[Johnson] Viking Thunderbolt

[Robert Nickels]

On 2/4/2021 9:27 PM, Steve H wrote:
> Also wondering if anyone knows did the Tbolt come designed for AB2 
> operation? 

Page 12 of the manual is the best resource for understanding the biasing 
vs excitation of the Thunderbolt - because the answer is "both", 
depending on the excitation level:

"The Thunderbolt operates in Class AB1 (zero grid current) up to a peak 
envelope power input of 1400 watts and is driven into the grid current 
region, Class AB2, to obtain the maximum rated power of 2000 watts. When 
grid current starts to flow, the Thunderbolt grid impedance drops thus 
presenting a variable load to the exciter. The exciter should be heavily 
loaded so that the changing grid load has relatively little effect upon 
the exciter. An exciter should have, preferably, an output of 40 watts 
or more, swamped down, when driving into the Class AB2 region. When in 
doubt, do not drive into the grid current region (limit audio level to 
the point where grid current just starts to flick upward) until careful 
checks are made to assure that there is no splatter."

For reference:

Class AB1:  Plate current flows for less than 360 degrees, but more than 
180 degrees of the grid input. No grid current flows.  33-45% efficiency.

Class AB2:  Plate current flows for less that 360 degrees of the  input 
cycle, but more than 180 degrees. GRID current flows, (input impedance 
changes when grid current flows) so input must be loaded to maintain 
constant input impedance. 40-50% efficiency.

---

It's important to consider the designer's intent with a piece of 
equipment like the Thunderbolt that was capable of being driven by 
exciters of the day that ranged from 10-20 watts to over 100 watts 
output.    When less drive is available (and for CW, class C) - the 
tuned grid input circuit is used, and when the drive power is greater 
than needed, external attenuation must be provided.  The manual spends a 
lot of time showing various combinations of attenuators for different 
exciters, because to have a clean signal the exciters of that era had to 
be loaded to rated power and then attenuated (in some kind of power 
waster!) to the appropriate level for SSB or AM linear operation.    For 
a range of excitation that is "in between", the "RES" or resistive input 
termination can be selected, such as when the Johnson Pacemaker is 
used.   This is a 350 ohm non-inductive terminating resistor that 
provides sufficient loading for linear operation.    As long as the 
excitation can be reduced to an appropriate level while maintaining high 
signal quality,  there's no problem using a grid-driven linear like the 
Thunderbolt with modern radios.

Bear in mind that when the Thunderbolt was introduced, amateur power 
levels were specified as input power, and nowadays output power is the 
norm.  A ham buddy uses his Thunderbolt for AM linear operation all the 
time, with good results, but he limits the drive to 8 watts, which 
produces about 150 watts AM carrier output, or 600 watts PEP on voice 
peaks.   It's also important to remember that when the Thunderbolt was 
introduced in 1957, hams were allowed to run a maximum of 1000 watts 
input.    That has been reduced by the FCC to 375 watts carrier or 1500 
watts PEP output which is consistent with all other emission modes.    
The T-bolt was sold as being capable of 800 watts input for AM operation 
which could produce output power at or above the legal power limit.   
All things considered I'd suggest setting it up for a maximum of 200 
watts carrier output if AM linear operation is intended.     A monitor 
scope is essential to verify the quality of your transmitted signal.

73, Bob W9RAN