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Diodes Accross Rectifier Socket?

Discussion in 'Shock Brother's DIY Amps' started by Columbus, Aug 8, 2009.

  1. Columbus

    Columbus Tele-Meister

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    I recently got a brown Vibrolux. When I pulled out the chassis, there were two pairs of diodes bridging pins 4 and 8 and pins 6 and 8. Why would somebody do this and is it OK to remove them? The rest of the circuit, with the exception of replaced electrolytics and a couple of resistors looks original and matches the schematic.
    The amp also takes longer than usual to shut off after the I flip the power switch...maybe 15-20 seconds of decreasing volume. Any ideas why this might be happening?
     
  2. jh45gun

    jh45gun Banned

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    Some one took a tube socket and made a SS rectifier. It is acceptable and is done. It has been discussed here before. I suspect you can just put in a rectifier tube with no issues. Seems to me if I remember right the SS rectification runs the amp with a higher voltage on the output tubes. It also allows for a tighter bottom end. JC knows a lot more about it than I do as does some of the others.
     
  3. Columbus

    Columbus Tele-Meister

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    Thanks for the reply. I was pretty confused when I saw the rectifiers since a 5AR4 was installed when I got the amp. The amp is plenty tight for my tastes so I just removed them.
     
  4. jefrs

    jefrs Doctor of Teleocity

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    I've seen the valve replaced by silicon diodes but not seen both run together. Not sure if running both together is a good thing though.

    I've seen a broken valve base with the diodes soldered into it, so it can be swapped out. Take a dead GZ54 and hold its base in the vise, chuck a rag over it for kindness. Now take a pair of bricks and bring them smartly together on the valve. It doesn't hurt at all unless you catch your thumbs.

    The amp turn off is discharging the electrolytic caps and is still live until they have discharged through the hot output valves which will still be conducting, hence your sound. We used to have a 50's tv set that took about half an hour to shut down.
     
  5. Ben Harmless

    Ben Harmless Friend of Leo's

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    I've heard of diodes being added to the tube recto socket as "insurance" against the recto blowing, but I'm a little unclear how they're wired up such that the diodes don't provide faster rectification than the tube - essentially making for a diode recto. I haven't really taken a close look at the implementation though, being that I generally prefer solid state rectos anyway.
     
  6. Columbus

    Columbus Tele-Meister

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    The first two filter caps that came installed are rated at 40uF 500V, the other three are 33uF. Planning on replacing all of them w/ 22uF caps. Maybe the overrated caps are the cause?
     
  7. jh45gun

    jh45gun Banned

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    OK I did not know both were together though the thought of them being a safety valve (No pun intended LOL ) might make sense. I thought you were talking about a tube base converted over with diodes to make a SS rectifier.
     
  8. jefrs

    jefrs Doctor of Teleocity

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    I would replace the caps with 47µF (50) or 33µF (30) instead of the smaller 22µF, otherwise they may not hold enough charge for smoothing. Smaller caps might work for a smaller amp. But you say that the electrolytics have already been replaced, if it ain't bust, don't fix it - 20 seconds to discharge is within normal range.
     
  9. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    This is likely done to make it a bridge rectifier.
     
  10. Columbus

    Columbus Tele-Meister

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    I have no idea how long the caps have been in there and the amp is a bit noisy, so I thought I would replace them.
    The amp originally came with 16uF filter caps. Would using 22uf caps really cause problems?
    Thanks.
     
  11. jefrs

    jefrs Doctor of Teleocity

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    Check the Vibrolux schematic for the right cap values. To me 16µF seems small for the first caps. My old 10W uses 50+50µF. My Epi BC30 uses a pair of 100µF. They are there to hold the volts up whilst the rectifier is going through the mains cycle.

    Noise is not created by the caps, provided they are working but bigger ones should give quieter operation.

    To kill noise -
    Mains hum comes from the transformers and the earth, also cable routing inside. Use a single earth point, a star earth. Cables should be twisted pair and cross others at right angles to reduce induction. Is the input jack shielded? Its ground to the star earth? Transformers should be raised off the chassis by a couple of millimetres, use washers.

    Have transformers been replaced. The mains and the output should be at right angles to each other and good design places them at opposite ends of the chassis.
     
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