[Johnson] Viking Thunderbolt

[Steve H]

Robert

Thanks for the detailed comments, very good reading for sure.

I finished up making the T-R switch with a dow key relay and some other odds
and ends I had laying around.
the DOW key had 2 sets of SPDT contacts basicly a DPDT, so I used those to
perform the blocking and biasing
states for the Thunderbolt cable. I also made a diagram here that shows the
pinout and wire colors, and uploaded it to a post about thunderbolt mods on 
amfone's website.

http://amfone.net/Amforum/index.php?action=dlattach;topic=18413.0;attach=67169;image


I do have a legal limit dummy load hooked to the Tbolt for initial tests
when that time comes.

today I'd like to confirm my bias cable hookup is proper.

Previously I had applied 115V to the Tbolt and
confirmed all filaments were working, and the vr tubes all lit up giving me
a good sign. I did notice when
I had the mode switch in LINEAR position, one of the VR tubes went out, so I
placed it in CW and it came up again.
I am guessing this is because I had no bias cable hooked up, and at some
point I plan on putting that 270k resistor
on the mode switch to resolve that issue.

For this initial test, I am not going to apply the external plate power
supply, and just want to confirm all seems well at the "idle" / standby
condition. Should I leave the mode swittch in CW, then tune and linear and
whats a normal reading on the meters
with respect to the grid and screen? I guess the "voltage" position regards
the plate, so that ought to be zero if
I read things right.

My station ground is hooked up to both the linear and
the PS. As someone previously mentioned,
that ought to be the first thing checked and rechecked before doing any kind
of testing.

--
Steve Hearns
KA2PTE





----- Original Message ----- 
From: "Robert Nickels" <ranickels at gmail.com>
To: <johnson at mailman.qth.net>
Sent: Friday, February 05, 2021 1:15 AM
Subject: Re: [Johnson] Viking Thunderbolt


> 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
>
> ______________________________________________________________
> Johnson mailing list
> Home: http://mailman.qth.net/mailman/listinfo/johnson
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Johnson at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html