[ARC5] [ARC-5] 803 circuits

[Neil]

> I simplified the circuit of the AVT-12 transmitter:

> http://home.netcom.com/~arc5/AVT12a.jpg

> It uses the large format RCA crystals.
> Don't know if your little HC-25s or FT-243s
> will hold up. The RFCs are like the ones
> used in ART-13s.
> 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,
> 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.
> 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.
> 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.
> 73 Dave S.


The publication 'RCA Transmitting Tubes' (1956) has this to say about suppressor
grid modulation:

"Operating conditions are similar to those used in screen-grid modulation, except
 that the suppressor grid is supplied with a fixed negative dc bias voltage in addition
 to the modulating voltage. This bias voltage is adjusted so that the plate current
 and rf output current of the modulated amplifier under carrier conditions are
 one-half those obtained in Class C telegraphy service with zero voltage on the
 suppressor grid. Under these conditions, the modulator is required to supply only
 a peak voltage equal to the suppressor grid bias, and does not have to supply
 power because the suppressor-grid is not driven positive. Suppressor-grid
 modulation has only limited application, however, because relatively few beam
 power tubes and pentodes have the necessary linear relation between suppressor
 grid voltage and plate current."

So firstly, it seems that an advantage of suppressor-grid modulation is that only a
simple, low consumption voltage amplifier is required for the modulator.

Secondly, it may well be that the 803 power pentode is one of the few pentodes
that have the linear relationship referred to. In addition, the grid modulation methods
(control grid, screen grid, suppressor grid) are prone to voice distortion if operating
conditions and bias of the modulated stage are not optimal. It may be that the 803
with very high plate voltage ensured better linearity than a lower voltage, higher
current tube of some other type when this modulation method demanded greatly
reduced plate current.

73 de Neil ZL1ANM