DHT Heating

[Deke609]

I'm interested in playing around with AC and DC heating on my SIIs. I use my amps with headphones, so the hum with AC heating is noticeable. So I installed the DC filament upgrade and no more hum. But some people prefer the sound of AC heated Directly Heated Tubes (DHTs). I don't yet know if I'm one of those people, but I'd like to experiment to find out, and for the fun of the experimentation.

I have two ideas - the first seems practical, the second may be pie-in-the-sky:

(1) Install a switch (or two) for switching between AC and DC heating.  Does anyone see a problem with this in principle?  I know that I'd be breaking one of the golden rules by having a lot of extra AC wiring -- and would try to mitigate that by going crazy with copper shielding.  If there is no problem in principle, what type of switching would be best: non-shorting "break before make" or shorting "make before break"?

(2) Increase the AC frequency above the audible band - this is the pie-in-the-sky idea.  But I've seen (but not heard) one example of this in a commercial tube headphone amp.  As I understand things, AC heating a DHT creates hum because the DHT, by definition, does not have a heater separate from the cathode -- instead it has a filament which is both the heating element and the cathode.  Because AC mains voltage has a frequency of 60hz in North America, there is a 60hz signal applied to the cathode of an AC heated DHT and the result is a 60hz hum (which may or may not be audible depending on the sensitivity of one's speakers/headphones).  But, if the AC voltage had a frequency above the audible range  (say 40 khz), there should be no audible hum from the AC heating regardless of sensitivity.  This strikes me as a pretty cool idea, so I assumed lots of tube DIYers would have tried it. But my search of some DIY tube forums turned up nothing.  There are commercial power supply units available, but they all supply WAY too much voltage and aren't geared to audio or electronics. Does anyone know of a resource for learning more about how this (increasing frequency of AC) could be done in a low power/voltage psu?

I'll add that most of my interest in idea 2 is for the learning/enjoyment factor. So even if it's complicated, expensive and has negligible sonic value, I'd still be interested in learning more about it and eventually taking a stab at it.

cheers and thanks,

Derek

[Paul Birkeland]

(1) Install a switch (or two) for switching between AC and DC heating.  Does anyone see a problem with this in principle? 

It's a lot of current.

going crazy with copper shielding.

That will mitigate reliability.  A non shorting switch is a good idea here.

(2) Increase the AC frequency above the audible band

Sure, you can try this.  Here's a good link for this: http://www.pmillett.com/hf_fil.htm

[Deke609]

Awesome! Thanks PB.

That will mitigate reliability.

Hah! Literally laughed out loud at that.

[Deke609]

  Here's a good link for this: http://www.pmillett.com/hf_fil.htm

@PB - I've hit a fews big snags with Peter Millett's high frequency heater supply. At the link you provided, PM has generously shared his schematic and CAD files for the board.
Problem: some of the spec'ed parts are from 20 years ago and so are now "obsolete" and no longer stocked.


I had hoped to to get a bunch of his boards made and use those, but I'm thinking this may not be workable (unless, by some miracle, I can find current production equivalents to the obsolete parts that have the exact same structure/configuration and pcb footprint -- that seems unlikely).


So I think my only shot at this requires: (1) identifying current production, functionally equivalent components; (2) rejigging, if and as necessary, PM's schematic to make use of the new components; and (3) generating a CAD or other digital pcb layout that can be sent to a pcb manufacturer.


Questions:


(1) is there a recommended resource for finding modern equivalents for obsolete parts?


(2) Is there a dummy-friendly program that will convert a schematic to pcb layout?


I got a copy of the free version of Eagle CAD to look at PM's pcb board and the idea of trying to design a complex pcb board freehand just blows my mind.  It looks like Eagle CAD has some sort of schematic-to-pcb functionality, but is there a simpler/more intuitive computer design program that you would recommend?


Many thanks again,


Derek

[Chris65]

Questions: (1) is there a recommended resource for finding modern equivalents for obsolete parts?

Pretty sure there is no panacea solution for finding modern equivalent parts. It requires due diligence, research & comparison of data. Some web searching may bring up some solutions that folks have found e.g *replacement/equivalent for xxxx*
Good luck!

[Deke609]

Thanks Chris. I was kinda hoping that that there would be an industrial need for an equivalent parts lookup database, but I suppose these days the useful lifetime of electronics is well less than 20 years, so the old stuff is just junked and replaced with new units.

[Paul Birkeland]

Pete might have some good ideas about this.  Which parts are you unable to find?

I would recommend Express PCB if you want something really easy to use to make your own board.  If you can find all the parts, there are oodles of PCB production services that can manufacture those boards for you.

[Deke609]

So far, 2 things:

(1) A power MOSFET by Fairfield Fairchild Semi, part # FDS6680

(2) Something labeled JWM8103 (I have no idea what this is) Ok - just figured out that this is an inductor, and so are the other parts I couldn't find. So no problem other than with (1) above. I'm sure I can find inductors with the right specs that will work. I just need to identify a replacement for the Fairchild MOSFET

I am reading off the PCB layout itself, because the BOM does not list many of the parts, or does so with parts codes that turn up nothing when I use them as search terms. I've since figured out how to generate a BOM directly from the schematic.  My noob learning curve seems to have a very prolonged flat beginning -- still waiting for the steep rate of learning to kick in!

[Doc B.]

There are all sorts of listings for the FDS6680 on ebay

[Deke609]

Many thanks Doc. i wrote too soon. I was searching Digikey and DigChip and came up with nothing. But Mouser helpfully lists variants for your search terms -- and there are two new variants in production. I'll compare the datasheets to ensure that the specs/size/spacing will work.


Apologies for not figuring this out myself earlier.


Thanks for all the help.


Derek