Elevated heater voltage on cathode follower

[Alex/Tubewonder]

Circuit?

It's basic, rectify your heater voltage, slam a massive 22000uF cap on it. regulate the DC to 6.1V using a 200mV drop out speced LDO Voltage regulatro, connect it's 0V output to the DC offset of your choice. Running heater at 6.1V is not a problem but if you're of anal persuasion you can always squeeze 2-3 additional turns onto the heater winding on your PT to build up more margin and regulate DC output to 6.3V within +-5mV. Don't forget to fuse the heater winding. You only want/need to use DC heater supply on the input stages. The rest can be fed AC from this same winding but don't try to refer the AC to GND.
Some customers don't like it. They don't trust the pilot lamp and need to hear hum & buzz to be sure the amp is ON.

[Chuck H]

Running heater at 6.1V is not a problem but if you're of anal persuasion you can always squeeze 2-3 additional turns onto the heater winding on your PT to build up more margin and regulate DC output to 6.3V within +-5mV.

Unless I'm mistaken, you lose a little current capacity when you rectify? And I know that heaters will drop with wire length too. In the end, if your running a bunch of tubes and using 22ga. wire, you could end up off tolerance. How does one "squeeze 2-3 additional turns onto the heater winding"? I've never attempted transformer surgery. But I often use Hammond PT's which are wired for a 115V mains voltage. And is a little high on top of that. Filament voltage off load is about 7.2V. So, just use Hammond iron for the PT and your good to go. You could do a lot worse. I've never had a problem with any Hammond iron. And I've abused them (accidentally) in ways that are known to toast normal stuff.

[Alex/Tubewonder]

Yes, you lose ca 30% of current capacity when you rectify AC. But I'm proposing to run DC heater supply on input stage(s) only, that would normally be not more than two 12A*7 type tubes drawing 0.6A.
As for winding additional few turn of heater winding on the PT. If the window in the core is not filled to the brink there is always room to wiggle in extra wire in without taking it apart. You don't need to use magnet wire, a 1 sqrmm stranded cable will do, you need only 3 turns.
I do this only occasionally when there's no other option. All my PTs are custom wound to my specs with a 7.5V heater winding that has a 6.3V tap, voltages spec's under full load.
Another way: if you're using SS rectifier and the PT has unused 5V/3A winding: connect it in series with 6.3V so you 11.3V AC, rectify & regulate down to12-12.6V, run preamp on series connected heater (don't use pin 9). You still have the 6.3V AC to run your power tube heaters.
As they say, there's more than one way to skin a bear (who'd want to skin a cat is beyond me).

[Cygnus X1]

I have one design where the filament is elevated by tying pin 7 to pin 8 on a 6V6 (or 6L6) socket.
Cathode biased single ended.
The issue I run into is that the filament circuit also has a center tap.
If I just use the conventional center tap to ground...it is too noisy.
And if I do not connect the CT to anything it is also too noisy.

And when I elevate plus CT to ground, then I end up with voltage on the ground circuit but it is quiet.
It plays hell with other powered devices connected to the amp.

Can I use a cap to the CT or something to keep voltage off the ground?

[Alex/Tubewonder]

It's either or, not both connections simultaneously.
Are you familiar with a hum balance adjustment with a potentiometer across filament supply and pot wiper to ground?
Well, skip wiper to ground and connect pot wiper to pin 8 on your power cathode biased pentode. The cathode reistor should be amply decoupled.
Adjust the pot for lowest hum. 500 Ohm/0.5W trimpot will do.

[Cygnus X1]

Thank you Alex.
I will try it.

[Alex/Tubewonder]

But remember to NOT connect heater winding CT to ground, just leave it alone.