[ARC5] Transmiitters: Parameters?

[Kenneth G. Gordon]

Neil:

All that you say below is true, but in our case(s) we are not using AM...at least not most of 
the time, but CW, and secondly, I suspect the reason ARC used two tubes in the final amp 
was mainly for redundancy in case one of the two died for some reason. Therefore, the two 
tubes are essentially one bigger tube.

In that case, the parameters in the manual for one tube, apply here for the two. For grid 
drive, it would still be 4 mA and not 8 mA, for instance, probably because the 1626 is a 
marginal driver tube for two 1625s, but is perfectly suitable for one tube, or for two acting as 
one.

Anyway, those are my musings on the subject.

Your thoughts?

Ken W7EKB

On 22 Mar 2016 at 18:03, AKLDGUY . wrote:

> > FYI, according to the RCA Transmitting Tube manual, in Class C as an
> > amplifier, with 600 VDC on the plate, the grid voltage SHOULD be -45
> at 4.0
> > mA. This is with an 11.2 kOhm grid resistor, but the grid-resistor in
> the
> > ARC-5 transmitters is 15K, which at 4 mA would give a grid voltage of
> -60,
> > which would then require more drive.
> >
> >I am thinking that the grid-current is low in the ARC-5, possibly well
> below 4..
> 
> 
> The -45 V and 11.2 kOhm that you quoted are the Telegraphy (CW)
> values,
> and are inappropriate for the AN/ARC-5 which did AM only.
> 
> I also have the RCA Transmitting Tube Manual (1956), open in front of
> me.
> 
> == Plate-Modulated RF Power Amplifier - Class C Telephony ==
> 
> For Typical Operation at 600 Volts plate voltage, my copy says that the
> DC
> Grid-No.1 Voltage should be -85 volts, obtained from grid-No.1 resistor
> of
> 21200 ohms or (quote) "from a combination of grid-No.1 resistor with
> either
> fixed supply or cathode resistor". (end quote).
> 
> You correctly quote the grid 1 current as 4 mA.
> 
> What does this mean? It means that at a bias voltage of 85 V, the grid
> 1
> current should be 4 mA when correctly driven at the peak driving
> voltage
> of 107 Volts.
> That means the grid resistor should be 85 / 4 mA = 21200 ohms.
> 
> Now, that is for ONE tube. The grid current when two tubes are in
> parallel
> (as in the ARC-5) is doubled to 8 mA, but the driving voltage and the
> bias
> voltage must remain the same. Therefore, the grid resistor must be
> halved
> to 10600 ohms.
> 
> It therefore appears that the AN/ARC-5 grid bias resistor is high at
> 15K,
> and that would result in low grid current, as you said.
> 
> I think there is probably some justification for reducing the resistor
> from
> 15K to 10.6K, but there may be some sound design reason for the choice
> of 15K. It may be that the designers didn't want to load the 1626
> oscillator
> too heavily, so used a compromise value.
> 
> I calculated out the value for Telegraphy (although the AN/ARC-5
> didn't
> do CW) to see whether the 15K was a compromise between AM and CW
> values that was unchanged from the ATA transmitter. The Telegraphy
> values are: bias voltage is -45 V at 4 mA, confirming the manual's
> value
> of 11200 ohms (45 / 4 mA = 11200), but that should be halved to 5600
> ohms for two tubes. So no, that's an even lower value and the 15K is not
> a
> compromise. It's astonishing that if the resistor in the ATA and
> SCR-274-N
> transmitters is 15K, there's an even worse discrepancy in CW mode than
> there is for the AN/ARC-5 in AM mode.
> 
> Those two earlier series used screen-grid modulation, and drive
> setting
> may have been a compromise between AM and CW.
> 
> 73 de Neil ZL1ANM
> 
> 
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