Re: [Amps] Johnson Thunderbolt questions

[Joe Giacobello <k2xx@swva.net>]

Clay, I no longer have my Tbolt but I never used that resistive position 
to drive it.  It sounds like that's what you're doing.  I simply 
selected the input for the band I wanted, tuned the grid in CW mode 
(Class C) to obtain the proper drive current, loaded it up in that mode 
and then switched to SSB.  When loaded to saturation, I could get about 
900W out on CW with about 20-30 mA of drive.

Modern transceivers do not need the swamping network.  Just feed it 
directly with your 730 and reduce the drive to obtain about 25 mA, 
equivalent to about 25 watts.

I replaced all of the rectifiers with solid state equivalents.  You can 
buy plug-ins for all of them or you can homebrew them.  K2AW's silicon 
alley sells suitable diodes for the HV supply.

Hope that helps.

73, Joe



Clay Curtiss W7CE wrote:

> 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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>  

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