Greetings Gentlemen.
I found Larry's experience with the 4cx1000 interesting. I built my
first hf amp (160-10m) using the same tube with a few problems that were
easily resolved. I did use grounded screen configuration (all supplies
isolated from chassis ground). When first put into service, this amp had
some instability on 15m, but using good bypassing technique for the
cathode/filament cleared that up(incorporated .01 uf disc ceramic with
.001 sm cap as the circuit element). I also had to rework the rf choke (
the stock Hammond choke I used needed a few turns removed to get away
from resonance near 20 mhz). The problem with hi c at the control grid
above 21 mhz was cleared by switching in some inductive reactance for
operation at 15/12/10 m when required.Terminating the control grid with
50 ohms and -50 v bias, 325 volts screen and 3kv anode voltage, 30 watts
drive gives 1100 watts out easily.The tank circuit (pi-network) I built
uses vacuum caps for tune and load with a roller inductor for 160-30 m
and a tapped coil for 20/17/15/12/10m. I haven't experienced any
instability problems with this amp, no evidence of parasitic oscillation
anywhere in the frequency domain. 3rd order IMD is well below -30db
(measured under dynamic mdulation conditions with a spectrum analyzer).
I also have tried the 4cx1500B in this deck with similar results. To
view some photos of this particular amp visit VE7AV's web site and look
at VE5RI photo archive (simply search for VE7AV with your web browser).
The point is that my introduction to HF amp construction was gratifying,
and I would not hesitate to use any tube from the 4cx family. I'm
currently finishing a deck using the 8169, and I hope it goes as well as
the first one!!
73,
Gerard VE5FF
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