Power a circuit from 6.3v heater tap?

[therecordingart]

So I'm going to be building up a couple of Firefly tube amps and powering them from a Hammond 269EX. That xfmr provides a 6.3v AC @ 2.5A winding for the heaters. I'd like to wire up a rectifier/regulator in parallel with the AC heater supply so I can power a low voltage circuit in the enclosure. It appears that my tubes will need roughly 1 amp total for the heaters so current-wise I'm ok, but I'm not familiar enough with tube topologies to know if this is a faux pas or not. Will I be setting myself up for issues?

[R.G.]

So I'm going to be building up a couple of Firefly tube amps and powering them from a Hammond 269EX. That xfmr provides a 6.3v AC @ 2.5A winding for the heaters. I'd like to wire up a rectifier/regulator in parallel with the AC heater supply so I can power a low voltage circuit in the enclosure. It appears that my tubes will need roughly 1 amp total for the heaters so current-wise I'm ok, but I'm not familiar enough with tube topologies to know if this is a faux pas or not. Will I be setting myself up for issues?

Like swapping ground polarity in PNP germanium is to us, powering "just a little circuit" from the heater supply in tube amps is a common solution, and a sometimes misery.  It works often enough that people keep doing it, but sometimes its ugly.

A lot depends on how the heater winding is grounded for the benefit of the tubes. It's almost certain to be if not tied to ground, then DC references somewhere. If it's not, you can get truly ugly hum and noise in the tubes it supplies. These winding are sometimes tied to ground through a center tap, sometimes through a fake center tap of two 100 ohm resistors in series, sometimes one end is grounded, sometimes the CT, fake or not, is tied to some elevated DC voltage.

All of these treatments have different consequences to the little circuit you're making a supply for if the circuit has to connect to the tube circuit's ground.

And often, you get away with just pure luck. 

[therecordingart]

Thanks, RG! I'll fart around and see what happens.

[amptramp]

I have a power supply intended for low-power tube applications that uses two 120V / 6.3V transformers.  One steps the line voltage down to 6.3 which is used to power the heaters and the other uses the 6.3 winding as the input and puts out 120 VAC that is rectified to provide line-isolated plate voltage.  They are identical transformers in this supply, but the second one can be smaller as it does not have to carry heater current.  The heater winding can be grounded to any point in the circuit or for low-noise applications, connected to a higher voltage to reduce the number of electrons that can go directly from the AC heater (rather than the cathode) to the plate.

[PRR]

Firefly uses the now-standard heater wiring: 6VAC *balanced* with a pair of resistors.

You REALLY want balanced heater power-- then if your heat wires at tube sockets are tight and neat, most of the AC hum cancels out of the audio path.

There's not a good for-sure way to get a little DC from the resistor-balanced heater supply. Pulling rectifier-capacitor spikes out of that affair really craps-up your "clean balanced AC".

If you want a *little* DC power, like 1mA, you can tap 8VDC from the final stage cathode resistor. Use an R-C filter (1K+100uFd) to clean the significant audio off that point.

For larger power you can drop off the main B+. Say a 4mA opamp. Run 7mA down from +265VDC to say a 24V Zener. 240V to drop at 7mA is a 33K resistor dissipating 1.68 Watts (use three 10K 2W in series). Yes, ugly.

Since you have way-ample heater current, you *could* just make ALL the 6VAC to DC and heat the tubes with DC. Run the 6VAC to a bridge rectifier and *BIG* capacitor (several thousand uFd). Get nearly 8VDC un-loaded, and hopefully near 7VDC with heater load. Since that's a bit high, and one cap isn't much cleaning, add about 1 Ohm (2W) in series and then another thousands-uFd cap.

[therecordingart]

Thank you guys! I now have some things to think about.