[ARC5] Transmiitters: Parameters?

[AKLDGUY .]

> 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