First try for DC heater supply - voltage too high

Discussion in 'Shock Brother's DIY Amps' started by NSB_Chris, Oct 11, 2021.

  1. NSB_Chris

    NSB_Chris Tele-Holic

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    I am building an amp using AnTek toroidal PT (two 6.3V 2A heater supplies). I decided to put a DC supply in for the input valve since 4xEF80 plus 3x12AX7 puts me just above 2A. I put in a simple bridge rectifier for one of the heater supplies with 4700uF smoothing cap. The AC supply with the light bulb limiter on is 6.7VAC and the DC supply is 8.5VDC (no tubes installed). For one 300mA tube draw, I would have to put a 7ohm dropping resistor but I don't think I want to mess with it. Would have to buy several more resistors to dial it in once everything is up and running.

    Thoughts from those that have implemented DC heaters and made them work?

    Guess I will go back to all AC heater supply for all tubes (with DC elevation of course).
    If I put everything on one heater supply then it would be about 2.1A assuming 300mV from all tubes. I am guessing I will have to parallel the two 2A heater supplies.
     
  2. sds1

    sds1 Tele-Afflicted

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    .
     
    Last edited: Oct 12, 2021
  3. andrewRneumann

    andrewRneumann Tele-Holic

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    That's interesting. I have a 6.3VAC 2.5A winding that runs the DC at 7.7VDC with no load (no tubes) and drops to 6.9VDC with just the DC tube installed. If yours dropped 0.8V like mine did, you'd still be 7.7VDC with the tube installed. Did you probe the voltage directly from pin 4-5 to 9?

    The only thing I can think is that my bridge rectifier is has higher forward voltage than yours. Maybe you can find a different rectifier that drops more voltage?
     
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  4. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Spitballing here.
    Can't a voltage divider on the DC supply bring it down to 6.3VDC?
     
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  5. NSB_Chris

    NSB_Chris Tele-Holic

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    Have to give that some thought. I actually did that on an LED supply but did not think of it here. With one tube, it will still have some healthy current going through one leg of the divider.
     
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  6. Ten Over

    Ten Over Tele-Holic

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    A VVR can work for DC heaters.
    Cap Multiplier Heater.gif


    This one uses the Base current for a voltage drop across the 100R resistor, but a voltage divider could also be used.
     
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  7. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Yep, a quick search suggests the voltage divider would be less desirable than a buck converter or a voltage regulator due to heat and efficiency issues.
     
  8. andrewRneumann

    andrewRneumann Tele-Holic

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    I think a series dropping resistor is simpler in this case. The dropping resistor is the upper leg of the voltage divider and the heater is the lower leg.

    I’m not sure if you would damage the tube by momentarily trying it at the higher voltage, but you might be surprised by how much having the tubes installed drops the voltage. Then you could get a more accurate read on how much dropping resistance you will need, if any. (Plus you will drop voltage across of the whole transformer by having the power tubes installed.)
     
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  9. NSB_Chris

    NSB_Chris Tele-Holic

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    I went ahead and wired all tubes on the one 6.3VAC heater leg for the first fire. Came right in at 6.3VAC under load of all the heaters. I am going to stick with that for now. Thanks guys!
    Now I have the issue that I don't seem to be getting any current through the power tubes. I will put that in a separate post.
     
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  10. SerpentRuss

    SerpentRuss Tele-Meister Ad Free Member

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    Glad you got to where you can test. A smaller Toroid could be used for the DC supply for one tube. I think the AN105, would put you right there.

    Would placing two, small-value resistors in series with that 2nd, secondary feeding the bridge, one on each leg to keep it balanced, work?
     
  11. NSB_Chris

    NSB_Chris Tele-Holic

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    Thanks! I will save that for future reference!
     
  12. andrewRneumann

    andrewRneumann Tele-Holic

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    Hey, was looking at your schematic on the other thread. I doesn't look like your DC heater winding has a ground reference. Could that be part of the problem?
     
  13. NSB_Chris

    NSB_Chris Tele-Holic

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    Everything is through the bridge. I could reference the DC negative side to ground but that should not affect the voltage level. Would it affect the noise level?
     
  14. tubejockey

    tubejockey Tele-Holic

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    I have used 7812s for a simple, cheap, and stable dc supply for heaters in the past. It works great. Rivera style.
     
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  15. andrewRneumann

    andrewRneumann Tele-Holic

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    I agree it shouldn’t affect the voltage across the heater, so it’s probably not the issue at hand. I still think you need to ground that heater circuit somewhere. 2 100R resistors in the typical fashion would work, or a humdinger. That will give you some current flowing through the bridge even with no tube and you should see 2x diode voltage drops.

    Interesting old article here that mentions a maximum heater-cathode resistance. Says 20K for a 12AX7. You currently have infiniteK. I’m guessing the concern is that the heater eventually accumulates a charge big enough to exceed Vhk max.
     
  16. David Barnett

    David Barnett Doctor of Teleocity Ad Free Member

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    If I was gonna do DC heaters, I'd regulate them.

    Otherwise, at that voltage and current, you'd need a lot of capacitance to smooth the supply.
     
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  17. andrewRneumann

    andrewRneumann Tele-Holic

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    Yes, even with 4700uF and a single tube, it is 100s of mV of ripple.

    Still that is better than 4.5Vpeak of ripple and is probably enough to drastically reduce AC current leakage and noise. No?
     
  18. bebopbrain

    bebopbrain Tele-Meister

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    To keep things quiet, I like starting with a higher AC voltage (another transformer?) and then a choke input power supply that is regulated to 6.3VDC. This avoids the current spike recharging B+ every cycle.
     
  19. 2L man

    2L man Tele-Holic

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    You could rectify and regulate 12,6VAC to 12,6VDC for three 12AX7 and they take only about 150mA each.

    Also connect two EF80 series and use 12,6VAC and all four take about 600mA. Total filament current come about 1,1A.
     
  20. NSB_Chris

    NSB_Chris Tele-Holic

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    Thanks for all the feedback. Excellent!
    If i had it to do over, since I have four 6.3VAC 2A heater supply secondaries, I could easily have used one for AC heater supply to the power tubes and then have two in series and regulate for a DC supply. Maybe next time.

    I think you are right. I abandoned the DC heater section before I found out if it had any influence on hum. If I do it again, I will regulate so that I have better control over the supply voltage.
    With this tube set, I ended up right at the 2A limit for one 6.3VAC secondary and hit 6.3V right on the dot!
     
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