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Diode for DC heater

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I am looking for diodes to convert an AC heater to a DC one. The current draw is ~2A and the winding is 6.3VAC, so something with minimal voltage drop would be good. Suggestions? With an old trabsformer, I just grounded the center tap and all was well, but I got a new transformer, and this is not working.

Also, if anyone has a favorite circuit for doing this that would be appreciated. I figured I'd use CRC with a couple of 10KuF caps unless someone has a better idea.
 
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Hi dsavitsk,
Hum problems? Bias your heater line up to 30 ~ 50 VDC above ground. Bypass the heaters to ground when doing this.

This eliminates any high frequency pickup from tring to rectify and filter your heater. The transformer should run cooler than rectifying the heater line also.

-Chris
 
I have lot of 1N5822 barrier Shottky diodes for such purposes.

bc348filament.jpg
 
I'd like to try this one for a SS amp project (idle current around 1 amp) and wouldn't have a HT supply. Got a version of that which could run with just the low voltage transformer? Maybe a doubler for the bias supply? Or does the required impedance at R1 have to be too high to allow a lower voltage?

Sheldon
 
Use PSUDII and run a sim on your proposed DC heater circuit... pay close attention to the resistance of the supply winding. You will find that it is very difficult to get 6.3VDC from an 6.3VAC winding... you need a HUGE cap.

Also, you are trading an innocent sine wave in your heaters for a spectral nightmare... high frequency stuff... from Schottkys especially. The capacitance from filament to cathode begins to look like an open door at these frequencies.

Don't get me wrong... I'm all for DC heaters. I just think you need to start with at least an 8-9 VAC winding first. Using 6.3 VAC is on the hairy edge of praticality at best.



:)
 
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Hi Poobah,
I'm very glad to see you around. Happy New Year dude!

This is what I've been trying to say all along. I'll add that I used to like DC heaters, but I've seen the light an I now think AC is the best way to go with the possilbe exception of low level stages.

-Chris
 
Poobah, you are right from theoretical point of view about spectral differences, but my amps are deadly quiet. :D

Sheldon, R1 to B+ is a simple kind of a current source, you may use CCS from doubled low voltage: you need more than output voltage + 6V + ripples + drop needed for CCS to work well. In most cases you will get it.
 
Hey-hey!!!,
Half measures availed us nothing...go full L-C. A few tens of mHy, a 10,000 uF cap and you're off. For 12.6V, a 18-0-18 does it well. SB5100 Fairchild Schottky diodes( one conducting at a time, full-wave ), allows voltage tuning with a second RC stage.

I've also been pleasantly suprised when AC has worked too...which is by far the easiest to try first.
cheers,
Douglas
 
Thanks for the suggestions. Problem has been solved. I had rewired an amp to use LED biased 5687's instead of the resistor biased 6N6p's. The amp had been perfectly quiet, and upon powering up with the new tubes it was very very noisy. Turns out I did something to the ground in the rewire process which is where the noise hailed from. So, the ground is fixed and everything is quiet.

But, to ask a question relating to an earlier response, how would one go about biasing the heaters up 30V? The amp is a single tube per side transformer coupled to the output, so there are no obvious places to get 30V from.
 
You connect the center tap of the heater winding to a 30 Volt source. If you lack a center tap... you can create a virtual tap by placing two equal value resistors in series across the winding. The center point can then be used as a center tap. The value of these resistors should be kept low relative to other impedances... but power consumption/dissipation is a limiting factor.

The 30 volt source can derived from a voltage divider off the B+ as well. The same constraints apply... low value is desired but burns power.

Sorry I can't give you some practical values... but, you got the logic behind it now.

Generally, if your heaters and cathodes are within the specified range for differential voltage... relax.




:)
 
Wavebourn said:
Sheldon, R1 to B+ is a simple kind of a current source, you may use CCS from doubled low voltage: you need more than output voltage + 6V + ripples + drop needed for CCS to work well. In most cases you will get it.

Thanks, seemed like that would be the case. With a lower voltage CCS bias source, D1 becomes optional as long as the double doesn't exceed the gate specs. for the FET? I know it couldn't hurt to have it, just seeing if I understand the circuit.

Sheldon
 
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