[Milsurplus] ARC 65 and ARC 21, plus ARC-38 and 38A

[Mike Morrow]

Scott wrote:

>The arc-65 used three 4-65's for about 200W PEP,

Hi Scott,

The RT-128A/ARC-21 uses two 4-65 PA tubes, plate-modulated by two 4-65
tubes, for a rated power output of 100 watts (AM/CW).  The RT-400/ARC-65
went to two 4X250F tubes in the PA stage, and the manual states that the
rated power output is 230 watts.  I suspect that this is average power, not
PEP.

>the ARC-58 used three 4-CX250's for 1KW PEP, and there were both dynamotor
and three phase power
>supplies built, depending on the installation.

The nice thing about the AN/ARC-58 is the receiver-exciter (R-761/ARC-58)
weighs about 53 lbm, and the amplifier-400 cps power supply (T-605/ARC-58)
weighs about 51 lbm.  The T-605 has three 4X250s in the PA.  There was no
dynamotor supply option for the AN/ARC-58, since the power supply was AC and
integral to the T-605.  Compare that to the 132 lbm of the RT-400/ARC-65 and
the approximately 30 lbm of the external dynamotor or AC supplies - a 60 lbm
savings for the higher-powered AN/ARC-58 compared to the AN/ARC-65.  Those
who are hollow-state purists would not be happy with the hybrid solid-state
and vacuum tube design of the AN/ARC-58, yet that no doubt was responsible
for much volume and weight reduction.

>I think RCA did the SSB upgrades to both the ARC-38A and the
>ARC-65

RCA did the re-design to convert the AM RT-128A/ARC-21 to the USB
RT-400/ARC-65, which made sense because the original AN/ARC-21 was an RCA
design.  It always seemed odd to me that, as you say, RCA also did the
conversion of the US Navy's Collins-made AM RT-311/ARC-38 to the USB
RT-594/ARC-38A.  I've always liked the RT-311 better than the RT-594.  Those
major back fits often have the appearances of "make-do" kludges, compared to
the "purity" of the original Collins design.  However, the frequency
stability specs for the RT-594 was much much better than the RT-311.

The US Navy's RT-311/ARC-38 (with three 6159s in the PA and two 6159s as
modulators) has a rated power output identical to that of the USAF's
RT-128A/ARC-21, at a weight of 67 lbm vs. 132 lbm.  The Navy's set wasn't
pressurized, yet I can't help but think that the RT-311 is a better set
overall than the odd RT-128A.  If only the RT-311 had direct frequency
setting dials instead of a code book and wheels, it would be almost perfect
for its era.

BTW, I have a USAF T.O. 12R2-2ARC21-512, dated 29AUG58, titled "Conversion
of AN/ARC-21 to AN/ARC-65."  It states that its purpose is to "Modify a
sufficient quantity of components of the AN/ARC-21 High Frequencty Radio
Communications Set to equip Strategic Air Command' B-47(), RB-47(), TB-47(),
B-52(), KC-97G, and KC-135A aircraft with AN/ARC-65."  The next several
pages are an itemized description of the changes required.  The T.O. says
that "Modification of RT-128()/ARC-21 results in approximately 20 percent
change in the components of the R-T unit."  The description and appearance
of changes would make one think that it were a much greater percentage.

73,
Mike / KK5F