6g6 build help

Discussion in 'Shock Brother's DIY Amps' started by Dsav54, Oct 24, 2021.

  1. Dsav54

    Dsav54 TDPRI Member

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    Hey y'all,

    I've begun my second build and have decided on a 6g6b with gz34 rectifier. Scored some great heavy iron from an old bogen for free, but have realized there's no bias tap. I'm planning on incorporating an adjustment bias pot as well as a master volume mod.

    I've searched for hours and read as much as I could about using one of the b+ wires to feed the bias circuit, but still not quite sure I grasp the idea. I've attached my layout below and I was hoping y'all might take a look and give me some pointers(is this the proper way to go about my -60v)

    Thanks soo much,
    Dave 20211024_121339.jpg
     
  2. Dsav54

    Dsav54 TDPRI Member

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    bogen-mx30a-07.jpg Also, does anyone have any other recommendations/alternatives to the master volume? I've heard mixed opinions on the ppimv, would a power scaling option be worth considering?

    Also attached is the schematic for the transformers I will be using.
     
    Last edited: Oct 24, 2021
  3. schmee

    schmee Doctor of Teleocity Silver Supporter

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    Yeah, just emulate any Fender vintage Tube amp from Deluxe to Super that has a tube rectifier. I built a 6G6b with rectifier last year.
    [​IMG]
    [​IMG]
     
    Last edited: Oct 24, 2021
  4. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Yes, take the voltage from one leg of the PT high voltage wires as in your diagram. The diode will rectify it to negative voltage. The resistors sort of make a voltage divider to reduce the voltage to your target bias voltage. You said there will be a bias pot included so it will make it adjustable. All is well.

    Since not being a tap, the source has high voltage. The 1000R resistor you show should have a value in the neighborhood of 100K to 220K.
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    I shy away from power scaling. The high voltage on the pot seems too much for safety reasons. OMMV.
     
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  5. Dsav54

    Dsav54 TDPRI Member

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    Awesome thanks soo much! Waiting on the last shipment of components and chassis, and I'll update with some pics! @schmee where'd you source that face plate?
     
  6. schmee

    schmee Doctor of Teleocity Silver Supporter

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    I got it on Ebay. Aluminum and nicely done. In fact, it's the first thing I bought then built the amp around it! :lol:
    Not sure if they are still selling on there or not, but be aware a BF style chassis' holes are not correct spacing for a 6G6b style!
     
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  7. Dsav54

    Dsav54 TDPRI Member

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    Hey Everyone, I've run into a possibly snag with this build and I was hoping y'all could share some wisdom. I tested voltage on my power transformer today, as I'm assembling everything. I noticed, what I thought to be a 5v secondary tap has infact 120v. I was hoping to use a gz34.

    Could someone with a bit more wisdom check out the schematic of said transformer I posted above and see where I might be mislead?

    What are my options here?

    Thanks soo much!
    Dave
     
  8. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    It appears the PT does not have a 5V secondary. You can use SS diodes for a rectifier or you can get a 5V filament transformer. A GZ34 doesn't have much voltage drop or sag compared to other rectifiers so there is not a big difference in performance between the GZ34 and SS.
     
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  9. 2L man

    2L man Tele-Holic

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    If you did test using 120V primary voltage, this 120V coil possibly is connected parallel with, it if it is so called "international" PT.

    EZ81 is 6,3V indirectly heated rectifier to Noval socket which is used up to 18W amps.

    When "sag" effect is wanted installing higher value current limiter resistors to rectifier anodes do almost the same what poorer rectifier tube does and they also multiply rectifier service life with significant factor.
     
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  10. 2L man

    2L man Tele-Holic

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    To test if your PT has two 120V primaries first connect them series and input 120VAC to other and then measure what voltage comes to end which is "in the air". It should be about double the inputted mains. Then change other mains wire to this other end and measure if double mains comes to other end.

    If voltage differes much the other coil might be output for low HV.

    If there does not appear double mains voltage to other end it is because coil phasing came wrong! There is 50% change for it if you can not quess how outputs are wired to coils. If you can see how enameled wires come out of stack they should be in logical order.

    When you know coil phasing next step is to connect them series and measure from center to both mains hot and neutral and both should have almost exact voltages which is half the imputted mains voltage.

    Then for 120V mains pair the primarys keeping directions like it was in previous step.

    I have scrap mains cable which I tack solder to PT and this take only few minutes. I always test PT but I always test OT winding ratios as well.

    OT Phasing test is the same if Negative Feedfack is wrong and cause oscillation.
     
    Last edited: Nov 30, 2021 at 6:27 AM
  11. NTC

    NTC Tele-Holic

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    Your schematic above is incomplete. One can't see all of the preamp nor the power cord connection. The output of the transformer should have made it obvious that there is no 5V tap. The use a voltage doubler to get the B+, rectify the output from another tap for -60V (there is the beginning of your bias voltage) followed by a bunch of heaters in series, and a 6.3V tap with only 3 heaters on it. The -60V supply even has a voltage divider for -23V to bias the tubes. You will need a lower (more negative) voltage probably.

    If you use that PT, you don't have much choice but to use it exactly as it was in the original amp.
     
  12. Dsav54

    Dsav54 TDPRI Member

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    I really appreciate all the feedback! Definitely should have caught this sooner, so I suppose I'm gonna try and get my hands on a proper PT before continuing.

    Am I right in thinking this transformer would be a better fit for something more so along the lines of say a hoffman 6v6 plexi?

    https://el34world.com/Hoffman/files/Hoffman_Plexi_6V6-V2.pdf
     
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  13. sds1

    sds1 Tele-Afflicted

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    You could add a small 5V transformer as well...
     
  14. NTC

    NTC Tele-Holic

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    The high voltage winding still needs to use a doubler to get the voltage up there for the output tubes.
     
  15. Dsav54

    Dsav54 TDPRI Member

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    Alright, so with that layout of the 2 diodes, 3 40uf caps and single 1.5k 4w resistor that should bring my b+ up to the 430vdc. Do those values seem solid? Also how would you reccomend grounding those?

    I should be able to feed a 5v transformer with the secondary that feeds the negative bias, correct?

    Thanks,
    Dave
     
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