Home Brew Outboard Modulator for Viking Invader 2000

[WQ9E]

John,

There aren't that many Invader 2000 rigs around.  Rather than chop up the 2000 why not just start from scratch and build a rig from plans on the web or in a handbook?  You could probably sell the 2000 for enough to fund most of the parts and then you will be building  an AM rig designed to do what you want instead of one designed to fit within an Invader 2000 case.

Another option is build a standalone AM modulator/final but use the Invader 2000 to supply excitation and to furnish the needed voltages.  That way you end up with a proper Class C final/modulator and a fully operational (and resalable) Invader.

The level of modifications you are contemplating are very involved and will likely involve other complications (such as instability in the final and overheating of the final tank components).

 

[AC0FA]

"Another option is build a standalone AM modulator/final but use the Invader 2000 to supply excitation and to furnish the needed voltages.  That way you end up with a proper Class C final/modulator and a fully operational (and resalable) Invader."

You got it Roger! Thats what I am trying to do!

Actually the 2000 PA rf deck is a completly seperate assembly built to plug in to the invader chassis where the original 100w invader 200 power supply was.
It can be easily removed and placed in a rack or other assembly the external power supply does have long cables.

The invader 2000 pa section could be duplicated home brewed and plate modulated. Without destroying the original 2000 PA section which could be sold or kept safe on a shelf some where. It would be kind of cool if the home brew PA fit in the original box but it will be ok if it dosent. 

It would seem the power supply has 85% of what I need for voltages. The high poop filament transformer is under the  pa chassis.

I have the outboard audio chain built the modulator tube sockets are in. The big honking 24hy modulation reactor and the 12uf 6000v cap is in the chassis.

I thought replicating the viking 500 pa section would be a good place to start.

I was thinking about push pull 811a's and maybe pulling one of the PL-175a's out of the invader and running it like a viking 500.  I dont have to get real fancy with the tank circut fixed tuned for 75m would be fine. Just needing some sugestions for the specifics.

I haven't really decided what tubes to run i pulled the relatively low audio chain voltages from the external invader supply already.

I haven't tapped into the high voltages yet because I havent arrived at a final decision about how the pa final will be configured.         

[WQ9E]

OK John,  Now I follow what you are doing.

I have both the 200 and 2000 versions of the Invader along with a 500 and Desk KW so I am pretty familiar with the features and drawbacks.  A neat setup for what you are doing would be to build the final and modulator on a separate chassis and install it in a Johnson case.  The 21x14x11 inch case style originated with the Valiant but was also used for the Viking 500 RF deck, the Pacemaker, Valiant II, Thunderbolt, 6N2 Thunderbolt, and both Invaders (with different color schemes).  So you have a lot of possibilities when looking for parts unit cases and panels.  You are probably aware that that Johnson sold a kit allowing you to later upgrade the Invader 200 to a 2000 although I wonder how many people took advantage of the upgrade path.  It came with a new front panel, power supply, etc.

You could use the Invader to provide RF excitation and audio drive just as the Desk KW relies upon the Ranger these functions.

With the fairly low high voltage and the cost factor I would think about either 813 tubes for a tetrode final or the readily available and easily cooled 572B tubes in a triode final.  If you use 811A modulators like the 500 I would try to find the classic US versions since you are going to be going well over the official voltage rating.  Johnson had the OK from RCA to run them at increased plate voltage but the Chinese copies might do interesting things.

I picked up another low power Invader 200 that is missing the power supply section so it is going to be modification material for a future project.  I haven't decided what to do with it yet since the front panel is nice and everything is there except the power supply.  It may turn into some sort of multi-mode rig yet to be determined.

[AC0FA]

Wow!
Thanks roger I just needed a fellow Johnson fan to help steer me straight.
So a pair of 572b's in the class C plate modulated PA final in a seperate case modulated by a pair of good old american 811As in push pull modulator with a
UTC VM-4 300w modulation transformer.

FB on using the 200 for an exciter I have tapped the audio at the volume control  pot in the invader and brought it to the Outboard audio amp I duplicated from the 500 up to the 811a sockets and tapped it in on the second half of the 12ax7 but I omitted the clipper and replaced it with a 6c4 then the 6au6 and the 6g4b and the 811a sockets.

I also plan to tap into the RF somewhere on the invader and use it to drive the grids of the outboard 811a's like the ranger but I am not sure where it would be best to tap in to it. The audio and RF mix at a low level in the invader bal mod. but in the 500 they dont mix at all till they get to the final.   

Would the 2 572b's require the same 250w class modulator as the viking 500 with the single 4-400 final? The viking 500 uses a Modulation Transformer with these specs
 PR) 23000 ohms p-p 2400vct (200-3500hz)
 S1) 8000  ohms (2000vdc @ 250ma) 1414vac 250w.
John AC0FA
 

[AC0FA]

Roger;
In an effort to answer your earlier question. I did search the specs for the invader 2000 to see if it was biased for Class C in cw mode.

I have found nothing in the manuals etc. The Thunderbolt the Courrier and the Desk Kw all use class C bias for CW. The Invder 2000 in CW mode has 1000w input and 800w cw output which would be an efficency of 80%. With that kind of efficency I would assume the invader is also biased in class C.

Transmitting AM In class C with a loosely coupled diode providing negative feed back to the driver tube and screen modulation of the 4-400s That might work out real nice too and I could use parts that are already there. It would take more drive. wouldn't be as linear but would put out more AM carrier. I dont know if the 6146's would have enough output to modulate the screens.   I suppose the 4-400a screens want 600v in class C phone.

John AC0FA
       

[AC0FA]

If I use both ideas for the home brew pa section. For the invader 2000. If I were to use the 572b's cathode driven and biased into class C that would give me an efficency of 66%. It would seem the invader is already biased into class C for the CW mode. All I would need to do is change the timing gears of the tank circut to give the roller inductor more L to increase the fly wheel effect for the class c bias. Wow! I am going to have to find some one WAY smarter than me to to help me out with this one.
John AC0FA