Elevated DC Heater Voltage

[schmidlin]

So I've decided to have DC heaters on my next pre, and also elevating them by about +50VDC.  So, does it matter which end of the heater DC I attach the elevation to?  I'm thinking attach to the neg, but neg to pos is throwing my mind into a swirl, for some stupid old-school reason. 

 

[Kingston]

How about the middle?

http://www.groupdiy.com/index.php?topic=41313.0

See how Sony C37 does elevated heaters. 100-ohm resistors wired that way also work for AC-heater biasing.

Or just bias the "negative" or "ground" and you don't even need those resistors..

 

[PRR]

> I've decided to have DC heaters on my next pre, and also elevating them by about +50VDC

Why?

With _AC_ heat, bringing the peak heater swing a few volts above the cathode voltage deters the heater-cathode leakage humm in a bad tube. Do you have sensitive cathodes at +40V? Will you use leaky tubes?

With DC heat, there's no hum to leak.

If you must DC heat, put all your heaters in series and feed say zero and +24VDC. Series reduces wiring and rectifier loss. This incidentally puts the majority of heaters positive of cathodes.

> neg to pos is throwing my mind into a swirl

That's just biggoted.

 

[schmidlin]

Why?  More is better, haven't you read? 

OK, on the prototype the max cathode voltage I have is +4 volts, so are you saying +6.3 DC to ground is elevated enough?  The tubes I am using max at a 90 volt heater spread, and I had room for elevation in the design, so....I figure why not.

Yes, no hum to leak in DC, but there is ripple, no?  I'm just using a bridge with a 10,000uf cap.

I like the series idea, but it looks like we may have this solved.  Many thanks!

 

[Kingston]

Yes, no hum to leak in DC, but there is ripple, no?  I'm just using a bridge with a 10,000uf cap.

There will be significant ripple with this method, and benefit is probably negligible compared to simpler elevated AC heaters. At worst you'll only have inserted nasty - and much more audible - 100hz+ harmonics to your audio with that rectifier.

Especially if you have a real hum problem this kind of pseudo DC might not be enough. Consider at least a CRC for filtering.

 

[schmidlin]

Thanks Kingston,

I just read an in-depth article that says pretty much the same thing.  Wish I found it earlier.

So now to punt.  I have about 2 squre inches on my designed board to get heaters quiet, now how to best spend it.  Not enough voltage (6.3 AC) to use a regulator unless I use a doubler, but then I may get too much DC.

That Sony C 37 looks like a good call, CRC and all.  Looks like I could make this fit.

I know we all want that magic-bullet, and often the real answer is try something and tweak it.  I guess I'm getting impatient and want to nail this board down.  P-to-P sure has it's advantages.  Maybe I should just elevate the AC version to say, 20VDC w/a cap to ground and run with it.

Thanks again, All!