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Decoupling DC heater supply between valves..benefit?

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Hi all,

Think of how you typically decouple preamp stages in a valve audio amp using RC networks set to an appropriate time constant.

Now if you were to apply that to the heater supplies of preamp valves, for each valve, do you think there would be any benefit sonic or other?

Never really thought of any mutual effects of heaters in a power feed chain, irrespective of how clean the DC feed is (regulated or not).

I haven't seen this concept used in any amp and think it would be fun to see if it could lower a noise floor, improve linearity, reduce crosstalk artifacts etc. Would cathode to heater capacitance have any bearing on the common supply?

I've discounted AC heaters even with balance pots as most preamps I think sound better with DC. Yep, controversial comment I know :)

Any thoughts or ideas?

6J6.
 
Hi Osvaldo,

DHT's would definately benefit the most from this idea, as obviously the heater/filament is also the cathode.

I was mainly referring to indirectly heated valves which I use for everything, and especially about heater to cathode capacitance interacting with the supply and other valves.

I think there is validity for both. Can't understand why I haven't seen it on any schematics..

6J6.
 
I believe definitively is not necessary to decouple them. At audio frequencies, capacitive coupling is small because of the low capacitance and low frequencies. So coupling is very very poor. And in case of high frequencies, at RF levels, it is normally no necessary to use DC at heaters.

But, no think implies that for security or simple pleasure you want to add decoupling, I don´t see any trouble.
 
The 6AK5 was designed for UHF amplification up to about 400MHz. It doesn't know that you only want it to work up to about 20kHz.

Failing to decouple the heater supply is unlikely to cause oscillation, except in some RF circuits, as the heater is mostly hidden inside the cathode, but it might spread the consequences of an oscillating valve. The solution is to stop the valve from oscillating by adding stoppers and ensuring that the cathode has a low inductance path to ground.
 
Thanks for all the interesting replies, some good points to think about.

Seems that with DHT's it's a definate benefit, and perhaps oscillator tubes used in audio. I'm thinking of my favourite little cheap small signal tube the 6J6, which started life as a mixer/oscillator. It's indirect but I guess we shall see if it makes any difference.

For the sake of a cheap capacitor, why not :)
 
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