[ARC5] 803 circuits

[mac]

Ken, David,

Oscillator circuit is not that weird.  RCA used a well screened  
pentode in a simple grid-plate oscillator circuit, not surprised they  
had to add a little feedback to get it to work.  Modern circuit with  
an RFC in the cathode would work better but guess that configuration  
hadn't been invented yet.  The use of a resistor in the B- lead to get  
some bias voltage was common for the period as was the use of a high  
resistance, high wattage screen dropping resistor.  Wastes some power  
but simple, straightforward and saves two power supplies.

The 803 plate modulates FB. Old Ashtabula Bill ran a TBW HF section  
for many years with an external high level HB plate modulator.

The 803 is a good tube but as a relatively high value of plate/ 
filament capacitance (29pf vs 14pf for an 813 for ex.)which limits  
it's usefulness on 10M and above.

Suppressor modulation actually works quite well.  In the phone mode  
the suppressor grid is always negative with respect to the filament  
and draws no current.  A simple one-tube speech amp/modulator is all  
that is required.  Cheap and simple.

Dennis D. W7QHO
Glendale, CA

***************

On Nov 17, 2010, at 9:58 PM, Kenneth G. Gordon wrote:

> 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
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html