Stromberg-Carlson Signet 22 PA to Guitar Amp

[Tone Snob Geezer]

Greetings experts and enthusiasts,

I’ve recently found my self in the possession of a Stromberg-Carlson Signet/22 PA amp. It came out of the local train depot and was absolutely buried in coal dust so I blew it out with compressed air and inspected it for any obvious problems and none were found. After a thorough cleaning I connected it to a speaker and ran it through the light bulb tester and it slowly came to life. I went from a 40 watt bulb to finally a 100 watt bulb and confident that nothing was majorly wrong I ran it direct. It seems to be working and was sensitive to a tap on the old style mic connector and after several minutes I noticed one of the EL84 output tubes was beginning to red-plate so I shut it off and discovered on the tube tester that the tube was shorted. I replaced the electrolytic filter caps and ended up replacing both OP tubes with matched JJ’s and replaced the 150 ohm 3 watt bias resistor that measured 127 ohms. I ended having had to add a resistor to balance the tubes, changed out the Mic 1 connector with a 1/4” jack and let it burn in for a few hours. The tone decent with both single coil and humbucker pickups but now I’m ready to begin its metamorphosis into a guitar amp.

I’ve repaired/restored rescued several vintage fender amps as well as early Philco consoles and AA5 table top radios over the years, and replacing bad parts (once they are located) is one thing but changing the tonal characteristics is something I've never done. There are several of these amps out there that have been modified into guitar amps but I have been unable to find out just what the tech did.

Your suggestions would be greatly appreciated!

[pompeiisneaks]

I'd say you could look to adding some cathode resistors to the preamp tubes to see how that impacts the tone. 2.2 to 3.3k resistors with those 220k anode and maybe add cathode capacitors? The coupling caps are also something you could play with, as well as that tone pot's value it's using now, it just says 2000 not familiar enough with older schematics to know if that's 2000pF or what.

Looks like a fun project!
~Phil

[xtian]

Agree: convert the input stage(s) to match typical Fender input style. Then reevaluate the sound.

[trobbins]

Perhaps an ounce of safety and protection would be a good start, like a proper fuse holder, a protective earth, ss diodes in series with each 5Y3GT plate, a suitable fuse on the power transformer secondary CT, EL84 screen stoppers, and EL84 cathode idle current sense resistors.

You may get some benefit from using a single ground-to-chassis connection and reconfiguring the ground nodes - that may be a pain for a bit lower noise and hum floor, but at least confirm the B+ CT connection is direct to first filter cap negative terminal and not to chassis.

If you can overdrive the PI and output stages, then well worth adding grid stoppers, eg. at least 100k to the PI, and maybe 1k to the EL84's (google to see what has been used).

Good to check the high frequency bandwidth to see if it is rolling off too low when tone pot is 'flat', due to C5 and C14. But widening that bandwidth is likely to require the feedback to be reduced or disconnected.

Given you have 3 input channels, then as already alerted, best to convert one of them to a standard cathode biased input stage with 1Meg grid leak, and say a 10k grid stopper, and with a shorting input socket. You could load the other mic input with 1.5Meg on the socket to give a nominal 1Meg input impedance, but still keep that stage as a grid leak bias for interest - you may need to vary the grid leak resistor to get the anode voltage to circa 70V idle.

Ciao, Tim

[Tone Snob Geezer]

Plenty of great suggestions and safety and circuit protection advice, thank you!

I failed to mention that I changed the cord and added a chassis ground, and I agree with the fuse. That is pretty sloppy.

I swapped out the ceramic .002mf tone control cap with a .47 just for kicks and I was able to roll off almost all the higher frequencies, which was kind of fun with the Jazz Bass but not very practical. The new cap introduced some noise so I had an old .002 paper cap that stuffed in there and quieted it down, and sounded better than the original ceramic disc. I'll continue to try different caps there though.

Tim, your comment about C5 and C14 make sense but would it make sense to also include C4? I have a signal generator and a Tektronix 2225 O' scope so I could check the range instead of just assuming it is bad, what frequency range would I be looking at?

I was prepared to get an onslaught of suggestions about changing out the old Pyramid molded caps in one fell swoop but apparently they are all likely ok? The ones I tested were all well within spec, but also intended for use in a PA.

I've seen some pretty expensive high-end audio caps for sale and wonder if they are tonally any better than standard replacements? I would imagine that shielding might be better but I'm open to your suggestions there too.

Thank you,
Paul

[trobbins]

C4 is the screen bypass and extends bass response - google will pick up a few pages on that. Any cap that doesn't leak is as good as the next cap that doesn't leak - I'd look for tone in your guitar/bass setup more than in one or two cap alternative makes.