[ARC5] Transmiitters: Parameters?

[AKLDGUY .]

All I can say is that the 15K is too high for both CW
and AM, although it's less wrong for AM (for which
the AN/ARC-5 was exclusively designed).

As I said, the value may be high to avoid loading
the 1626 oscillator, or, it may be a carryover from
the ATA value where screen-grid modulation was
used and different parameters apply.

73 de Neil ZL1ANM


On 3/22/16, Kenneth G. Gordon <kgordon2006 at frontier.com> wrote:
> 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
>>
>>
>> -----
>> No virus found in this message.
>> Checked by AVG - www.avg.com
>> Version: 2016.0.7497 / Virus Database: 4542/11860 - Release Date:
>> 03/21/16
>>
>
>
>