Hi Everybody, I've had this amp for about 20 years and believe it or not, it is brand new, never used. I am converting it to a guitar amp for practice and recording. I have little experience with tube amps. Here's some specs 10watts, using a 12ax7 pre and a 7868 power tube. has a High impedance mic input w/volume control and a phono input also with a volume control. I've already installed a 1/4" phono jack in the mic circuit and it works just fine with my guitar to a point. I ordered a Jensen mod 1050 speaker. In the mean time I have it hooked up to a couple 8 ohm 6x9 triaxl car speakers . I know, not a good choice for a guitar amp but it's all I had lying around. I have a few questions maybe someone would be kind enough to steer me in the right direction. I can post the schematic if need be.
I can see on the schematic that this amp has only half wave rectification (solid state). Any advantage to change it to full wave rectification? Also on the schematic it appears that the 12ax7 tube is actually 2 preamps in one tube. If thats the case can it be used as a second gain stage? Acording to the schematic it's used as a preamp for the phono input. I'm speculating here so please correct me if I'm wrong. I think the guitar is over driving the 12ax7. I'm not sure because of the speakers I'm using. Any way to match the output of the guitar (Single coil strat) to the input of the 12ax7? Any other suggestions or tricks would be greatly appreciated.
Thanks, Have a great day!
I can see on the schematic that this amp has only half wave rectification (solid state). Any advantage to change it to full wave rectification? Also on the schematic it appears that the 12ax7 tube is actually 2 preamps in one tube. If thats the case can it be used as a second gain stage? Acording to the schematic it's used as a preamp for the phono input. I'm speculating here so please correct me if I'm wrong. I think the guitar is over driving the 12ax7. I'm not sure because of the speakers I'm using. Any way to match the output of the guitar (Single coil strat) to the input of the 12ax7? Any other suggestions or tricks would be greatly appreciated.
Thanks, Have a great day!
I'm in the process of converting a Stromberg-Carlson myself. I don't suppose you could post the schematic or at least pictures of the amp?
Meanwhile here are some reading that I've found useful:
http://www.rru.com/~meo/Guitar/Amps/PA2Guitar/old2new.html
Meanwhile here are some reading that I've found useful:
http://www.rru.com/~meo/Guitar/Amps/PA2Guitar/old2new.html
Thanks for the info...here is the schematic. Do you have one for yours? I consulted with an old friend who is an Electronics engineer . and I've got some excellent info from him. I'll post an updated schematic with possible changes to make to alter the sonic quality. I used the mic input but it was a little bass heavy. I changed the very first cap from the mic input from a .o1 to a .oo1 cap. More to come. Says file s too big. I'll have to crunch it down and repost. I'll try tonite.
I scanned the schematic from the Stromberg-Carlson AU-57 chassis here. Not too easy to read, but still readable:
An externally hosted image should be here but it was not working when we last tested it.
Wow nice piece to work with. Have you hooked the guitar to it yet? My amp is much simpler only 2 tubes. I'm still working on putting up the schematic. I've been busy with other stuff. I'll try later today.
I've gotten good results from the AU-57 used as a bass amp with my Alembic Epic. I like the tone as-is. Not bad with guitar, but very noisy with a single-coil pickup. I plan to take one or two of the preamp stages and bump the voltage up to about 110V and use 12AX7's instead of 12AU7's. That should give me some good overdrive. I'll then adjust one input for a keyboard, and then leave one as is for the bass guitar or clean guitar stage. I'll have a very versatile amp in the end versus the solid state SWR I was using prior.
I'm also going to beef up the power supply to make it stiffer, recap the electrolytics, and possibly replace some caps with orange drops, just cause I've got 'em.
You may only need to change a couple input resistors to be ready to go. The link I posted should address that. If you want overdrive you may be best off using an external stomp box where a single 12AX7 may not be suitable by itself. Most schematics for 12AX7 distortion stomp boxes I've seen have a transistor stage that overdrives the 12AX7. The Matchless hotbox uses two 12AX7's which is a better design in my opinion.
I'm also going to beef up the power supply to make it stiffer, recap the electrolytics, and possibly replace some caps with orange drops, just cause I've got 'em.
You may only need to change a couple input resistors to be ready to go. The link I posted should address that. If you want overdrive you may be best off using an external stomp box where a single 12AX7 may not be suitable by itself. Most schematics for 12AX7 distortion stomp boxes I've seen have a transistor stage that overdrives the 12AX7. The Matchless hotbox uses two 12AX7's which is a better design in my opinion.
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