I'm in the processing of bringing a Johnson Thunderbolt amp up and can't
quite make sense out of what I'm seeing. I'm hoping there is someone out
there with experience using the Thunderbolt (or a pair of 4-400A's) as an AM
linear. I've searched the list archives and found lots of old threads about
Thunderbolts, but none of them addressed my situation.
My Thunderbolt appears to be a completely stock, factory-built unit (with
the exception of a 120 ohm resistor in parallel with the original 350 ohm
swamping resistor). The HV power supply is putting out 2200V to the plates.
I've been able to get a good match to my IC-730 by using a small matchbox.
I've also driven it with a Ranger. The additional swamping resistor lowers
the input impedance to about 89 ohms. The 40 watts out from the Ranger
gives 59V RMS at 89 ohms which is what 10 watts will yield at 350 ohms (the
original AM/SSB drive spec). Since the grids of the 4-400A's are biased
at -75 volts in linear mode (Class AB1/2), that sounds just about right.
In CW mode (class C) I have about -165V on the grid, and I can tune it up to
1000 Watts input, which is giving about 600 watts out. So far so good.
When I switch to linear mode, I can't get more than about 125 watts of
carrier out (500 W PEP on voice peaks) before the waveform starts to flat
top. I can tune it up per the manual instructions and get about 350 watts
of carrier out (360mA plate current), but there is very little headroom so
I'm still only getting 500W PEP. The original specs for the Thunderbolt are
800W input on AM. Assuming 35% efficiency for AB1, I'd expect that I sould
be able to get about 280 watts of carrier and about 1000-1100 watts PEP.
I've tried using it on SSB and I see the same 500W PEP output limit. I've
also noticed, that when operating this way the plates start to glow red
after about 20-30 seconds of AM carrier (the plates never glow in CW mode).
Am I missing something fundamental? I have no experience with class AB1/2
and also no experience with 4-440A's. My first guess is that the tubes are
the problem. I suspect they are the original Eimac tubes and about 40-45
years old. Do 4-400A's tend to loose linearity as they age? Or are my
power assumptions incorrect? I assume that I should see higher power peaks
in SSB then in CW. Or does the change from class C for CW to AB1/2 for
AM/SSB and the reduced efficiency, mean that 500-600W PEP out is the best I
can hope for?
Also, some of the old message threads talked in generalites about modifying
the power supply to increase plate voltage (solid-state rectifiers, removing
the choke, more capacitance, etc.). Has anyone actually done this who can
report on the results? I'd like to put this great old boatanchor to use in
a vintage Johnson AM station and would really like to have at least 250
watts of carrier with 100% modulation peaks.
Thanks,
Clay W7CE
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