Hiwatt DR504 adjustable bias

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Mattbedrock, Feb 25, 2012.

  1. Mattbedrock

    Mattbedrock Silver Supporting Member

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    Gentlemen - I have a Hiwatt DR504 build going on (thread over on amps & cabs). I am building to the Mike Huss layout but want to change to adjustable bias.

    Here's the schematic:

    [​IMG]

    The baseline is a 27K resistor to ground. I changed to a 10K resistor and a 25K pot. Measured, I can get between 10K and 33K of resistance to ground. I hope that will get me within a reasonable bias range. Here's a picture of the build in process. I am still a ways away from completion.

    [​IMG]

    Am I on the right track? Please and thanks.

    Matt
     
  2. Stonecutter

    Stonecutter Member

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    Yeah, that should work. Just set it to the 33K setting when you first install the tubes. Tweak the 10K up if the tubes pull too much current, or down if they don't pull enough.
     
  3. gldtp99

    gldtp99 Member

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    His name is Mark Huss-- your bias pot plan should work just fine
     
  4. Mattbedrock

    Mattbedrock Silver Supporting Member

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    Oops, sorry Mark.

    Thanks for the input.
     
  5. gldtp99

    gldtp99 Member

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    When building a fixed bias amp i always test the bias circuit before i install any output tubes---i test the bias circuit for DCV (make sure the expected neg DC voltage is present) and sweep with DMM--- usually this is done on "Standby", as most fixed bias circuits operate without B+ voltage present in the rest of the amp, or most of it depending on the amp design----and it's safer this way--- safer for me and safer for the output tubes.......................gldtp99
     
  6. donnyjaguar

    donnyjaguar Member

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    I made one of these. The bias circuit on Hi-Watt is inferior with high ripple content. You'd be better to replace it with the Marshall type, IMO.
     
  7. Mattbedrock

    Mattbedrock Silver Supporting Member

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    I decided to change the fixed resistor to 15K and the pot ended up measureing closer to 20K than 25K. I was getting 33K+ resistance to ground in total.

    Ok, well, I fired up the amp for the first time today. No one was killed. That's the good news.

    I measured -39VDC on pin 5 of the EL34's from my bias circuit. When I checked bias with my biasrite, I was cooking at 462 plate volts and 59ma bias - about 100%! Tubes were glowing pretty red, so I shut her down. Hope I didn't do too much damage to them.

    I guess I'll switch up to 27K on the fixed resistor and see if I can get the bias down into the 30's. Wow - thought I'd have plenty of range with 20+15K.
     
  8. donnyjaguar

    donnyjaguar Member

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    Sometimes I think I'm talkin' to a wall....
     
  9. Mattbedrock

    Mattbedrock Silver Supporting Member

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    Donny -
    My apologies. I did read your post and thanks for the input. I was not in love with the Hiwatt bias circuit either. In terms of ripple, I was comfortable that the 100uF/150V cap to ground I was using would clean up any leftover A/C that had leaked by the diode. But my intentions were to build a fairly representative version of am early 70's Hiwatt. The resistor value (27K) and resulting bias of the original Hiwatts were likely a combination of the Partridge transformers and the exceptionally stout Mullard EL34's of the day, of which I am using neither. The increased resistor values that I ended up using allowed me to get the bias values into a safe range for today's EL34's, (not to mention 122V A/C at the wall).

    In fact, I am not a fan of either Marshall or Hiwatt bias circuits. I am used to the Fender style of a dedicated PT tap for bias that supplies a voltage nearer to the real need, rather than ratcheting down the main output through resistors.

    Anyway, the amp sounds great and I am happy with the results. Again, thanks for you input - I do appreciate the help that tgpr's are always willing to share.
     

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