Stromberg-Carlson AU-29 mod project - advice/help needed, please...

[Rich K.]

Getting a 1950's vintage Stromberg-Carlson AU-29 PA amp from a guy I work with. Right now, it's stock, but I want to convert it into a guitar amp (seen some Youtube videos featuring these amps already converted, and they sound SWEET!). Aside from replacing the microphone connector (the old-fashioned 2-conductor screw-on type) with a 1/4 inch jack, and re-capping it, and possibly changing the speaker output connectors (it's currently an octal-type socket with taps for different impedances) what mods do I need to make to get a decent-sounding guitar amp?

[sluckey]

First task is to find a schematic. Then post it here so people can give you ideas.

[Rich K.]

Schematic here: https://www.tdpri.com/attachments/strom ... pg.612610/

[Stevem]

Since the amp is single ended output I would replace the 5Y3 recto with a 5AR4.

This will give you more voltage for more gain and also allow you to up the first filtering stage to 60 uf and get rid of more of the 120 HZ idle hum that single ended amps are known for , Plus give the amp a tad more punch when cranked up!

In terms of the filters themselves replace that 4 section original can with 4 separate filters each with a separate ground lead.

Drill a hole near the original can filter to mount a long 6/32 machine screw and solder on wire rings to the end of all of the 4 filter leads and just stack them up on the 6/32 screw to ground them all.
Be sure to use a 6/32 nut with a built in lock washer to do this.

If the amp is built on terminal strips then many times these old buggers use the mounting post as a local grounding point and they have bad grounds due to age, if that's the case then if there bolted down be sure that the bolt it tight, and if they are riveted down drill them out and use a machine screw to hold it tight.

Also I would add a 100 ohm 1 watt resistor off of each leg of the 6.3 volt filiment wires to ground to form a artificial center tap and kill off 60 HZ hum that can get into the audio signal path.

With the stepping up of the power supply voltage I would replace and coupling cap off of any preamp tube plate or screen with 630 volt rated orange drop type cap, the output tube cathode resistor I would replace with a 7 watt wire wound ciramic type also.

For the best use of the OT and the best sound from the amp I would use the orange 15 ohm tap, but for sure no less then the 8 ohm tap to feed your speaker(s) .

[thetragichero]

wow a pa amp with no extraneous nonsense in it!
should be a fun little project

[Rich K.]

wow a pa amp with no extraneous nonsense in it!
should be a fun little project

1950's tech at its best!