• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

filaments power supply

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two questions for you experts about filaments voltage:

1- is it necessary to stabilize it?

2- is it convenient to have a "soft start" system or is it not relevant?


Remember to add up your heater supply for your power tubes. I think when you look at the current required you might be better to use AC for the power tubes and DC for the gain/driver stage. However it depends on which power tubes you are going to use and if you are using Single Ended or Push pull!

Without a soft start inrush current is very high!

Regards (Not an expert!)
M. Gregg
Last edited:
Thanks for your reply Mgregg,

...In fact if I have to supply all amplifier filaments with stabilized voltage I should need of a 8A power device! It means something like six 7806, or alternatively LM317 (but in this case I don't know if more devices in parallel is possible)...
Using voltage regulator just for preamps would be easier of course, but I would not compromise in this way the signal quality on the output stages using AC voltage...
what would be pros and cons of AC and DC main?

Disabled Account
Joined 2010
The sound difference between AC and DC heater supply is difficult to explain! AC heaters can sound more open than dc. However DC I think sounds better on gain stages, it also reduces noise "Hum". It is a personal choice.

If you try to power your output tubes with DC you will have a lot of heat in the amp and large heat sinks. If I was going to power an output stage with DC, I would either use separate regulators for each tube or pair of tubes. Ripple is going to be a major problem. If you work out the wattage dissipated in each reg. Calc Volt drop across reg times current you will see how much heat and wasted power you will have!

I have seen people that use DC Output heaters use a relay circuit that changes the polarity on heaters at each start of the amp. I have never looked into this, it is suppose to reduce stress on the filament. It would be interesting to hear if others think there is any merit to this or if it is just a myth!

You will need very large caps in the supply. So as I said DC for gain AC for Power.

You will need some kind of inrush limiter or watch your house lights dim at start up and power switches weld. A timed relay contact across an inrush current suppressor for each 4 output tubes is O.K.

M. Gregg

Disabled Account
Joined 2010
yes the tube heaters draw 8 amps, but the voltage is only 6.3 so the power is what, 50-60 watts, an incadesent light bulb draws more.

I agree it is O.K. to power from D.C, however a 60W light bulb in an enclosure is hot. Also it is based upon Volt Drop across the reg. so you don't want to start with a high initial heater supply voltage. Then you get ripple problems.:)
Well, 50-60 watts of continous drawing is not so low to dissipate...
Anyway the major problem would be to find 8A regulators or to build circuits with separate regulators.
I think Mgregg has raised up relevant points about DC maining for filaments, particularly about power tubes...
So seems that really the best solution would be the "mixed" one: DC for gain and AC for power, or alternatively unregulated DC for everything...
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