The AM Forum

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John,

Is the Invader 2000 final actually biased for Class C in the CW position?  I seem to remember that it uses the same screen and bias voltage in CW as SSB.  You might have to change the operating conditions.  Another thing you need to check is whether the plate tune capacitor spacing is going to be sufficient for the additional peak voltage from high level modulation. 

John,
       Unfortunately, you cant take szht and make ice cream! I have done a few I2Ks and trying to modulate the finals is a lost cause. The plate tank circuit wll not take the voltage of plate modulation without arcing over. That is one of the biggest problems in modding one for more output. The final tank components wont take much of an increase in voltage. If you could up the plate voltage they would scream and the finals would be a lot happier running at a higher voltage / reduced current scenario. The only real way around it is to possibly screen modulate it, or do the mods that I have listed in my article on the AM Window site. Tell your "old timer" buddies that things are not always as simple as they appear.

Here is what you need to do. I've allready laid it out for those who want to do it. If done right, it is a fun rig to use!! But most of them never worked very well from the beginning.

http://www.amwindow.org/tech/htm/invader2000.htm

Well Slab Bacon;
How do you do!
 I did all your mods and she does run beautiful!
 I even picked up a Hallicrafters SX-101 and did all of your suggested mods for that.

I was a little worried about taking out one of the big honking bleeder resistors from the swinging choke input power supply The one that is the size of a banana.

The old timers said i would be pulling large chunks of iron out of my forehead if i did that Hi HI.

The invader 2000 is putting our 800w PEP am on 40 and 80 But she really does need more strap. The linear setup is working fine for me good hi fi audio and all.

I didnt realize the doctor was in!  Tell me how to increase the plate voltage so she screams. 200w carrier is the most I can get out of her. I havent seen the
250-300w carrier that yours does.   Maybe we can investigate screen modulation could I pull from the 6146 supply?
John AC0FA       

John,
        Do whatever you can to increase the screen voltage on the finals. Every little bit makes a noticable difference!! Removing the one bleeder will give a little more plate voltage to the finals. It's now down to one of those "every little bit" scenarios.

Pull the bleeder, be sure your HV filter caps are good (and fresh) Get the screen voltage as high as you can (swap out some of the VR tubes for higher voltage ones)
And play with the final biass for most output vs efficiency. And be sure you have good line voltage coming into the rig.

If it is working good, the PL-175s WILL get a little "red in the face" so dont be worried if you see it happening.

And tell your buddies that after you pull the one bleeder out, you're gonna hit em over the head with it!!  ;D  ;D  ;D

Sounds good;
Screen voltage is 575V with using the 105V tap on thre screen transformer. Line voltage is good with 250V AC  not far from the power company.

Which voltage regulator tubes should I swap and which tubes would you suggest I replace them with?

I have changed the transmit bias regulator tube specified in your article " Change the transmit bias regulator (V203) from an 0C2 to an 0B2."
Are there any other changes  I can make to the the bias that would help?

Sorry for 20 questions. I have been restoring and repairing boat anchors for years but I am not an electrical engineer. Some times the math gets to be too much.
If I need to make a change to a tube socket to use a different tube please let me know.

Sure is nice to hear from a fellow boat anchor enthusiast who actually owns an invader 2000 and knows how to modify it. We might be the only ones left.
I put in the switch to by pass the 3.4 kc filter for double side band am with carrier
she sounds beautiful nobody is believing me. They swear I have a viking 500 with audio processing.

John AC0FA
 

John,
       It's been a while since I did one. (Part of the reason I wrote the article is to have a back up for my sometimes foggy memory) (I can blame it on the drugs I did back in the 70s) I dont remember what I did to the screen supply. I do remember that I picked the lowest voltage input tap on the transformer. I will have to look at the skizmatic when I get home tonight and reply about it tomorrow. It must have been pretty simple because it doesnt stick out in my mind (like the stray RF problem or the miserable lower VFO box)

Those tubes will take a lot more than 575v on the screens. The more the merrier.
If it wasn't for that damned continuously tunable final tank it would be a breeze. Just change the plate transformer to something that will give 3kv and away you go. but that little wimpy plate tuning cap wont even consider taking it. A plate tuning cap of the same pF value but higher voltage (wider plate spacing) would be the ticket, but wouldnt physically fit in there. Not to mention getting it timed to the roller inductor tank coil. (A possible future project)

After I did mine, the audio was so good that I ran the EQ set dead flat and just used the processor as a mike preamp and compressor/limiter. It sounds good no matter which mic I use on it. Somewhere there is an audio clip of the train at the Gaithersburg fairgrounds being picked up in the background from my Invader. It sounded fabulous!! I think Paul (VJB) may have it ? ?

Sounds good;
3kv plate transformer one amp? I have a whole box of tuning caps some are 560pf and big as a bread slicer. Not opposed to using 1/4 inch copper tubing and switching 2 or 3 three seperate coils for each band.

Maybe it needs an OUTBOARD plug in tank circut with heavy components like a johnson kw matchbox with a big bread slicer cap a 1/4 inch copper tank coil fix tuned for 75 meters and a seperate 3kv plate transformer that can be switched in and out just for 3885. I would need some one smarter than me to come up with the dimentions. But that would be an easy project to actually build. Tune to the am window clamp the lead of the outboard big bertha output circut where it intercepts the piss weaker 2000 output circut. Flip the high voltage switch that disables the 2500v plate supply and engages the 3kv one amp supply and you are cooking with gas.   

The 500 uses only one tube and the same plate transformer as the 2000. Oh well food for thought. Looking forward to your post and thanks a million for all the help.
John AC0FA 

Which tubes are you using in the final? I'm a little unclear here. I plan to try out a couple PL-175's in my Desk KW and what you are doing here with the screens, may apply to the Desk KW.

Thanks. Not a hi jack ;D ;D

I am actualy using PL-175A which puts out even more power by about 10% than the 4-400a.For all practical purposes they are the same tube.

I was thinking of plate modulating the the invader but slab bacon takled me out of it. Now I am trying to get more power by increasinf the vltage to the plate and screens from the supply I have in the invader.

John AC0FA   

John,
        Sorry, but I didnt get a chance to dig out the skiz last nite. The war dept kept me busy pretty much from the time I got in the door. I'll dig it out tonight.

Terry,
         the PL-175 is Penta Labs pentode replacement for the 4-400. They require a little less drive and put out a little more power than a 4-400. they are nice tubes if you can get your hands on them. They work very well for AB1 or 2 grid driven ssb leanyour service. (I think that is what they were actually designed for) They are also getting a little hard to find and pricey when found these days.

the Invader 2000 actually specifies them for the finals and they will put out a little more power in an Invader than the 4-400s do in a T-bolt. But for the most part they are drop-in interchangable.
   

I have been following this thread with interest. I have an idea not yet discussed. I must say however that I have no experience with this rig, nor have I ever touched one.

Here it is:

  Take the HV plate transformer and rewire primary to power it from 115vac instead of 230vac. This will drop the loaded B+ to ~ 1Kv from 2Kv before. Then replace the finals with a pair of 7094's biased and driven for class C plate modulated operation. Then add the HB outboard plate modulator as described in the original post in this thread.

   So looking at the 7094 CCS Plate modulated specifications, at 1KV,  250ma, we get 165 watts out. With two of them we get 330 watts RF out. The lower B+ will give the Pi-Net plenty of headroom for positive modulation beyond 100%.

    I realize that I'm probably full of it, but a first pass doodling session on a paper napkin says this idea might work. The pi-net might need to be tweaked though for the lower plate impedance.

Jim
WD5JKO

Bacon;
No problem I have also been out and about. I have also been inspired by an article written by w8ji http://www.w8ji.com/ the man who designed amplifiers for ameritron. He was improvong the circut to his WRL globe scout 65A  which was one of the few ham rigs left to use heising modulation.

I was also thinking about using heising plat modulation for the invader 2000 If we consider using only one 4-400 tube for the final and the other 4-400 tube for the heising modulator circut (positioned to directly modualte the b+ of the single final)   not sure what you would even call the tube in that circut.

It works out more in line with the power supply and tank coil capabilities of the invader. The heising wouldnt push the b+ as high as a modulation transformer we might get 300w carrier.

There hasnt been a good recent example that i have found to base things on.
in a high powered rig. Using 4-400 tubes. The design is from the 20's i just thought it might be cool. I wanteedto know if any one has tried it.
John AC0FA   

     

jim wd5jko;
is that 300w carrier output that is the minimum power level i am looking / hoping for.
John AC0FA 

Jim,
     that is a pretty cool idea!! It would prolly work. I totally forgot about those tubes. They are definately a low impedance tube. They are also a bit pricey, but would prolly be the ticket. I guess you would need a second pair to modulate them.
As far as the tank circuit goes You could accomplish some of it by just retiming the gears that couple the tuning cap to the roller inductor to get the needed L/C ratio.

However....................... Switching back and forth between AM and slopbucket would require changing the finals back and forth between Class C (for AM) and
AB1 or AB2 linear for SSB service. This would require changing the Q of the final tank each time you switch modes as the class C will need more of the "flywheel effect".

A lot to ponder............................

The difference between 200 and 300 watts carrier output would not be worth all this effort.

Ya know you're right. It would be less than 3 db Hmmmmmmmm..........................
It is just a numbers game.
Also I had another thought about Jim's idea. I dont think a pair of 7094s would have anywhere enough plate dissapation for the ssb linear service. IIRC they are somewhere around 150w each.

I thought of them as a small jug version of an 813 with higher frequency capabilities.

How much is the factory tank good for before it makes R.F. into light?

You get much easier quality control with low level followed by a linear amp.

Not only that but it's already proven by a guy who likes Chryslers  ;D

Unless you have a good supply of 7094's I wouldn't build/modify anything to use them since they are pricey as replacements and not that great a tube for most purposes.  My Halli HT-41 will probably end up with some sort of sub when one/both 7094's give up the ghost.

There is one of them thar 2000's on epay right now. Looks a bit beet up!

Pretty much but they won't take anywhere near the voltage an 813 will - 1250 on AM and maybe 1500 for SSB. I put one in a Globe Champion 300. It was good for about 200 watts plate modulated AM.

I was thinking of a conversion of the PA section of the invader 2000 with two 4-400a tubes in linnear service. To the plate modulated single tube PA section of the viking 500

I have been pouring over specs and transformer ratings and such. Both rigs use many of the same components and are actually the same power class except for AM the 500 whips the invader. I think the 500 puts out 325w++ carrier from a single tube.

The 500 is a really rare sought after rig the last one I saw fully restored was 3900 for the power supply and 3900 for the rf deck. The seller got what he asked for.

I was thinking of a poor mans 500.

The things that I see on the 500 that would require extra effort are the 150 VR strings and screen dropping resistors that are required for SSB. But the 2000 has a seperate coax output for the 100w level that would be fine to drive another linear like a thunderbolt ssb am cw.

If the PA section in the invader 2000 could be designed and plate modulated like the 500 and fixed tuned (for simplicity) for AM only and screamed in the AM window from 3875 to 3890 or 7290.

That would be quite a beast. Have your cake and eat it too. It looks (to me) like all the hard parts are there except for a 250w modulation transformer. They just need to be configured properly.

We could then have a reason to bring some of these invader 2000's back form the parts pile. Right now they are not real expensive like Globe Kings 500 or Viking 500's.
A real nice I2K cream puff is 900 and a parts rig is 300. I have seen them around.

If some one has a hurting 2000 that has been sitting for years and a 500 parts rig.
 It would really take the guess work out of it. Only so much you can see from pictures.

John AC0FA     



     

    How about build one homebrew? If the modulation transformer is a problem, then use cathode modulation. Here is a circuit that cathode modulates a Linear amplifier like a SB-200, or a SB-220, or one of the many with 4 X 572b amplifiers:

http://kx5jt.net/myPictures/wa0zhh.jpg

   A cathode modulated triode amplifier (driven and biased class C) will have both grid and cathode modulation. The RF amp efficiency varies from 33% (for 100% grid modulation, 0% cathode modulation) to 66% (100% cathode modulation, 0% grid modulation). So using this method with say a SB-220, getting > 300 watts AM without overheating the power supply is possible since the efficiency might be ~ 50% whereas a SB220 on AM in linear mode might only be 30-35% efficient. All those numbers are paper napkin doodles - SWAG.

  Doing the conversion to the Invader 2000 as I mentioned would be more tricky if you intend to keep SSB capability. If you drop the SSB mode, instead of the 7094's, maybe the 4D32 (maybe 3 of those with 1 kv plate) would do 300w AM. I seem to recall Brett using three 4D32's with 1200v B+ plate modulated. He says the 4D32 can take it. The 4D32's can be had NOS for $ 20 bucks each (last I checked).

Jim
WD5JKO

FB Jim;
I dont intend to keep SSB (in the PA side) of the 2000 at all. The 100w exciter and the 1200w PA are cramed into the same box and are able to operate independently. The exciter has a coax connector on the back seperate from the PA coax connector.

The exciter has a couple of 6146's, "That I am not to crazy about either. They seem kind of whimpy and kind of sweat to push out 20w carrier.
Any more than that they seem to fade fast but with the 10:1 one relationship between the exciter and PA. If i can get more carrier 25w-30w with a better tubes in the exciter it just might equal 250-300w carrier in the PA.)

Anyhow, the invader puts out real good audio and 120W ssb and has no trouble getting out and getting heard on 40 and 20 where my shorter antennas are more efficent. I don't really need an amp there most of the time.

 
Where I really need the amp is 80 meters with my (compromise) Vertical . I don't have the realestate or the height for a good 80 antenna.

The 3 x 4D32 finals sound good the price is right the power requirements are good. I would need a schematic which shows how they could be spliced into my invader sockets voltages and stuff. I am good at fixing and restoring things that already exist but i am not an engineer. Sounds like a good start Jim.   

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).

 

"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.         

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.

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
 

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
       

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