vintage PA amp conversion

[srinva]

Hi all,

First time poster here...

I have a very clean, completely stock 1951 stromberg carlson au-33 pa amp that I'd like to convert to a guitar amp. The amp is in working condition at the present time and contains 6sj7 preamp tubes (it's 3 mic inputs and 1 phono input) 2 6l6 output tubes and a 5u4 rectifier tube. there's a couple others (drawing a blank here). I have the schematic. I plan on using it to drive a 2x12 speaker cab (70's kasino loaded with CTS speakers). I'd like a little guidance and hand holding thru the process. I know it needs input and output jacks, reworking the tone controls, possibly cascading the gain stages (i want both bluesy overdrive and snarly distortion if possible), recapping and removing some key tone sucking components. all opinions are welcome...thanks in advance.

SR

[hasserl]

Typically on amps like this the mic inputs use transformers, so to use them for guitar input takes some rewiring to remove the transformers and give the input a more typical arrangement for a guitar amp. But the phone input is typically just fine for guitar input. If you've got a schematic for the amp is there any chance you can post it somewhere so we can take a look at it. It'd make it much easier to help.

[srinva]

thanks...it doesn't have transformers on the mic inputs. 6sj7 preamp tubes...at least on the mics, not sure on the phono input. I do have some pics, altho my camera is in the other car at the moment. I'll post the schematic and pics top and bottom of the chassis and a complete tube lineup. thanks again...back in touch soon.

sr

[srinva]

here's what i have so far...the pic of the schematic wasn't good enough...i'll try photocopying or scanning.

[srinva]

ok...last night I wired a 1/4" input jack into it and just used the original PA speakers. It all works but the bass and treble do very little for the sound, it also goes wah, wah, wah, wah...when you strike a chord and let it ring...sounds kind of like a tremolo. I also hear harmonic overtones. I obviously have a few issues to work thru...where should I start? Thanks in advance...

SR

[hasserl]

Firt thing is to get a look at a schematic. Can't do anything without some insight into how the circuit is put together.

[Enzo]

Here is a link to the schematic

http://www.theused.com/manuals/strom...u-33_ps-33.pdf

Back in th0se days mic inputs used those screw-on connectors. Hi-Z.

I'd be thinking the filter caps are dried out and need to be replaced.

[srinva]

Enzo,

Thank you very much for posting that schematic. I've looked and looked online to no avail and getting a pic or scan of the one I have hasn't worked very well. Yes...I did remove a screw on mic connector and installed a 1/4 " insulated jack there. It plays but it has overtone that is an octave above the played note. (second order?) it also has an oscillation in the sound...I'd call itvolume related. The caps are definately original and some have some wax beads on the outside...so, I assume they have leaked and are junk. Are all of the big yellow can looking ones in the picture electrolytic? If so,should they be replaced with electrolytic ones as well? What are the things in the silver cans on the top down view? I assume that they are caps as well...

SR

[Enzo]

Yes, caps all over. The silver cans are multisection electrolytic caps. You can replace them with separate e-caps. See the parts list on the schematic for each section value. You would solder the new cap directly to where the wires from the cans go, then disconnect he cans. Leave them in place for appearance. The cans are C21 and C10.

Looking at the underside photo, the three paper sleeved caps across the bottom end - one per preamp tube - are C1,14,17 and are 50uf 25v e-caps And the one mounted between the power tube sockets is also an e-cap, 50uf but 50v there. All these e-caps ought to be replaced by fresh new electrolytics. New ones will be considerably smaller.

The two sleeved e-caps upper left, the bright yellow ones, look to me like someone already replaced one section of a can cap with them. But frankly they look old enough to need replpacing themselves - this is an OLD amp.

All the waxy caps? Get rid of them, nice fresh film caps in their place. Those are paper caps, meaning there is impregnated paper dielectric inside. Not to be confused with caps with paper on the outside. Outside paper is just the wrapper.

[srinva]

Thanks a ton...I'll start a parts list. Are there any resistors that should arbitrarily need to go? I'll test them and add any that need to be replaced.

The powwer tubes are different brands and size...one is a smaller 6L6GA sylvania and the glass is loose in the base...altho it seems to work. The other power tube is a sylvania 6L6G...it's bigger. I know that tube matching isn't critical but the loose one kinda bugs me...should it be replaced because of that?

What about adding bias pots to this amp? Good idea at this time...or wait?

I'm just trying to see the big picture and order parts once. Probably not realistic, i know...

I do have excellent schematic reading and soldering skills. I teach automotive technology at the college level and have done a ton of reading on tube amplification. I wouldn't teach it, but I can understand signal flow and know what most components do. Thanks again for the help so far...

SR

[Enzo]

The thing is ancient, how about a nice new pair of 6L6s - your choice of brands.

Bias pots now or wait? How about: wait. This amp is cathode biased. Bias pots can wait until you have rewired the amp for fixed bias operation. Not sure why or when you'd do that though.

Those powr resistors are wire wound and ought to last a lifetime. You want to check the small ones for drift, go ahead. The resistors in this are 10% tolerance, which is nice since in that day 20% were more common. So when checking them keep in mind they may be off the mark a ways but still in tolerance. And even if past tolerance, might still be OK in the application they are in.

[joshester]

I just picked up one of these amps today too and in searching for a schematic I ended up here! A lot of good info here but when I click the link for the schematic you posted it's telling me that the server needs a user name and password to enter. Any clue how to get around this? Thanks!!

[Jazz P Bass]

Stromberg Carlson AU35 Schematic

Here is the schematic.

[bluto]

I just picked up one of these amps today too and in searching for a schematic I ended up here! A lot of good info here but when I click the link for the schematic you posted it's telling me that the server needs a user name and password to enter. Any clue how to get around this? Thanks!!

Which model specifically, an AU-33?? I have an old AU-32 myself I'l be converting for guitar. I found the schematic at Made In Rochester
you need to scroll down quite aways and mine was in a weird dejavu format but I was able to download a free dejuvu reader.
I just finished converting a similar Lafayette PA and it turned out great, hope to start on my AU-32 soon

[joshester]

Yeah, I've got the 33.