Combination DC and AC Filaments?

[Mandopicker]

I have studied up on the opinions of DC filament supplies, (pros-cons)and also have experience using them in SE amp designs with great success, but I have not used it in a PP amp. My question relates to the idea of using a combination design with a PT that has a CT'd 6.3 volt supply. So...

If I was to design a PP amp and wanted to use a DC Filament supply for the preamp tubes and AC filament supply for the output tubes...assuming I have a Power Transformer with a CT'd filament supply...do I leave the CT grounded? (for the power tubes?)

I would be using a bridge rectifier (25a) self contained for the preamp tubes with its own filtering (9400uf x 35volts); which would have its own ground through a .1 630volt cap.

Or given this setup, does the ground at the rectifier assembly act as the CT for all tubes if the main CT is lifted?

Any thoughts would be appreciated.

[Robert M. Martinelli]

Hi.
I think the arrangement which should better fit the bill would be a well-smoothed DC for the preamp tubes and AC for the power tubes.

The rationale for this is that P-P output stages are hum canceling by design, that's the reason why the +B for P-P doesn't have to be as smoothed as the preamp +B. Whatever residual ( heater ) hum you could have ( e.g. in case of severely mismatched output tubes "ruining" the P-P symmetry ) could be killed by adding a hum balance pot, an inexpensive, useful and easy-to-perform mod ( of course this does not help in case of hum induced by bad lead dressing or unwanted electromagnetic coupling between stages ).

Should you add the hum balance pot, make yourself sure you disconnect the 6,3VAC CT from GND, or that pot could go south for a long vacation....

HTH

Best regards

Bob

[mac1amps]

FWIW, I thought I'd add that it's somewhat cheaper to go with (2) 100 Ohm resistors to ground (from each 6.3 VAC winding, and yes, still lift the CT). With the 100 Ohm pots, there's a tendency to have to keep re-adjusting the pot for that "sweet spot" of minimum humm. Also, in the case of a heater/cathode short in the output network, you burn up a couple of 45-cent, ½ Watt resistors, as opposed to a $3 potentiometer.

[Mandopicker]

So to clarify, if I use the combination Filament method...regardless of whether I use the balance pot for the output tubes...ungrounding the Filament CT tap is necessary. (Keeping in mind the bridge rectifier will be grounded to the chassis.)

Thank you so much for your ideas and insights.

[mooreamps]

I use a separate filament power transformer for the power tubes ; and elevate it to 100 volts dc. I still twist the wires very tight using a hand drill, and route this wiring away from all other wires. I believe the using of ac for power tube filaments helps avoid the in-rush current on cold start up, and relieves a good bulk of the filament current off the dc supply for the preamp tubes.


I have read you don't need a ground for the filament power, if the power amp is P/P. I've never wired one up that way, I always tie my filament power supplies to ground regardless of what the project is.







-g

[defaced]

Why elevate the power tube heaters? I have always read that elevating the heaters only reduced cathode coupling and the heater to cathode voltage in CF stages.

[mooreamps]

Why elevate the power tube heaters? I have always read that elevating the heaters only reduced cathode coupling and the heater to cathode voltage in CF stages.

The high static dc voltage helps encapsulate the electro magnetic radiation from the wires, and from the filaments. I still wind the wires very tightly, mostly as it helps keep the dressing inside the chassis from looking to wild.

[defaced]

Thank you for the explanation. I will be giving this a go on my new build.