[AMPS] AM-6155 to 4CX400 : more info

[John Lane]

I got quite a few requests for information so I'll just post what I have
here.
In general I followed the notes of N3AHI in the overall conversion except
as follows:
1)  I left the screen supply unmodified (per rational of K1WHS)
2)  I modified the metering circuit using 2 series, back to back pairs of
1N4148's across the meter.  I verified correct bias and screen voltages
with a Fluke 77.  This mod corrected the low plate current reading.
3)  Cleaned the plate blocking cap and coated surfaces with Dow-Corning #4
dielectric compound but did not use teflon replacement.
4)  So far I have not had a problem with bias instability so I haven't
played with any active regulation.  As previously noted I suspect these
problems are pathology, not genetics.
5)  The plate contact finger stock had to be slightly enlarged to fit the
smaller anode diameter of the 4CX400.
(per WZ1V, also consult his  4CX400 222 Mhz conversion notes for the
AM-615X series.)
6)  I use the 1992 'Handbook input circuit for their 144 Mhz 8930 project.
7)  I notice the following sags at full output:  Ep:  2200 to 1900, Es: 
390 to 350, Eb
 -44 to -42.
8)  I bias the tube a little closer to B conditions (90mA at idle) to
offset the slight sag at full out.
9) I could possibly get another 100 or so watts out if I ran the HV supply
with the taps set to 205 volts but I'm not sure the results are worth the
extra stress on the supply.  I'm leaving it at 240 for now.
10) Loading of the amp is very light.  This seems to give lowest screen
current and highest output power.  I'd be interested in knowing what others
have observed with this.
11) On the air reports indicate good quality but I haven't checked it on
the Tek 2710 yet.
12)  I see no reason why this shouldn't work on 432Mhz or on the '6154. 
Price for the 4CX400 is about $140 everywhere I checked.  Anyone know of
any deals out there? 
					-- W8KX

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