Replace a Vacuum Tube with Diodes

Discussion in 'The Projects Forum' started by Wyl, Apr 30, 2015.

  1. Wyl

    Thread Starter New Member

    Apr 30, 2015
    1
    0
    I have a Conn 813 electronic church organ, which uses dozens of 12AU7 vacuum tubes, as well as a vacuum tube amplifier, one of whose rectifier tubes accidentally shattered during a move.

    My project is to replace the broken tube with solid-state diodes, of which I have several dozen.

    The amplifier is a Pritek (Chicago) Power Amp Model 4270, 115W output, peak 180W.

    The broken tube is a 6CA4, a full-wave rectifier, whose spec sheet I've attached. Apart from the heater pins, which I won't need, the pins used are 3, the cathode; and pins 1 & 7, plates # 1 and 2.

    I'm guessing I'll poke two diode cathodes into pin 3, and their anodes into pins 1 & 7, assuming they'll handle the voltage. If not, I'll just put some diodes in series, do the same thing.

    But I've read something that suggests I'll need a resistor somewhere to suppress some voltage transients, which I don't understand.

    Can someone provide some guidance on this idea, and explain the voltage transients and suggest a resistor?
     
  2. Ramussons

    Active Member

    May 3, 2013
    557
    92
    You can use a pair of 1N007 diodes with their Anodes as Pin 1 and Pin 7 with the cathodes joined at pin 3.

    Yes, a resistor is advisable in series with Pin 3 to reduce the turn on surge.
     
  3. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,500
    380
    hi Wyl,
    A major problem to watch out for when replacing a valve/tube rectifier with a solid state rectifier is the much higher rectified voltage you will get at on the capacitor smoothed DC rail.
    You will need to have a series resistance equal to the valves internal plate resistance in order to drop the DC rail voltage to its specified value.
    Also consider the resistor wattage.
    E

    EDIT:
    I should also point out that the full DC voltage will be placed on the other tubes and circuitry long before the tube heaters have warmed up.!
    You may need a power thermistor is series with the equiv plate resistor.
     
    Last edited: Apr 30, 2015
    absf likes this.
  4. #12

    Expert

    Nov 30, 2010
    16,257
    6,757
    You are going to end up with a nice fat resistor, or 2, like about 10 watts. (Page 2)

    You are going to be rectifying more than 100 volts extra B+ compared to what the tube does. That's why you need a resistor (or 2) in series with this circuit.

    That extra 100 plus volts is going to hit the filter capacitors long before the power tubes warm up and load down the B+ voltage. Sometimes this makes capacitors go, "pop", and that's why Eric said to use a thermistor to delay the arrival of full voltage.

    Be careful, and measure your voltage results while wearing a protective face shield. You can get into trouble redesigning this kind of power.
     
  5. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    the resistor will not protect the filter caps till the heaters warm up in the amp. I have had this happen when changing from tube to solid state rectifiers before. there is a company that makes tube substitutes called "tubesters" which have the internal compensation. by the way, that rectifyer in the second post is supposed to be a 1N4007, not a 1N0007.
     
  6. Ramussons

    Active Member

    May 3, 2013
    557
    92
    yes :oops::oops:
     
  7. neonharp

    New Member

    May 10, 2015
    13
    2
    Well , that can be done , see some of the of the other posts here about some of the things you need to consider when substituting silicon diodes for tubes . What I would do would be to just get a new tube ! they are available for purchase for about $12 US .
    Here is one vendor : http://www.thetubestore.com/Tubes/6CA4-EZ81-Tube-Types/JJ-EZ81-6CA4
    There are others
    Cheers , take care , and have a good day !!!
     
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