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Old 27th April 2012, 11:48 PM   #1
bst is offline bst  United States
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Default What's the simplest way to adjust filament voltage?

Well, once again I've been digging through boxes of stuff in the basement. I came up with some good candidates for a new project, but also a new question.

I dug out two sleeves of NOS PCL85s, as well as a pair of Stancor 8604 filament transformers. The tube heaters each draw 17.5 volts @ 300mA. The transformers are rated at 20V C/T out, at 1 amp.

I'd like to build a "spud" stereo amp, using just one PCL85/18GV8 per channel. I want to keep the filament supply as simple as possible, avoiding DC supplies if I can.

At first glance there are two simple ways to drop the AC heater supply voltage: just put a dropping resistor in each leg of the secondary, or, use a pair of back-to-back zeners as a "clipper" limiter.

As a third option, if necessary, I could rectify and filter the 20 VAC, and use an LM317T for each tube to limit the filament current to 300 mA.

These amps will be used as low-wattage 'hi-fi' amps, so I need to keep filament hum below an audible threshold. Any thoughts on the best approach to obtain my necessary filament voltage?
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Old 28th April 2012, 12:03 AM   #2
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Easiest? Resistors.
Least noise? LM317.

Try it with the resistors first.If you have the heaters elevated,and have taken care with the wiring,the hum should be pretty low. But,if it isn't,and you want to get more exotic,rectify,filter,and regulate with a LM317.
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Old 28th April 2012, 12:07 AM   #3
tomchr is offline tomchr  United States
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In my 6LU8 spud, I used AC heating with good results. It's not the ultimate in hum-free design (you'll need DC heating for that), but it's a pretty decent amp regardless.

I'd drop the excess voltage across a resistor. The zener clamp you mention will blow the zeners unless you fit them with series resistors - but then you're back to the dropper resistors anyway.

~Tom
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Old 28th April 2012, 02:09 AM   #4
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A "gotcha" is the current capability of the heater winding.

Assuming that the RMS current capability is 1 A., end to end, bridge rectifying the 20 VAC will be good for approx. 500 mA. into a cap. I/P filter That's not enough for 2X PCL85s in parallel and you don't get the 35 VDC needed for series wiring.

OTOH, bridge rectifying and choke I/P filtering may work very well. Little if any dropping resistance will be needed to make things 17.5 VDC and the 600 mA. draw is not a problem. I make a 30 mH. choke as being enough at this low voltage. A LCRC filter, with the R tweaked, should make for a nice 17.5 VDC heater supply and bye/bye hum worries. Rated at 35 mH. and 2 ADC, the Triad C-56U seems highly suitable.
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