DC heating directly heated output valves

[mikeyyy]

Here's my situation:

I want to use 4 valves in a sterio PP configuration. The heater ratings are 1.75A 2.5V each. My power tranny has a single 5V tap but multple 6.3V taps.

I am wondering that since I can't really use the 5V tap as it can only power half the valves in series, I need to use the 6.3V taps, build a DC regulator and power the tubes that way. My concern is that the tubes' cathodes are directly heated and I am wondering what the effect is of the DC regulator being directly connected to the cathode. Any takers?

 

[ErikdeBest]

I have never seen it being done, directly heated valves being fed in series. I think the reason is simple: the four cathodes will be all at a different potential, with a difference of 2,5V from one to the other. That means that if using auto bias each will be biased differently (I don't know about fixed bias...I would say that this may work, but don't know) . It has a reason that amplifiers using DHT's heaters in parallel require matched valves.

Erik

 

[andyjevans]

My preamp has DHTs in a differential pair, and the filament supply - a current source - feeds the pair in series. The CCS attaches to the mid point between the two filaments.

It works - I'm listening to it as we speak.

I haven't tried anything more advanturous than that.

 

[ErikdeBest]

Hi Eusebius

Something like this (beautiful, but incomplete) picture?

Erik

 

[andyjevans]

Exactly like that!
there's a bit of a difference in the voltage on each filament but they are pretty close.

 

[jlsem]

It has a reason that amplifiers using DHT's heaters in parallel require matched valves.

You are better off using a separate filament winding (or in your case, separate DC filament suuply) for each DHT in a push-pull output. It makes for better and easier biasing/balancing.

John