Announcement

Collapse
No announcement yet.

Harvard 5F10 voltage check

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Harvard 5F10 voltage check

    Just completed, I think, a Harvard build, bringing it online the voltage maybe normal, but want to double-check with the experts here.

    The power transformer is a Mojo757EX, and the voltages are very close to spec.
    Heaters: 6.3V
    Yellow: 5.19V
    Red: 352V to ground on each.
    Brown: 50V

    For the fixed bias, a 50K pot is inserted where the 56K resistor would be. A 10K resistor was put in series.
    -27V bias was dialed in.

    For info, the output transformer is a ClassicTone 40-18022.

    After setting the bias voltage, I inserted the rectifier, NOS RCA 5Y3GT. No other tubes.
    B+ 480V!

    The original layout has B+ at 305V, and I was expecting ~330V..

    Is B+ 480V correct since no other tubes are in the amp? Therefore OK to continue?
    Or is there a potential problem that needs correcting? All the resistance measurements are OK..

  • #2
    Answered my question - with tubes:
    B+ 375V - 373V - 299V - 175V, going down stream.

    Since the heater voltage is 6.3V, I'm thinking of adding a couple or more zener diodes in series at the center tap.
    What is a good operating B+ for a 5F10 circuit? 360V? lower?

    Comment


    • #3
      Those voltages look right to me. It's just the wrong PT for a Harvard. That isn't to say it's the wrong PT for a "Harvard like circuit". It just won't be a Harvard. You can expect a Vp of about 380. Safe for modern 6V6's so no problem there. Can you say "Hey! My Harvard is loud enough to gig with!"

      You probably want to bump that bias voltage with the higher Vp. The 50k pot/10k resistor bias circuit mod will only allow you to dial in a hotter than stock bias. That won't work. Replace the 6.8k series resistor with a 1k and change your 10k series resistor to a 33k. You'll probably need -40V +/-
      "Take two placebos, works twice as well." Enzo

      "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

      "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

      Comment


      • #4
        Originally posted by pbiagi View Post
        Answered my question - with tubes:
        B+ 375V - 373V - 299V - 175V, going down stream.

        Since the heater voltage is 6.3V, I'm thinking of adding a couple or more zener diodes in series at the center tap.
        What is a good operating B+ for a 5F10 circuit? 360V? lower?
        A good operating voltage for a 5F10 is 305Vp. Probably more like 340Vp with modern AC mains. After that it's not really a 5F10 anymore, is it? Lower voltages have an affect on the tone and feel of the amp. If you're after a particular sound you should get as close as possible to the voltage of the amp you're trying to replicate. If you're not chasing a specific sound, and you're just asking for safety reasons, you're fine as you are with no need to lower the voltages.
        "Take two placebos, works twice as well." Enzo

        "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

        "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

        Comment


        • #5
          Thanks Chuck for the responses.

          The primary concern was safety and not ruining something.
          I would like get close to the Harvard sound, but am not really sure what the differences would be ... I can hear some experimenting in my future.
          I'll probably add a couple zener's but from memory that will take the B+ down 10 or a little more volts, nothing huge. I suspect they stiffen the response but that's a guess.
          I really like the idea of being able to tow this amp on the Paris metro, so gigging would be great! (Parking is hard.)

          Silly question - would a cheap? step-down transformer for the red winding make any sense?
          Or is that just getting weird - best to replace the transformer? (although this one was expensive)

          It's late here, so will be new year's day or after before I go further.

          Comment

          Working...
          X