50s Intercom Tube Practice Amp

[wovenbone]

Hi - question, question, questionquestionquestion:

I've had an old tube intercom kicking around for a while now, always considered whether it'd be possible to turn it into a guit practice amp. I have basic/intermediate electronics knowledge (depending on what I'm working on), but not as much w/ tubes and analog circuitry like this.

I DO know that this follows ye ol' widowmaker design, à la:

http://lectrolab.wordpress.com/lectrola ... idowmaker/

However, I see two transformers. Isolation/output or...? Since I'm a novice, and a just-beginner at this tube amp thing, anyone have any thoughts/direction?

Schem attached, thanks much.

[Invertiguy]

As you noted, you'll definitely need an isolation transformer between the wall plug and the rectifier. The transformer at the input of the amp appears to be there to match the impedance of the speaker, which doubled as a microphone in this setup, to the grid of the 12AU6, which is set up for grid-leak bias. I'd just cut it out entirely and install an input jack in its place, and it's probably worth considering rebuilding the first stage as a cathode-bias circuit, since grid-leak, while arguably OK for very small signals, has a tendency to drift all over the place with larger signals or even just as the valve ages. It was done for cost-efficiency, not optimal sonic performance. Either way, with only one gain stage before the output tube you're not going to get much in the way of gain out of this amp, but it's still a nifty little project!

[M Fowler]

Pretty much the All American 5 (AA5) amp minus a few tubes.

Need 120v to 120v power transformer. Tino Zottola book has the schematic and info building using these tubes.

[Stevem]

That output tube only needs 8 volts of signal to drive it to full output so that Pentode preamp tube should make that easy!

[wovenbone]

Fantastic! Thank you all for the info. There may be one less widow on the planet and one more guitar amp. Time will tell...