[Collins] 30S-1 final

[Dr. Gerald N. Johnson]

On Mon, 2007-09-10 at 20:15 -1000, pete wokoun, sr. wrote:
> The 30S-1 I'm working on has a 4CX1000A final.  The tech manual also lists 
> this as the final.  I've heard and read about some amps having a 4CX1500B in 
> there.
> 
> Is this a later model change?  I see the tubes specs are almost identical 
> except for the plate dissipation and filament current.  Can  they be 
> interchanged in the 30S-1 or do you HAVE to use what's stenciled on the 
> bottom side of the top cover, ie the 4cx1000A?
> 
> pete KH6GRT
> 
I don't think the 4CX1500 was ever a factory mod.

Comparing the two data sheets there isn't a great deal of difference
other than plate and grid dissipation ratings and screen current
characteristics. The heater currents are the same.

Because of better cathode to grid alignment, the 1500 claims less grid
interception of the electron stream which allows better linearity with a
bit of grid current and some grid current is allowed (grid dissipation 1
watt).

Typical RF amplifier characteristics are with 225 volts on the screen of
the 1500 and 325 on the screen of the 1000. Since tube gain is directly
proportional to screen voltage, a mod that raises the screen voltage
will make the amplifier have more gain but may upset the tuning meter
and amplifier stability. A different screen voltage also can mean a
different plate load line is needed to keep the minimum dynamic plate
voltage is always above the screen voltage, else the screen gets
overloaded by gulps of current that used to be peak plate current
suddenly switching to the screen, and its not made for those several amp
peak currents.

The typical data also shows the 1500 screen tends to run much more of a
negative current than the 1000.

Other than the more negative screen current (which isn't a problem in
the grounded screen supply situation where the "screen" supply is also
carrying the total cathode current) it looks as if the 1500 makes a
decent and probably more linear replacement for the 1000 and the 1500
tolerates more grid drive without adding distortion or damaging the
grid.

I'm always concerned with plate matching and plate tank components.
Raising the output from 1 KW to 1.5 KW raises the tank coil and loading
capacitor currents about 50% and that raises their power dissipation
more than double. The 30S-1 may have conservatively enough rated parts
to handle that increase. Not all finals can stand that, and the 1.76 dB
difference between 1 KW and 1.5 KW is far less than the second by second
changes in propagation and not something measurable with most receiver
S-meters. In the rare situation where propagation is steady and the 1 KW
signal is at the noise, that 1.76 dB increase may improve received
signal to noise ratio several more dB than the increase in power. In all
other situations it only runs up the electric bill.

If it was me, I'd run the 4CX1500B at 4CX1000A ratings and expect a very
long tube and radio life.
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer