Re: VTTC - audio modulation

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Herwig Roscher by way of Terry Fritz <twftesla-at-qwest-dot-net>" <herwig.roscher-at-gmx.de>


Dr. Duncan Cadd wrote:
> The oscillator, if it's a power oscillator, will operate in class C

Duncan,

In order to minimize distortion I moved the operating point of the 
Armstrong oscillator towards the class A operating point using an 
external control grid bias supply. Of course the exact class A 
operating point is not suitable for modulation, as a nonlinear part of 
the VT characteristic needs to be used here. Despite I lowered the 
number of grid coil turns before, distortion was large unfortunately. 
Too much turns still?

> What you want is linear modulation of that class C stage
- That is what I am looking for indeed. I wonder whether it is 
possible in principle to amplitude modulate an *oscillator* because 
usual modulators require carrier signals with constant amplitude. 
Wouldn't modulation cause the oscillation to stop?

>my apologies - I haven't actually forgotten
- Never mind. Coiling is a hobby and professional tasks are much 
more important.
 
> It could be that the mere act of connecting your modulator to the 
VTTC is causing the modulator itself to distort
- I didn't try a multistage approach, consisting of oscillator, 
modulator and class C amplifier yet. The main reason is, that the 
Armstrong power oscillator I'm using is selfstarting and 
*selfregulating*. I'm afraid that a class C amplifier, driven by a 
separate and frequencystable oscillator, will behave unpredictably 
when varying load, temperature, natural frequency of the secondary 
coil etc. Of course this multistage solution would permit an optimal 
modulator design. However i shy away from the additional expense 
and the increased complexity as well.

> because with the VTTC operating in class C, it will offer a
> widely-differing load impedance to the modulator stage
- If the tube was cut off, the modulator would see a high impedance 
and if the control grid was driven into grid current, the impedance 
would become quite low. Right? Therefore a catode follower should 
be use to drive the toob.

> http://members.aol-dot-com/uhaumann2/plasma/eplasma.htm
- Thank you. Very interesting indeed!

> Full plate HT on the grid is not recommended :-) 
- My tube survived a strike to the control grid, blowing a fast 200 A 
peak diode, which is connected from the control grid via a 800 V 
gas discharge voltage protector to GND.

> You can locate high rf potential on such a plate choke with a 
small neon bulb held on an insulating rod
- As my plate voltage is 10 kV DC I'm "a little bit" afraid from 
getting close to the system when it's in operation. But it's an 
interesting modification you have made. Way to go?

> If you have a working Q&D VTTC
- quick and dirty?  :-))

> http://home.freeuk-dot-net/dunckx/wireless/qdvttc/qdvttc.html
- Thank you VERY MUCH for your advice and hints, Duncan. Step 
by step I'm becoming familiar with all the problems, connected to 
the relatively simple looking VTTCs. A long way to go still, but I'll 
make it with your and the list members assistance.

Personal regards,

Herwig