[hagtech]

It took some time, but I've finally solved the DHT RF heating challenge.  At least to my satisfaction.  I was hoping it would eventually come to me.  The technology is called "HUSH", for Hagerman Ultra-Sonic Heating.  It's a really cool circuit that produces a high frequency sinewave with enough power for a 300B or 2A3.  Secondary coils are center-tapped so it can fit into existing amplifier designs.

I got a prototype board working, but at slightly lower power.  



The scope waveforms are the transistor collector voltage (top) and differential output voltage (bottom).  Very clean switching resonant circuit produces a reasonably low distortion sinewave.  This test circuit was running at about 40kHz.



HUSH will first be introduced in the CLARION, a stereo 2A3 SET built using my new "C" series chassis (to match Clarinet).  It looks like a real tough fit, the output transformers are huge.  This will be one extremely heavy amplifier.  Lots of iron.  The high voltage is doubly choke filtered.  Driver tube will be 6H30 (hey, it's the best).

After that, I am thinking of making a small HUSH-only board, for use in existing amplifiers as an upgrade.  But I'm not quite sure how it should fit.  Right now it operates off of 6.3Vdc, because that was convenient within the CLARION.  Is that reasonable for the small HUSH?  I'm thinking most DHT amplifiers won't have a spare supply available, except for the existing 2.5V (or 5V) windings.  Is it best to include a separate power transformer?  What is best topology that would be amenable to retrofits?  Oh yeah, single HUSH will be able to drive two separate tubes.

jh

[andrew]

Hi Jim,

  I'd go for a separate HUSH-only board, saves me figuring out the problem - it was on my list - so thumbs up as far as I'm concerned.

 As a far as the supply goes, I think  a separate TX would be the way to go, unless you can figure out how to re-use the existing supply, which would be much better. I would have thought that even if an amp has an existing 6.3v supply for the input/driver valves, in most cases you be hard pushed to get enough oomph out of that supply to drive two DHTs and the DC on the small signal heaters as well.

 Thinking about this some more, the best solution would be to definitely reuse the existing supply - that makes the HUSH a drop in solution - and more attractive to the potential purchaser as they have to buy less expensive ironwork. Though it might be best to cater for variable input voltage (up to a point) and several common output heater voltages - if that's at all possible.


  Oh, and can we have an option of one DHT per HUSH for those with monoblocs?

  So not much really then