Ultrasonic heaters

[DHT112A]

I made a couple of these and they worked quite well during initial build and installation. When I finished the amps and hooked them up (monoblocks) they both developed a static noise. I have traced it to the ultrasonic heater. I cannot seem to remove this static noise. Does anyone have any ideas what may be causing this and how it can be remedied. Other than the static there is no hum and the amps sound very nice.javascript:smilie('')

[cogsncogs]

I made a couple of these [snip]

A couple of what? And just what is an ultrasonic heater?

And... welcome to diyAudio forums!

Cheers
Wayne

Edit: By ultrasonic heater do you mean this?

www.pmillett.com/hf_fil.htm

[alexmoose]

Hey guys, I just read what an Ultra Sonic heater is (through the link) but why would you need one? what affect does it have?

[wfmali]

Hmmm, guess Pete knows what he´s talking about, but honestly, I also don´t get it. You would still need the iron... I used switching power supplys to heat up my latest monos with 4xEL34 each, switching freq is 135kHz - I had to pay even more attention on how to run the wires to not get artifacts of that freq coupled in... but other then that, it works really good and saved quite some pounds....
Cheers,
Marcus

[DHT112A]

Ultrasonic heaters to heat the filament in a 300B

[kevinkr]

I assume your ultrasonic heating scheme is generating emi which is getting into other parts of your amplifier circuitry. How well shielded is the generator circuitry and how clean is the waveform? You probably don't want a squarewave for this purpose. You could low pass filter the generator output with some inductors and caps just above the fundamental to clean things up.

The P. Millet article drew some interesting conclusions from the experience, in terms of noise and sonics the results were totally inconclusive in comparison to dc heating. Additionally getting the filaments to heat evenly was a problem due to resonance effects in the filament. (He was heating at a couple of MHz)

Another thought, is this noise audible with all sources, no sources or just digital. (Think beat notes here.) What is the output frequency of your generator design?

I frequently use constant current heating and usually add a couple of large chokes in series with the filaments in my designs. (Several mH minimum) I don't currently use common mode chokes although some do. I also add several thousand uF on both sides of the pair of chokes..

Good luck.

[Giaime]

Hello Kevin,

but...

heating DHT tubes with DC is possible, but as I understand there will be a different DC potential across the cathode, which is not good, some parts will be worn out much faster than others.

I don't understand yet why RCA and other companies said in their datasheets that for example the 2A3 tube could be used in DC or AC heaters, without any significant performance modification: I could be wrong, but I think that this DC difference in parts of the cathode can strongly affect tube parameters...

[kevinkr]

Hi Giaime,
In practice there doesn't seem to be much discernable difference, the grid bias voltage is a little different for dc heating vs ac heating, but not much.

I have been running 300B's on dc for years, and have never had a filament problem with WE, SV, JJ, Vaic, Valve Arts or even Shuguang. I've built dozens of 300B amplifiers, both SE and PP.

I have a pair of 300B PP which have ac heating, as long as the tubes are highly symmetrical in construction and well matched they don't hum, otherwise they do.

I run 2.5V dht's (output tubes only) on ac primarily because I am lazy, cheap and I have not noted serious hum problems doing so.

It is pretty hard to get quiet operation with 5V, 7.5V, and 10V dhts though, so all of these designs have dc heating.

Some argue that dc heating sounds worse, but I have not noticed any significant difference except for lower hum. I do recommend constant current heating for 300B's although this must often be tailored to the specific brand of tube, and prevents ready interchange of tubes without modifications to the current sources. (Unless of course you make them adjustable.)

[poobah]

Giaime,

As far as uneven wear on the cathodes is concerned, the electrons are in a cloud around the cathode. They are held in place by the grid. As long as the tube is used within its limits... I would ignore any stories about uneven cathode wear.

[Giaime]

Dear friends,

thank you all for your answers. Please note that my objections aren't derived from reading what tube gurus say on the net, it's just a physical observation, am I an engineer or not?

But...

electrons are in a cloud, so for normal operation there will be no difference between AC or DC heating.

But when the cloud is momentarily gone, as in a strong positive peak signal, maybe overdriving the tube, there will be a finite time constant for the electron cloud to be re-created, and no matter how little it will be this time, it surely (if I understand well) higher if I heat the cathode with DC: because heavy electron emitting areas are reduced.

Could this be a problem, let aside premature cathode wear (maybe an issue for old NOS pricey tubes)?