[ARC5] [ARC-5] 803 circuits

[Morton Jones]

----- Original Message ----- 
From: "Neil" <neilb at ihug.co.nz>
To: "ARC-5" <ARC5 at mailman.qth.net>
Sent: Friday, November 19, 2010 4:49 PM
Subject: Re: [ARC5] [ARC-5] 803 circuits


>> 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
> ______________________________________________________________
> 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
>