Vintage Stromberg Carlson Signet 33 (SAU-33).

Discussion in 'The Squawk Box' started by Natima, Jan 12, 2019.

  1. Natima

    Natima Senior Member

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    So I just picked up a Stromberg-Carlson Signet 33. It's a dual 6L6 33W push-pull ampifier with a 5U4GB Rectifier.

    I was wondering what, if any, vintage guitar amps share the most similar circuitry.
    I see some similarities to a bassman, but most of the resistor values are way higher in the Stromberg.

    What would be my best path to getting this thing rocking for a guitar?
    [​IMG]
     

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  2. Soul Tramp

    Soul Tramp Speaker Snob V.I.P. Member

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    I haven't looked closely at the schematic, but looks like there's enough there to morph it into a single channel BF Bassman. How is it put together (point-to-point, eyelet board, etc.)?

    The voltages are a tad low. If you converted to a diode rectifier it would bump them up.
     
  3. rabidhamster

    rabidhamster Senior Member

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    R1 and R25 to 1meg
    add cathode resistor and cap on V1a cathode (pin 3 currently straight to ground), do similar on V1b - This will tailor your sound significantly. Read up on how running both inputs in parallel will change the tone based on which values you choose here. You can swipe values from just about any Fender, or come up with your own. I might use something like 1.2k and anywhere from a .68 to a 40uf cap on the cathode, the cathode cap will make a HUGE difference in EQ. .68 will be a nice lead tone, with not a lot of bass. 40 uf will be fat and "blatty" when you overdrive things, but very Fendery. Briefly, the Cathode resistor will set the bias point of the tube, you'll select the value you prefer depending on if you want your amp to be as clean as possible, or a hotter more overdriven tone baked in. Most people select a value that will get the highest voltage swing positive and negative for this first stage.
    Small-value 1-5 watt resistor (pick one over 50 ohms, I'd use 5 watt to be safe, dont go too large) DIRECTLY on v3b (phase investor) pin 9 - resistor as close to the socket as you can get it without ruining things. Carbon Composition will make a difference, carbon film is fine. Metal film is quietest and sterile sounding. This is called blocking resistor and its designed to help prevent blocking distortion (google it) when cranking the amp - sweet touch-sensitive overdrive is the idea.
    You might find you also need blocking resistors on pin 5 on the Power tubes as well, but you can probably play that by ear.

    3 prong mod before all the other stuff, remove the death cap if there is one, recap the electrolytics if it needs it, and that should be all you really need to do to get it working for a guitar amp. Once thats done, you can take the time to decide if you want to change the EQ or not. Theres a program called LTSpice that will let you model changes in EQ selection, but you may prefer to find amps with similar circuitry that you know you like and swipe their EQ section because you wont need to really know why it works to make it work.

    This is what immediately jumps out at me I'll try to remember to come back and review the schematic again for more suggestions when I have time.
    If you kill yourself its not my fault or if this advice kills you its not my fault yada yada standard high voltage warnings and such, use at your own risk, never be inept, dont swim after eating, any fish may have bones, don't drink the water in Mexico. Any phrase I used that sounds like nonsense is googleable.
     
    Last edited: Jan 12, 2019
  4. Natima

    Natima Senior Member

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    Any reason to leave C1/C16 there after converting those to cathode bias?
    I was tempted to take the take the output of R4 and run it into either C16 or the grid of V1B. The hope would be that Input 1 could utilize both halves of the dual triode, and input two would be lower gain. Would that work???
     
  5. rabidhamster

    rabidhamster Senior Member

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    Yeah you could ditch those caps and use a more typical cap to ground for high freq interference. Actually now that I think about it 3.3 meg to ground is fine instead of 1 meg, it's all the same at that value.

    You certainly could cascade into v1b with appropriate the voltage divider but you may not need that much gain with 4 gain stages before the cathodyne phase inverter, even with active EQ You shouldn't really be hurting for gain, plus with the lower voltage of this amp you'll already have a dirt-prone circuit to begin with.
     
  6. Natima

    Natima Senior Member

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    I double checked and the 470k on the phase splitter is indeed connected directly to pin 9 and IS in fact my grid stopper.
    Now, am I correct in thinking I can literally just wire up the old Phono volume pot in place of R18 and have a Pre Phase Inverter master volume?

    If so this is my current plan.
    Remove C1 & C16
    R1/R25 -> 1 Meg
    - R29
    R31 -> 27k
    1.5k + 25ufd parralell on V1A+V1B Cathode
    Phono pot replaces PI Grid stop/R18.

    Lastly, I tested the can caps for voltage... and I feel like I MUST be doing something wrong... because I was reading 0mV on every lug. Meanwhile my meter was reading 8mV in the thin air. Just want to be sure I'm safe.... I have to replace the 30/30 filter cap.
     
    Last edited: Jan 17, 2019 at 8:57 PM

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