Stromberg Carlson Signet 70 conversion into a Bass Amp

[didit]

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.

I'm of two minds -- can easily agree with taking an easy path forward. However caveat, just a few more volts on your power tubes and more importantly larger bulk filters places the power stage closer to later stage bass amp designs. A 6500 is spec'ed for 800V max and 600V is a common operating point for them, which should net out about 80W.

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[johnnyreece]

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.

I'm of two minds -- can easily agree with taking an easy path forward. However caveat, just a few more volts on your power tubes and more importantly larger bulk filters places the power stage closer to later stage bass amp designs. A 6500 is spec'ed for 800V max and 600V is a common operating point for them, which should net out about 80W.

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This was kinda my thinking, too, but there is something to be said for trying it the easy way, and if that's not enough power, modding it in the future.

[Organ Bender]

However caveat, just a few more volts on your power tubes and more importantly larger bulk filters places the power stage closer to later stage bass amp designs.

I'm not sure I follow you. Why are we talking about more volts and larger bulk filters? Closer in what sense?

[didit]

However caveat, just a few more volts on your power tubes and more importantly larger bulk filters places the power stage closer to later stage bass amp designs.

I'm not sure I follow you. Why are we talking about more volts and larger bulk filters? Closer in what sense?

higher plate voltage and larger bulk capacitance provides both a bit more headroom and quite a bit more total power through heavy transient loads on the power amp. Bass players tend to create heavy transients.

The 5U4 drops 35V or more that's all wasted in heat radiating off its plates. Plus there's a max. upper limit on filter capacitance -- 40uF if memory serves. The schematic shows 20mF. Using silicon rectifiers would easily enable increasing that to 80-100uF and you'd also gain 60-70V on the plates headroom. The strength of drive coming from the PI (triode in the 7687) might make this somewhat moot. We'd need to measure it, or model it, to confirm. I've found through experimentation that even when the PI isn't fully pushing limits of the power tube control grid there's still tighter dynamics at the transformer primary. I've somewhat oversimplified, but in the context of adding just ~10-15% believe it's still safely in the ballpark.

Does that help? Maybe grab spec sheets for the tubes involved and spend a while reading through.

While we're here:

Don't recall seeing recommendation you put in an adjustable bias. The schematic has fixed (top of C21, junction of R23/R4) with -47V as target. You will doubtless find it helpful if not absolutely necessary to add adjustable control to ensure your 6550s are operating in a safe but effective dissipation range. Also, can we assume you've got a 7687/6U8? These, as has been mentioned, are no longing being made so advise you hunt for at least one or maybe two spares. They are less scarce than unobtainable 7199s --however, still $30-40 for NOS from reliable sellers.

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[Organ Bender]

The 5U4 drops 35V or more that's all wasted in heat...

I am aware that the solid state rectifier will provide more voltage than the tube setup. Although I don't have the design skills to calibrate the output section accordingly. I asked mostly because the way you were putting it I had no clue what you were referring to. But that's ok, I am glad you took the time to explain and show me your willingness to guide me.

For now I will focus on the preamp section. I might decide to dive in the power amp part later. I will let you know. And I already made a LTSpice simulation, so this will be available.

Don't recall seeing recommendation you put in an adjustable bias

Great suggestion. I did not measure the actual operation of the 6550s, but I was ready to make the bias adjustable if needed.

And yes, I have a 6U8 in place. I think I already have a spare one.

[didit]

A pair of survivor 6550s should be cautiously cared for. If you’re planning to tinker with the preamp using the power stage as a mule then suggest making certain your’s are biased correctly. Martin Manning has captured an excellent adjustable design in schematic with notes. Searching for that would be my recommendation.

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[martin manning]

Martin Manning has captured an excellent adjustable design in schematic with notes. Searching for that would be my recommendation.

IDK about notes, but in this ODS schematic is my go-to bias circuit https://ampgarage.com/forum/viewtopic.p ... 08#p365408

[didit]

Martin Manning has captured an excellent adjustable design in schematic with notes. Searching for that would be my recommendation.

IDK about notes, but in this ODS schematic is my go-to bias circuit https://ampgarage.com/forum/viewtopic.p ... 08#p365408

Informal notes across various threads where you've shone the light on it ..

[Organ Bender]

If you’re planning to tinker with the preamp using the power stage as a mule then suggest making certain your’s are biased correctly.

I was implying measuring the actual bias and make it adjustable if it was off. I will measure it, anyway, at each significant stages of the modification

[Organ Bender]

Martin Manning has captured an excellent adjustable design in schematic with notes. Searching for that would be my recommendation.

I just spent half an hour looking for a somewhat specific info, it appears that Martin Manning is everywhere, hahaha!
Why not just go with the classic adjustable bias mod? Say I replace the 12K resistor with a 25K pot in line with a 6.8K resistor?

[sluckey]

Say I replace the 12K resistor with a 25K pot in line with a 6.8K resistor?

yes

[martin manning]

As long as you are doing it, why not build a fast, low-ripple, and fail-safe bias supply so your expensive components will survive a bias pot wiper losing contact or a crack in the resistive element? If your starting point is the SC schematic your proposed design is good.

[Organ Bender]

As long as you are doing it, why not build a fast, low-ripple, and fail-safe bias supply so your expensive components will survive a bias pot wiper losing contact or a crack in the resistive element? If your starting point is the SC schematic your proposed design is good.

Yes the starting point is the SC schematic.
When you say "fail safe", I don't see what is the fail safe part. I refer to the schematic in the link you provided. I mean I don't see anything significantly more protective than what I usually see in tube amps or basic adjustable bias mod. Could you extrapolate?

[martin manning]

The fail-safe feature is that if the pot wiper loses contact, the resistance to ground goes to its maximum, and the bias voltage goes full negative. If the element cracks you get the raw negative voltage. Fender typically took bias voltage from the pot wiper, which leaves the grids floating or at ground potential if it fails.

[Organ Bender]

Hi everyone.

I just finished testing the Sunn's preamp implementation. Everything went quite easily with no issues. I played a while and it sounds really nice as is.

Here are some observations and question that some of may have something insightful to comment :
1) All the HT voltages are about 30V higher than the schematic. How much raise in voltage with old tube amps is considered normal? Should I do something about it.
2) I did measure the bias the best I could. Since my meter does not show decimal when over 500V, I had to guess by looking at the toggling pattern between 559V and 560V. So it is somewhere between 45% and 70% dissipation. I will do a more precise measure later, but for now at least I know it in a safe zone.
3) Since HT4 and 5 are not used anymore they stay at 357 and 353V
4) I said it sounded nice as is, but I still have to dial a bit to get a proper bass sound. There is a bit too much of high freq to my taste, i.e. there is too much fret clacking noise. Compared with my Ampeg Micro stack, with which I play rather flat, here I have to dial the treble down to like 2/10 and flip the bass boost. So I'd like some recommendation on where to install a lowpass option that could tame this just enough.
5) Some of the PI caps are a little out of spec (like 30%), should I bother replacing them? See 2nd pic, the socket to the right

Here are some shots

IMG_20260723_111505.jpg

IMG_20260723_111516.jpg

IMG_20260723_111521.jpg