running multiple heater supplies in parrallel

[black_labb]

for my big bass amp i am running 2 power transformers. one with 2x5A of 6.3v and one with 2x3A of 6.3v. i am thinking of running the 2x5A and 1x3A to the power tubes and the second 3A elevated to the preamp. is running the filament supplies from different trannies in parrallel a bad idea.
my understanding is one voltage may drop so the other provides more current to compensate, but i'm not sure if this is dangerous or not. the transformers are both torroidial trannies from the same manufacturer, the one for the grid's and preamp is a lower voltage and VA rating than the one for the plates.




on a unrelated note, i was thinking about how some amps run the heaters as cathode resistors. if the amp needs the heater heating to start conducting, how do the heaters heat if the tube isnt conducting yet due to the heater off? do they include some circuitry to get the heater going on startup?

[Glasgow]

You should be fine running parallel heater circuits if what you mean is sending one 6.3v heater winding to some tubes and another 6.3v winding to other tubes (given you don't overrate any winding). You wouldn't want to hook multiple windings in parallel to individual tubes.

if the amp needs the heater heating to start conducting, how do the heaters heat if the tube isnt conducting yet due to the heater off? do they include some circuitry to get the heater going on startup?

I am naive to the technique of using the heater for a cathode resistor. Please, someone describe its merits and pitfalls.
As far as the heaters getting to go, they don't need the tube to be conducting to warm up. Only the cathode-plate path needs them to be hot to conduct. In other words, it is not a two-way dependency.

Brian

[black_labb]

You should be fine running parallel heater circuits if what you mean is sending one 6.3v heater winding to some tubes and another 6.3v winding to other tubes (given you don't overrate any winding). You wouldn't want to hook multiple windings in parallel to individual tubes.



I am naive to the technique of using the heater for a cathode resistor. Please, someone describe its merits and pitfalls.
As far as the heaters getting to go, they don't need the tube to be conducting to warm up. Only the cathode-plate path needs them to be hot to conduct. In other words, it is not a two-way dependency.

Brian

the cathode resistor/filament design isnt very common and doesnt seem to be used these days. the advantage is you dont need a filament supply and the power for the cathode resistor isnt wasted as it is used to heat the filaments.

just realised i can use the second heater supply into a voltage doubler to run a fan, and 2x5A will heat 6 ky88/6550's fine.

[Glasgow]

Oh,

I think I see the chicken and egg dilemma now: The heaters wouldn't have a separate supply and would be heated by the cathode current, but the cathode current wouldn't flow without being heated. Hmm, I wonder if by sacrificing cathode longevity and using a high enough plate voltage, you could suck enough electrons from the cold cathode to get a reaction going?

Anyone have any insights?

Brian

[Enzo]

In all the cathode resistor as tube heater circuits I have seen, the tube is not running its own heaters with its own cathode current, if that is the confusion. What they do is run a preamp tube heater or two as a cathode resistor for the power tube. The power tube heater itself is conventionally powered. This trick will only work where the current demands of the heaters in question can be met by the cathode current and voltage in the power tube circuit.

The merit of this would be you get some "free" power for your preamp tube heater. You then need less power transformer current overall, and the preamp tube heaters are running on DC. That cathode current will be flowing either way, so why not heat a preamp tube filament instead of heating a resistor? A pitfall would be that the cathode bias level of the power tube will be dependent upon the preamp tubes selected having similar heater characteristics. It could get ugly having to specify something like preamp tubes MUST be RCA, NO GE. Though I do recall on the old Conn 6T5 12-wheel strobe tuners, in the powr oscillator circuit that drove the motor, they specified ONLY RCA for certain tubes as other brands may not reliably oscillate.

[Glasgow]

Ah!

Thanks for the insightful and clear explanation.

Brian

[black_labb]

In all the cathode resistor as tube heater circuits I have seen, the tube is not running its own heaters with its own cathode current, if that is the confusion. What they do is run a preamp tube heater or two as a cathode resistor for the power tube. The power tube heater itself is conventionally powered. This trick will only work where the current demands of the heaters in question can be met by the cathode current and voltage in the power tube circuit.

The merit of this would be you get some "free" power for your preamp tube heater. You then need less power transformer current overall, and the preamp tube heaters are running on DC. That cathode current will be flowing either way, so why not heat a preamp tube filament instead of heating a resistor? A pitfall would be that the cathode bias level of the power tube will be dependent upon the preamp tubes selected having similar heater characteristics. It could get ugly having to specify something like preamp tubes MUST be RCA, NO GE. Though I do recall on the old Conn 6T5 12-wheel strobe tuners, in the powr oscillator circuit that drove the motor, they specified ONLY RCA for certain tubes as other brands may not reliably oscillate.

makes sense. thanks