Plexi Build- PT with multiple taps vs one, for switching between tube and solid state rectifier?

Discussion in 'Shock Brother's DIY Amps' started by itsGiusto, Aug 23, 2019.

  1. itsGiusto

    itsGiusto Tele-Meister

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    I just put in an order with Ceriatone - I plan to build a Plexi50 (1967 specs). I asked Nik to give me the JTM50 chassis, however, so I could put a tube rectifier in and make it switchable. I also plan on putting a second bias pot on the same switch so I can bias for the tube-rectifier separately from the diode rectifier. The main problem with switching from diodes to tube recto, of course, is that B+ will drop significantly when I switch to the tube from the diodes. A few questions:

    1. Is it possible for the setting bias pot to make up for this loss? Or does the bias pot only control the voltage on the power tubes, meaning all the other tubes will still be receiving less voltage than they should be?

    2. Is it worth getting a transformer that has both a 625v tap for the diode rectifier, and a 690v tap for the tube rectifier? This one seems to have that capability: http://www.classictone.net/40-18054.html
    If I did this, would I still hook up the diode/rectifier switch after the rectifier, between the rectifier and the filter caps, like in this diagram? Or would it be better to move the switch to be before the rectifier, to stop the actual flow of current from the tap to each rectifier?

    [​IMG]

    Would running both rectifiers from different taps simultaneously (but switching after the rectifier) be a waste of energy? Or would it bad for any other particular reason?
     
    Last edited: Aug 24, 2019
  2. itsGiusto

    itsGiusto Tele-Meister

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    Also another question:
    3) if I got the above transformer (http://www.classictone.net/40-18054.html) would I be better off using the included 50v bias tap for the bias instead of taking the bias from the normal high voltage tap? What benefits are there to using this instead of the normal tap? What changes would have to be made to the standard Plexi bias circuit?
     
  3. dan40

    dan40 Tele-Afflicted Silver Supporter

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    The 50 watters tapped their bias voltage from one of the PT's secondaries instead of using a dedicated bias tap like the 100 watters. If you decided to use the bias tap, you would need to lower the value of the dropping resistor feeding the bias circuit. Take a look at the 100 watt and 50 watt schematics to get an idea of the resistor value needed for the bias tap. You may need to adjust up or down slightly to get the correct bias voltage after you finish the amp.
     
  4. itsGiusto

    itsGiusto Tele-Meister

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    Thanks! What's the positives and negatives of doing it with a dedicated bias tap on the same winding (which is what http://www.classictone.net/40-18054.html provides) vs tapping it the original way, off the same tap as the rectifier?
     
  5. dan40

    dan40 Tele-Afflicted Silver Supporter

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    The original Marshall way works just fine in the 50 watt circuit. The bias tap in the 100 watters was done for a specific reason because the power supplies are slightly different in the 100 watters and the tap was necessary. In your build either way would work equally well. If you want to wire it like the originals, simply heatshrink the end of the bias wire and tuck it away safely.
     
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  6. Ten Over

    Ten Over Tele-Meister

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    Your drawing seems to be missing a leg.

    You could design a bias circuit for the 50V tap and put the different values on the '67 Lead board, but I think it would be easier to just use the bias circuit that comes with the kit. One less wire used, also, from a transformer with 17 wires coming out of it.

    Note that the first filter capacitor after the tube rectifier has a maximum of 32uF. This means that the power supply will need to be wired slightly different than the '67 Lead. No big deal and I don't like the raw rectified voltage going through the HT fuse before the first filter, anyways.

    I presume the bias circuit would stay the same when switching rectifiers since the plate voltages don't change much from tube to diode rectification.

    Ceriatone 67 Lead Rectification.png
     
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  7. itsGiusto

    itsGiusto Tele-Meister

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    Ah, cool, yeah, I didn't realize. I thought the diode rectifier and tube rectifier would share one leg.

    Interesting, but I'm not certain I understand this part, could you explain further? Why is there a limit of 32uf? Is that based a limitation of how much current the rectifier tube can supply or something?

    So in your diagram the filtering is cut half in at the OT center tap? Does this have any affect on the operation of the amp or tone?

    Instead of including those RC filters, could I just have one of the 32uf caps be lifted from the circuit on the same switch as the diode/tube rectifier switch? That way I'd get to keep the circuit stock when in diode configuration?
     
    Last edited: Aug 27, 2019
  8. Ten Over

    Ten Over Tele-Meister

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    The manufacturer lists the maximum reservoir capacitance as 32uF. The Ceriatone JTM50 uses 32uF for the reservoir capacitor with a GZ34. I'm sure Nik would have used 64uF if it didn't exceed the GZ34 maximum, especially since he is leaving half a can unused.

    Millions of amps use a similar reservoir capacitance and they seem to work fine. The classic stacked 70uF's that Fender used works out to 35uF for the reservoir capacitor. Nonetheless, I have redrawn the power supply to get 64uF when using diode rectification.

    Ceriatone 67 Lead Rectification 2.png
     
  9. itsGiusto

    itsGiusto Tele-Meister

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    Cool, thanks! But why the 470k/1M resistors to ground? I don't see those in most plexi schematics.
     
  10. Ten Over

    Ten Over Tele-Meister

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    You will come to appreciate those once you've been shocked by an amp that has been off for over an hour. You could probably leave the 1M bleeder off since it takes some special talent to wind up with those capacitors charged and the 500mA HT fuse open. But then again, it's really easy to install those bleeder resistors right there on the can capacitors.

    The Ceriatone JTM50 has a 220K/3W bleeder resistor installed on one of the can capacitors.
     
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  11. itsGiusto

    itsGiusto Tele-Meister

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    Cool. So none of the filter caps are capable of causing problems (by nature of being too high-valued) for a rectifier tube except the first one? I wonder why that is.

    Also, when you have this sort of switching-configuration, will the tube rectifier still be "active" and pulling current when the diode rectifier is switched on, and vice versa? Does this cause strain on the PT, or have any negative effects?
     
  12. Ten Over

    Ten Over Tele-Meister

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    The first capacitor (aka reservoir cap) is the one that causes the rectifier tube problems because the other ones have some sort of current-limiter between them and the rectifier tube.

    The tube rectifier will not be pulling current when the diode rectifier is switched on. There is no circuit between the cathode and the plates, so there is no potential between the cathode and plates, so there is no reason for electrons to move from the cathodes to the plates.

    The diode rectifier does have a circuit when the tube rectifier is switched on, but it draws less than 1mA after the capacitor is charged up.
     
    Last edited: Aug 27, 2019
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  13. itsGiusto

    itsGiusto Tele-Meister

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    Cool. I wasn't sure if the 5v rectifier heater wires would cause there to be a circuit.
     
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