HF AC DHT Filament Supplies

Ray P · Post by **Ray P** » Sun Feb 17, 2013 8:47 am

I know high frequency AC supplies for DHT valves has been discussed here, and elsewhere, before but haven't been able to find much in the way of practical solutions, the nearest would seem to be this;

http://www.pmillett.com/hf_fil.htm

Alternative routes such as DC are more common; I know Tentlabs do a module and Andrew has mentioned something in the indirectly heated filament supply group buy thread.

Unfortunately, DC isn't an option in the amp I'm planning to build and given the possibility of some audible hum being present I would like to explore the HF possibilities further. I will need to supply four 300Bs and, due to the amp config that needs to be 10V @ 2.5A.

Does anyone have any pointers.

Ray

Andrew · Post by **Andrew** » Sun Feb 17, 2013 9:33 am

Why is DC not an option, Ray?

I have a prototype DHT Heater sitting in front of me on the bench as we speak.

Andrew

Ray P · Post by **Ray P** » Sun Feb 17, 2013 11:12 am

It's the topology of the circuit Andrew; its an OPT design using four 300Bs in a novel configuration.

Ray

Andrew · Post by **Andrew** » Sun Feb 17, 2013 11:39 am

Ray P wrote:It's the topology of the circuit Andrew; its an OPT design using four 300Bs in a novel configuration.

Ray

Hmm, I'm curious now, can you show me a sketch of that part of the circuit?

Andrew

Ray P · Post by **Ray P** » Sun Feb 17, 2013 12:10 pm

Andrew, I'll PM you some more info a little later; just been given my next task!

The design is in Bruce Rozenblit's book 'Tubes and Circuits' so I'm reluctant to be seen to be giving away his IPR on an open forum.

Ray

chris661 · Post by **chris661** » Sun Feb 17, 2013 12:14 pm

Hmmmm...

I know its solid-state, but...

If you used a 555-timer as a frequency generator (I'm told its possible), then connected its output to a chip amp of some kind (one that'll do 25w@4ohm comfortably - LM3886?), that'd do the job. You could pretty much use any frequency you like.

You'd need a dedicated power supply (needn't be massive) for the chip amp, and you're set.

Chris

Mike H · Post by **Mike H** » Sun Feb 17, 2013 12:37 pm

Ditto!!

- EDIT: was meant to be a follow-on from Andrew's post

Crikey that's elaborate (Peter Millet's)

Coincidentally I have recently being toying with the idea of a buck converter DC supply using MC34063A, basically what's on page 8 here -

http://www.rapidonline.com/pdf/82-1088e.pdf

May need some extra external power transistor help depending how much current you want or to allow a safe margin. I figured a series resistor will help limiting initial current surge while filament is cold, ergo actual V-out is filament Volts + resistor drop.

What I like about it is much less heat generation so no big heatsinks, I mean to say this little 8-pin DIL chip can do 1.5 Amps on its own!

Mike H · Post by **Mike H** » Sun Feb 17, 2013 12:50 pm

PS: I meant of course one buck reg per filament as the converter chip is < £1 each.

PPS: only needs one inductor, and I was thinking of this sort of thing:

http://www.rapidonline.com/Electronic-C ... ctor-31000

andrew Ivimey · Post by **andrew Ivimey** » Sun Feb 17, 2013 1:47 pm

a 555 is square wave - would it matter? I'd have thought iti might.

There is an 8038 (I think) IC which gives you sine, square and sawtooth outputs - pr'aps the sine wave would be best?:

chris661 · Post by **chris661** » Sun Feb 17, 2013 2:04 pm

andrew Ivimey wrote:a 555 is square wave - would it matter? I'd have thought iti might.

There is an 8038 (I think) IC which gives you sine, square and sawtooth outputs - pr'aps the sine wave would be best?:

Ohh-er, didn't realise.

I suppose an RC filter would take off the HF mush, but if there's an IC that'll do it better, go with that IMO.

Cressy Snr · Post by **Cressy Snr** » Sun Feb 17, 2013 2:48 pm

Maybe I'm missing something here, but we go to great lengths (or we should) to keep RF out of our circuits. Yet here we are talking about injecting AM radio frequencies straight into the cathodes of directly heated triodes.
What a strange world.

BTW this is not meant to be a slight on HF heater supplies or anything else for that matter. I'm just curious, thats all

Mike H · Post by **Mike H** » Mon Feb 18, 2013 3:38 pm

Long time ago someone e-mailed me saying he'd had great success, and had I tried it. (I hadn't.) 100 kHz I believe it was.

Dave the bass · Post by **Dave the bass** » Mon Feb 18, 2013 4:04 pm

Andrew wrote: I have a prototype DHT Heater sitting in front of me on the bench as we speak.

Andrew

Quick question from the chubby kid looking at couple of DHT's....Are you hoping to do a kit for the DHT heater board MrL? If yes, what sorta filament voltage and current would it be able to handle?

DTB

Andrew · Post by **Andrew** » Mon Feb 18, 2013 6:55 pm

Dave the bass wrote:
Andrew wrote: I have a prototype DHT Heater sitting in front of me on the bench as we speak.

Andrew
Are you hoping to do a kit for the DHT heater board MrL? If yes, what sorta filament voltage and current would it be able to handle?

DTB

Yes, I am, I need a few and so there's no sense in not doing it as a group buy, what VA do you need?

Andrew

Dave the bass · Post by **Dave the bass** » Mon Feb 18, 2013 8:10 pm

Andrew wrote: Yes, I am, I need a few and so there's no sense in not doing it as a group buy, what VA do you need?

Andrew

0.00000001W up to 1234567890MW....... thats fair enough innit!

Meanwhile back in reality, maybe a board that can be adapted to work with everything from a 26 up to say2A3/300B types?

DTB

audio-talk

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