more energy than you''ll get back 

[PRR]

Amps is amps. The whole thing "is" a guitar amp, dressed for intercom duty.

As you say, a g-amp demands isolated wall-power. A guitar won't drive the low-Z transformer but will drive the volume control. The speaker/mike is very likely 45 Ohms for better mike-action on very long (and cheap) wiring. You can series several normal speakers to get near 45 Ohms. Or you can use the 22k:4 transformer for Fender reverb.

[Latole]

For isolating transformer, I often use two low power transformers find at thrift store for cheap or surplus electronic store
Here two 120  to 24 volts used to make one isolating transformer

120 ( from wall outlet ) to 24 at secondary to second transformer secondary 24 to 120 volts to amp circuit

[neddyboy]

I haven't come across a widowmaker amp (so far) that has been worth the trouble of an isolation transformer. This thing was just a bunch of fishy low-rent looking stuff, including a very weathered 20 watt resistor, an all-metal case (zap), and a few others. I disassembled it! Now I have some parts to fool with.