First try for DC heater supply - voltage too high

NSB_Chris

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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.
 

andrewRneumann

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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?
 

Ten Over

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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.
 

andrewRneumann

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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.)
 

NSB_Chris

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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.
 

SerpentRuss

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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?
 

andrewRneumann

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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?
 

NSB_Chris

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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?
 

andrewRneumann

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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?

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.
 

andrewRneumann

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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.

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?
 

bebopbrain

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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.
 

2L man

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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.
 

NSB_Chris

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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 still think you need to ground that heater circuit somewhere.
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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