Matchless rectifier question

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by telephone, Feb 28, 2012.

  1. telephone

    telephone Supporting Member

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    Just received a 30/15 from a forum member. Upon firing it up I got no sound. Next, I noticed the rectifier was not glowing and was cold to the touch. I figured the rec. was dead and swapped in a fresh 5ar4. This time I had sound but the rec. was still not glowing, cold as well. After pondering this I finally opened up the amp and noticed a couple of things of interest.

    First and foremost, the center of the tube socket which mates to the male guide pin on the tube had expanded to the point where I could insert the tube in the improper orientation. This explains why the filaments didn't light and the tube was cold.

    Next, I noticed two pairs of diodes soldered to the recto socket. This explains why I was able to play through the amp with a non-functioning rectifier tube.

    Of course, I will replace the socket but, I'm wondering if those diodes need to be there at all. It's possible the amp had been eating 5ar4s to the point where the diodes were added as a failsafe.

    Another question; With diodes in place along with the 5ar4, which rectifier is actually doing the work?

    One other thing; None of the Matchless schematics I've been able to find show these diodes across the recto socket.

    Here's a pic

    [​IMG]
     
    Last edited: Feb 28, 2012
  2. SatelliteAmps

    SatelliteAmps Member

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    Those diodes are rectifying before the tube. I would remove them, move the B+ back to pins 4 & 6 and call it a day.
     
  3. telephone

    telephone Supporting Member

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    Roger that. Thanks.

    Ever know of any amp manufacturers to implement this setup? Does the "amp eating rectifiers" theory have any validity?
     
  4. guitarcapo

    guitarcapo Senior Member

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    I'm thinking someone either ordered it for diode rectification from Matchless or modded it. I usually use a solid state plug in if I want that. I think the idea after the mod would havebeen not to use the rectifier tube at all.
     
  5. SatelliteAmps

    SatelliteAmps Member

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    I doubt anyone ordered it that way. The only way it will work is with a tube rectifier or SS unit plugged in.
     
  6. telephone

    telephone Supporting Member

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    I'm happy to report I have rectified the situation. Diodes are gone along with the faulty socket. Quite a nice difference with the 5ar4 doing the job. While I had the chassis opened up I installed a 1M switching volume pot. Sometimes it's nice to switch out the bright cap.
     
  7. dass101

    dass101 Member

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    It looks like this mod is the one Merlin described here (the hybrid approach) http://www.freewebs.co.uk/valvewizard/bridge.html

    I'm curios what has changed in the amp tone, I have something similar in my amp and I'm contemplating modding it.
     
  8. trobbins

    trobbins Member

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    The mod inserts two series ss diodes in series with each valve diode. You still get the voltage drop across the valve diode - so the B+ level and the sag characteristic should be almost exactly the same.

    The ss diodes provide a form of failure protection. However the rectifier noise may be a bit higher at the time each diode turns off. I wouldn't call it a bad mod.
     
  9. telephone

    telephone Supporting Member

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    Here's an excerpt from an email Phil Jamison sent regarding the subject.

    "The diodes are a factory built thing. It's a simple protection circuit to keep AC from spiking and blowing fuses or the rectifier tube... The problem would often happen right after flipping the Stand By, you would get a spike and it would blow the fuse, even if there is nothing wrong with the rectifier tube. And replace the fuse and it works fine.

    This issue with the black socket is not uncommon. A whole bunch of us amp builders were getting the sockets from the same supplier. Now, I'm using a new socket that does not do that."
     
  10. wyatt

    wyatt Member

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    I use the diodes on all my builds.

    This seems to be fast becoming standard with several boutique builders for similar reasons (5AR4's blowing when swithed off standby).

    I see it's now being encourage by R.G. Keen and others as smart amp building.
     

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