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  • Stromberg AU-32 cascaded mic sections...

    Having fun with an idea picked up somewhere? It is for the old PA's with multiple mic inputs. This one was a Stromberg AU-33 with three mic in's, 3 vol controls and a phono with a vol.. It combined 2 tubes into one section.. the shielded wire was lifted from the input jack ( already converted to 1/4) and connected to the plate of the previous section, I think it's called cascaded . The tubes that this amp uses are 6sj7 pentode types. The sound level increase from this was cool. This mod also has converted the master volume pot of the first section into a gain/distortion pot, and for the section that follows, it is now like a master volume control. I've had to change the plate and cathode resistors on both sections experimenting with vaues to move the gain into useable levels but all by ear. So my concerns are that by doing this it may have a negative somewhere down the road... I've played this amp for hours monitoring the under hood temps and sniffing for signs of escaping electrons, with no real issues. Anyone tried this, I know I got this idea somewhere but not sure where. All in all it is a good way to use the unused sections and avoids just having three separate inputs with one useable. Thanks to the guy that did the white papers on modifying preamps and the one on converting PA's to guitar. Was a roadmap of sorts for this fun project.
    Last edited by tboy; 06-06-2011, 03:59 AM.

  • #2
    Unlike with triodes where the gain can be easily manipulated by changing the plate and cathode resistors, the gain of a pentode such as the 6sj7 is more easily manipulated by changing the screen (g2) resistor value.The closer the screen voltage is to the plate,the higher the gain.......This is a very generalized statement but hopefully useful to you in your experimentation with the gain of your amplifier. SG

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    • #3
      Were you referring to this?

      I think this is the article you were alluding to...

      Converting Integrated/PA Tube Amps into Guitar Amps.

      and the follow-up...

      Notes on Converting PA Amps to Guitar Amps

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      • #4
        Thanks for that SG...I am reducing the first gain stage at present, trying to get the range of the vol pot distributed along at least 80 per cent of the pots sweep. It now becomes too fuzzy/gainy after 12 o'clock, would like it closer to 4 or 5. Before I attempt anything else I will look at the screen values and do some measuring..great tip, thanks again.

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        • #5
          Yes.. and this too "Modifying Preamp Circuitry", unknown author ( Modifying Guitar Amp Preamp Circuitry ) I think this still works. Good stuff.

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          • #6
            sorry for double clik. OP
            Last edited by hrbay; 06-07-2011, 03:03 AM. Reason: double post

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            • #7
              Thanks SG, the screen grid tweak worked great..

              Just a fllow up, ... raised the screen R about 25 % to cut the gain on one, lowered the Vp and it now does not hammer the second tube 'till about 7 on the pot rotation. Thanks for the help.

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              • #8
                hrbay,
                Appreciate the follow up.I went through the same process you have with a 6SJ7 in a Masco MA-8n conversion some time ago.The signal pentodes have been out of favor in musical preamps for some time now for a number of reasons.This was not always so.It seems a lot of musical amps/PA amps employed them into the early 60s,although sometimes strapped as triodes.Preamps employing 6SJ7s and later EF86s (6267) and 5879s were common.Although they can tend to be noisy and microphonic, they are a interesting and useful alternative to the ever present 12A*7 triodes.Glad you found the info useful.Again, thanks for the follow-up. SG

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                • #9
                  schem of homebrew S/C

                  This is it so far.
                  Attached Files

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                  • #10
                    Thanks for sharing your schematic. It's really nicely drawn......I only wish my own were of such clarity. SG

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