[ARC5] 803 circuits

[Kenneth G. Gordon]

On 17 Nov 2010 at 0:00, David Stinson wrote:

> I simplified the circuit of the AVT-12 transmitter:
> 
> http://home.netcom.com/~arc5/AVT12a.jpg

Holy Cow! What a weird oscillator circuit! I have not seen many oscillator 
circuits that required a grid-to-plate "gimmick" to get enough feedback to 
make it "go". The only other one I know of is in the Paraset WWII spy 
transmitter which uses a 6V6. I suppose the 837 is one of those "well 
screened" tubes of the period. But the Navy used the 837 as an oscillator in 
many different rigs. It is a good tube.

> The 803 suppressor is modulated at 50 volts
> through the secondary of the modulator output transformer.
> That voltage is developed by taking it off the top of
> a honking-big 750 Ohm, 75 Watt resistor in the B- lead.
> Whatever they gained in a smaller mod transformer,

...and no modulator power supply....

> they wasted in those two big 75-watt dropping resistors
> for the screen and modulator, not to mention the 
> huge ones in the dynamotor chassis to divide down
> to Osc. B+.  I still don't see the advantage
> of running Kamikaze B+ just to burn it off 
> in King Kong voltage dividers.

Me either....but they must have had at least one reason for it.

> If I ever get a chance to put an 803-like rig on the air,
> it will be a serious candidate 
> for reduced B+ experimentation.

IMHO, the 803 is a darned good tube. Very reliable with very few bad habits. 
I once thought of using a pair of those in a push-pull Hartley oscillator 
transmitter. I have a number of 803s with ceramic bases here.
  
> I'm too old, clumsy and forgetful to be twiddling 
> around 2000 volts. I've lived over many 
> 3 or 400 volt shocks, but 2000 volts is instant death.
> No thanks.

Agreed.

I have NEVER liked suppressor modulation. I cannot figure out why several 
different military rigs used it.

I think the TBX-8 used suppressor modulation too.

So-called "cathode" modulation (actually a form of grid modulation), or even 
controlled carrier (screen) modulation would be more effective, less trouble, 
and more efficient in my experience.

Surely, there was at least ONE very good reason for the use of suppressor 
modulation in so many rigs.

The designers of the period were no dummies.

Someone must know the story. I don't. I can only guess.

Ken W7EKB