[sdiy] Tube amplifier question

[René Schmitz]

Hi Jürgen and Motohiko,

jhaible at debitel.net wrote:
> What really caught my attention is the way the heater current
> is created in this circuit:
> http://www.asahi-net.or.jp/~cn3h-kkc/claft/12au7srp.htm
> 
> Using a capacitor as an almost no-loss almost-ideal current
> source is a brilliant idea! But I would be frightened and worried
> about safety issues, with the mains voltage so close to the
> cathode of the tubes. (?)

I think besides the risk of shock (the output transformer would save 
you), this is a no-no, since there is a specified maximum resistance 
between filament and cathode that shouldn't be exeeded. Rfk is in the 
order of 20kOhms usually. Also there is a maximum Ufk. This has to do 
with the capabilities of the insulation of the cathode, which isn't 
perfect. This means that the heater circuit should be galvanicaly 
coupled to the rest of the circuit.

In tube TVs they used series "off the net" heating, often also with caps 
for reducing the voltage. But then the "GND" of the chassis was tied to 
one pin of the plug, and so was the heater. (There was no transformer...)

Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159