[ARC5] More on 801 Vs. VT-25

[Mike Morrow]

Dave wrote:

> It appears that when the Military called for the VT-25,
> also known as the "10Y,"...

Here comes the confusion in my mind.  That says that the VT-25 is a 10Y, not a 10.  So then...what is a VT-25-A?

> ...the tube makers just grabbed the tooling for the 801 
> and called it "VT-25."  I base this on comparisons between
> a NOS 801 and a NOS VT-25. Both have thoriated tungsten
> filaments. They operate identically in both the transmitters.

This implies that a 10 or 10Y is the same as an 801.

> I guess the Army called "foul" or something, because
> sometime during the war the VT-25A appeared.

SCR-AO-183 and -283 (Order No. 6120-NY-41, 06-18-1941) are the earliest sets in this series that use the VT-25-A.

> The VT-25A is so different from the VT-25, it's not even
> the same tube, IMHO. It has an oxide-coated filament and
> longer plate for more dissipation.

Which brings up the question again.  What is the commercial equivalent to the VT-25-A, if there is any?  It seems clear that it is NOT the earlier 10Y, regardless of what several references have stated.

> The VT-25A will deliver about 30% more power out than the
> VT-25/801.  VT-25s are commonly called "short plate" for
> the "801 in disguise" and "long plate" for the VT-25A...

It would be informative to compare full tube data sheets for VT-25 and VT-25-A to verify that the improvements in VT-25-A were "officially" claimed in the manufacturer's literature.  I've never come across that data.

With respect to the SCR-A*-183/-283 sets, it appears that NO advantage could be taken from the improved characteristics of VT-25-A.  Transmitter tuning procedure did not change with the new tube.  The transmitter is always tuned for VOICE emission, and as such PA tube plate current at resonance is limited by the available plate modulation power from the VT-52 tubes.  The PA is tuned for max antenna current with the PA tuning capacitor and the appropriate antenna tap on the PA coil.  Then, if a PA plate current test meter is available, the tap is lowered and PA stage retuned until final PA plate current is less than 35 mA for 75 percent modulaton, or 25 mA for 100 percent modulation.  (If no PA current meter is available, the instructions suggest tuning for antenna current less than maximum by about 5 percent...a percentage that should likely have been increased for transmitters using VT-25-A if the new tube allowed any significant increase in PA power over VT-25.)

As a result of these modulator-stage-based PA tuning limitations, PA stage power is the same, regardless of which tube was in the PA socket.  Perhaps the VT-25-A would offer increased output if the PA stage were tuned for max antenna current in CW operation, but that would never happen in military operation due to adverse low modulation effects on VOICE and TONE emission, in addition to the associated receiver not being designed for unmodulated signal reception.

So...it appears that no additional RF power in VOICE or TONE mode may be enjoyed from the use of VT-25-A instead of VT-25 in a properly-tuned BC-A*-230.  However, it remains very uncertain to me that VT-25-A is a 10Y, or that it has any exact commercial equivalent.

Mike / KK5F