[ARC5] [Bulk] BC456 vs MD7 modulation

[Jack Antonio]

On 10/14/2014 11:37 AM, J Mcvey via ARC5 wrote:
> I noticed that one of the differences between the two is that the BC456 is a single tube  class A and the MD7 is AB.
> Is there a noticeable difference in the AM modulation?  One wonders why the change was made.

  Was it cost, or power conservation /efficiency?


As far as the what, the AN/ARC-5 transmitters are plate modulated,
the ATA and SCR-274N transmitters are screen modulated. Thus the
requirement for the push pull modulator in the MD-7/ARC-5

The AN/ARC-5 was the Navy driven "new and improved" version of the ATA 
and ARA sets, and started showing up in the middle of the war.

Why they made the change? I'd like to see some hard evidence as to the
reason the changes were made, but the following is a guess on my part:

The ATA and SCR-274N manuals take great pains to stress that
transmitter tune up is to be made in CW mode, and to *not* retune
the transmitter for maximum power in voice as this results in
poor modulation.

I speculate that in practice in the field, this item was overlooked
and the Navy wanted to eliminate that possible error.  Like I said,
this is a guess on my part, and I would like to see some hard
evidence on the matter.

The transmitter components of the AN/ARC-5 are not plug compatible
with their corresponding ATA and SCR274N counterparts. I have seen
it said that the larger plug on the AN/ARC-5 transmitters is to
accommodate the higher peak voltages involved, but I surmise that
another important reason was to prevent the inadvertent mixing of
ATA and AN/ARC-5 components in the field. After all, they do *look*
almost identical, and I would guess that both ATA and AN/ARC-5 systems
were both in use during the latter half of the war.

Jack Antonio WA7DIA/4