Stromberg Carlson Signet 70 conversion into a Bass Amp

[Organ Bender]

Hi everyone,
This is my first post here.

I got this Stromberg Carlson Signet 70, a tube PA from the 60' with a pair of 6550 tubes. It's been found in the garbage, and apart from having been questionably repaired in the past, it is functioning ok. I redone the repairs and mods (the added ¼" jacks wiring was knotted instead of soldered, quite interesting, haha)

I am hoping I could convert it and make it in a decent bass amp. As it is it can sound beautiful, but the headroom is so narrow, I can't push it more than at 2 without having some not so musical clipping, and not so loud volume. I think with 2 available 12ax7 tubes and plenty of high voltage taps there is something nice, and hopefully not too expensive, that I can do.

There are two amp circuits I am tempted to experiment with. See schematics below
- The Ampeg SB12. This is basically a 12" version of the B15 but with 12AX7 instead of the usual 6SL7, and with the exact same tone stack.
- The Sunn 200S.

Here is what I was planning to do :
1) Replicating only the preamp section of either or even both, since I have the space and tube sockets
2) Keeping both the original PI and power amp section
3) Use the 330V tap to power the preamp sections

I'd like some advice/recommendation on how to approach this project. This is my first time trying to mess with amps this way, I am here to learn!

Note that although I hope to get the best out of it, my main goal is to use it as an experimental project to learn more about tube design and amp voicing in general.

SAU_70_Schematic_2.png

SB12_alt.png

Sunn_200s2.png

[johnnyreece]

No opinions one way or another, but wanted to chime in to say that's an interesting rectifier arrangement. Took me a minute to sort out what was going on. Are you planning on keeping that same setup? I would think that the lower screen voltage wouldn't be ideal on a bass amp, as I'm guessing you're looking for maximum clean power and not earlier breakup.

[Stevem]

I have both a Sunn Sorado ( same as the 200) and a B15NF, and strictly for Bass the Sunn is a better amp.

We just need to come up with a work around for that 6AN8 tube since it’s only available NOS as is the 7199 that’s used a a sub these days.

[Organ Bender]

Are you planning on keeping that same setup? I would think that the lower screen voltage wouldn't be ideal on a bass amp, as I'm guessing you're looking for maximum clean power and not earlier breakup.

As I said, I hope to get away with as little work as possible, but I still aim to have something worth of a bass amp. This is where I need advice from knowledgeable people. I did not dive much into PI analysis yet. So any thought will help.
What screen voltage were you referring to?

We just need to come up with a work around for that 6AN8 tube since it’s only available NOS as is the 7199 that’s used a a sub these days.

It is a 6U8 that is used in the Signet. Are you implying I should implement the Sunn's PI circuit in there? Or just mod to 7199 wiring anyway?

[Organ Bender]

Here are the schematics reuploaded for better quality

SB12_alt.pdf

Sunn_200s2.pdf

[johnnyreece]

What screen voltage were you referring to?

On the Stromberg Carlson schematic, there are two rectifiers. The 5U4GB which powers the plates at 530V, and a 5Y3GT that powers the rest (starting with the screens) at 380V. I don't really have experience with 6550 tubes, but the lower your screen voltage, the lower your output power (generally speaking). However, looking at the datasheet, it seems there's plenty of power available for the 6550, even with the lower screen voltage. Again, I don't play with 6550 tubes, so someone with experience on those might chime in.

[Organ Bender]

at 380V

Thanks for the clarification. Since you were talking about the PI at the beginning, I was searching there so I was confused. I'll see about output power later I think.

[johnnyreece]

at 380V

Thanks for the clarification. Since you were talking about the PI at the beginning, I was searching there so I was confused. I'll see about output power later I think.

I believe it was Stevem who was referencing the PI tube. I was only talking voltages. I hadn't seen that kind of rectifier setup before, and I thought it was kind of neat. Good luck on your journey! I imagine you'll be able to cook up something cool with what you've got there.

[didit]

Hi everyone,
This is my first post here.

[…]
I am hoping I could convert it and make it in a decent bass amp.

When you say “bass amp” it’d be helpful to understand what in specific detail you mean, as a B15 is quite different than a Sunn 200s. Both are, in context, cool amps.

Best ..

[didit]

Scrapped an unexpected duplicate)

[Organ Bender]

When you say “bass amp” it’d be helpful to understand what in specific detail you mean

I just mean a cool amp with which I can play bass and make use of that old PA I got for free. I picked the SB12 for it's close to the B15, which is a kind of legend and is designed with 12AX7 preamp tubes ; and the Sunn because these amps are cool and the design seems easy to implement in my Signet chassis. Tone wise, I will be happy with either one of them.I am not sure what specific you want to know...

[didit]

OK. Suggest you start out roughly following the Sunn design. It’s definitely closer, and going to require less changes to get working.

You say you’ve tidied up from questionable mods and added 1/4 jacks. This suggests you have the original or at least likely stale bulk filter capacitors for the power supply down the chain.

My recommendation would be to strip off the elaborate multi-rectifier set up, replacing it with suitable silicon-based design along with getting fresh electrolytic bulk filter capacitors. Would you need more guidance on details or can you sketch the steps and circuit layout from my brief advice?

Best ..

[sluckey]

If this were my project, I'd leave the power supply and power amp stock and add the Sunn 200S preamp. See attached...

[jbrew73]

I agree with sluckey on leaving the power supply and power amp alone. You will have to deal with some really high voltages if you ditch the tube rectifier setup.

Pick either preamp and apply it as shown above. You might have a little too much gain but once you get it running it will be easy to lower it to the level needed.

[Organ Bender]

If this were my project, I'd leave the power supply and power amp stock and add the Sunn 200S preamp.

That's exactly on of the scenarios I was mentioning. I like your collage!

My recommendation would be to strip off the elaborate multi-rectifier set up

I already installed a fresh capacitors for the power section. If I was to opt for a solid state rectifier design I would indeed need guidance, thanks. But I don't see why I would bother to put work in there if it is working as is and it supplies the voltages needed for the amplifier.

All right, since I need to start somewhere, I'll go for - let's call it - the "Sluckey design", haha!
Will be back in a couple days with the results.