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Wide range of B+ for EL84 amps

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  • Wide range of B+ for EL84 amps

    I just picked up a very clean aviation band fm radio that I was considering gutting and converting to a guitar head. It has a 200-0-200v PT with a non center tapped 6v winding which powered several tubes. I know the B+ would be too low for an 18 watt Marshall, but I noticed that the Hi Octane amp from AX84 uses a 200-0-200 PT. Are there any other common designs that use that PT? Any comments on the Hi Octane amp?

  • #2
    I was planning to use a full wave bridge rectifier.

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    • #3
      No. If you use the PT with a full wave bridge you'll end up with 475vdc-525vdc!!
      That's asking too much for a pair of EL84s.
      Also, is this an old PT that says it is a 200v-0-200v secondary or did you measure it with no load?
      And, we have no idea what the high voltage "current" rating of that PT is.
      It might be old made for a 115vac-117vac primary... and you'll find most outlets are in the 120vac-124vac range these days.
      If yes, that is going to change that actual B+ rail voltage when assembled.

      Just use it with a full wave rectifier and some good 100uF@350v filter caps on the front end of your B+ rail and be done with it.
      I'm sure you'll be OK with 6BQ5/EL84s at a little lower voltage if you make it a fixed bias amp... or you could run the amp with 6AQ5s instead.
      I bet a FW rectifier (two 1N4007s) with that PT (if the high current current rating is enough) will get you around 260vdc to 290vdc range with two preamp tubes and the power tubes running class AB in fixed bias and make a fine sounding 12-15 watt amp.
      Spend your money on a more efficient speaker!!
      Bruce

      Mission Amps
      Denver, CO. 80022
      www.missionamps.com
      303-955-2412

      Comment


      • #4
        Thanks for your response Bruce. It's a nice clean green metal box radio from 1963. I measured the AC secondaries while operating in circuit. As it sits it has a single ended power amp but I would guess that the there is enough B+ current for a PP amp. I have a PP EL84 OT in my pile of stuff. The tubes are a 6x4, 6AK6, 6AL5, 12AX7, 6BJ6, 6BJ6, 6U8, and a 6AC5. The thing looks brand new. I would probably fix it if it was collectible. Got it for nothing and am hoping to make a nice lunchbox head out of it. Maybe a Spitfire, AC 15 or 18 watt light? It's just that they seem to have higher voltage secondaries. It's a Regency MR-10b.

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        • #5
          Originally posted by olddawg View Post
          Thanks for your response Bruce. It's a nice clean green metal box radio from 1963. I measured the AC secondaries while operating in circuit. As it sits it has a single ended power amp but I would guess that the there is enough B+ current for a PP amp. I have a PP EL84 OT in my pile of stuff. The tubes are a 6x4, 6AK6, 6AL5, 12AX7, 6BJ6, 6BJ6, 6U8, and a 6AC5. The thing looks brand new. I would probably fix it if it was collectible. Got it for nothing and am hoping to make a nice lunchbox head out of it. Maybe a Spitfire, AC 15 or 18 watt light? It's just that they seem to have higher voltage secondaries. It's a Regency MR-10b.
          My first Voxy Lady amp was built with a 230vac secondary PT from a piece of test equipment and the OT from a Princeton Reverb.
          It made about 12-14 watts. Full wave SS diodes, two 12AX7s with two EL84s in fixed bias. Two input jacks, Vol, Treble Bass and Master Volume.
          Fun little amp and just ripped into a small 1x12 cabinet.
          I'm sure you can do something very very similar.
          Bruce

          Mission Amps
          Denver, CO. 80022
          www.missionamps.com
          303-955-2412

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