viking invader 2000 increasing screen voltage mod

[AC0FA]

Slab Bacon KB3AHE
Mentioned increasing screen voltage in the 4-400a will make a big difference.
In output power. In original stock condition in class AB2 with no other mods than increasing the screen voltage. The max screen voltage listed in tube data for AB2 is 800v.
 I have an idea of how to increase the screen voltage supply for the finals in the invader 2000.
 I wanted to run by you.

Would it be possible to tap into the 800v power source intended for the plates of the 6146 finals and route it to the screens of the 4-400a’s.

In stock configuration the  2 x 4-400a’s run 2250V to 2500V on the plates of the 4-400a finals at 500-600ma. The stock screen supply provides 575v to the screens the bias is -150v.
Would the 4-400a’s be able to take 800V on the screens with only 2250V-2500V  on the plates?
would I need to make any other changes?  If so what and how?

The plan
The 200 low power HV supply is of course seperated from the rf deck and piggy back bolted to the external 2000 HV supply.  It generates 800v intended for the plates of the 6146's peanut whistle finals in the 200 exciter.

Invader 200 supply 800V I plan on using for the screens of the invader 2000 
It comes from the invader 200 exciter multi tap power trasformer (T101) , 5R4 rectifier (v101),
5-25HY choke (L101), the circut also has 180mf and 300k paralel to ground.

Tap the 800V DC power source after the swinging choke (L101) and the paralell resistor and capacitor string (C175-177) (R175-177) to  ground . and send it to the screens of the invader
2000 4-400a finals by connecting to the output of the disconnected original screen supply.
after the voltage divider of (R309) 1.5k and (R310) 3.5k.   

The invader 2000 screen supply I plan on by passing is configured as follows.
It provides 575V to the screens from a seperate screen supply transformer (T303), a 5U4GB(V303), 250 ohm resistor (R313) a 5HY choke (L302) and a voltage and capacitor circut which ends with (R309) and (R310) ands sends voltage to the screens of the  2 4-400a's in the final.

John AC0FA

[The Slab Bacon]

John,
        Around 750v+ is about where you want to be. 800v wont hurt anything but you might want to leave a little room for cushion. Just be sure that it is switched off in standby or you will prolly cook the screens out of the tubes with no plate voltage in standby. If you dont want to switch the screen voltage, to something to slam enough negative blocking biass on the control grid to cut the tube off hard during standby. Also, your resting plate current at resting unmodulated carrier will be higher. (the plate iron will take it).

Also, I had the same problem with my rectifier plate cap leads. I just removed the transformer end bell and slid a several layers of heat shrink tube over them. When I cooked the heat shrink, the old hard insulation, became soft and supple for a few minutes allowing me to shape it the way I needed it before it cooled and got hard again. That was quite a few years ago.

[AC0FA]

Wow thanks Slab;
I will tap into the screens at the external power supply. I might even leave the low screen power (tune setting) alone and just tap into the high power side of the circut. I can jumper in a way that uses all original switching etc. To turn off the screens allong with the plate etc.

So I have no reason to change the bias or anything else? I will only need to change the screens.

Cool beans I will give that a try.

I Might even screen modulate like your big 4-1000. I suppose the invader would need a 250w class transformer for that. Secondary windings for the screens and grid?

John AC0FA     

[The Slab Bacon]

If you do decide to scream modulate it, you will prolly only need 15-20 watts of audio and a small mod tranny.

My 4X1 rig is modded with a pair of 2E26s and an ARC-5 Mod transformer. (Used that combo because I had them)

[W2PFY]

If you do decide to scream modulate it, you will prolly only need 15-20 watts of audio and a small mod tranny.

I think a better approach would be to use a cathode follower scheme for cleaner audio. There must be many examples of that out there somewhere. One BC transmitter used two 4CX5000 in the final and screen modulated them using 3ea 4-65 tetrodes using no transformers. Maybe I'll be able to dig that schematic up. seems to me, that one 4-65 would be enough for a 4-1000 yube.
JMHO    

[ke7trp]

I do not know this rig but why not use a Screen Dropper from the plate supply?  Then the screen is modulated in phaze with the plate?

C

[The Slab Bacon]

I do not know this rig but why not use a Screen Dropper from the plate supply?  Then the screen is modulated in phaze with the plate?

C

'Cause it is a SSB rig with a pair of PL-175s as linear finals. It just happens to make excellent sounding AM after modifying it. He is just looking for a way to get more snot out of it. The problem is that there is not enough HV to the final plates to really make it sing for it's supper. 

[ke7trp]

Oh.  I see.  Not plate modulated!

C

[AC0FA]

Thanks for the good info.
Assembly line wearing me out will do some mods this weekend and find a neat seamless way to route the 800v. I need to look at all three schematics taped to the wall. It looks like a good splice will be the 1.5 amp fuse to the socket at the top of the heavy duty supply into the front plug of the 200 piggy back supply and jumpered to the output plug at the back.

As long as I am in there I can bring the original screen voltage 575v out the back of the 200 supply for the plates of the external 25watt modulator tubes 2e26 class. Far less power than the 811a mod transformer cheaper too. Lots of
UTC CVM-1 etc. Is there another cheaper more common tube that will work  in a push pull (class b?) modulator to modulate the screens? Thanks Slab

John AC0FA
       

[The Slab Bacon]

John,
       Dont forget that even with scream modulation, you re not get any more P.E.P. power out of it than what the final can produce. So the whole adventure may just be a moot point. Before building an outboard modulator, I would work on getting the peak power up as high as I could. Once you do that, you may find that an external scream modulator wont allow it to produce any more than it does without it.

Work on getting the screen voltage up, see if you can bump the plate voltage up a few hundred volts or so, maybe with a big variac wired into the primary of the plate transformer. Every little bit will make a difference on the putpoot wattmeter.
However........................................

You have to get away from the chicken band "every watt counts" mentality, and start looking at the DBM / S meter scales. Like Steve and others have said, if it doesnt give at least a 3db gain, noone on the other end is likely to notice it anyway. 3db is doubling your outpoot, and most receivers are calibrated either 3db/s unit or 6db/s unit, so anything past what you allready have may be somewhat a waste of time. If you are getting 200+ out of it, It might not be worth all the work for another 50 or so.

But the knowing that you have the extra horsepower under the hood sometimes gives that feeling of satisfaction, even if you arent using it       

[W2VW]

Good point. Some audio compression would yield a more effective signal than wringing a few more watts out of the tx.

There's pretty good control over unwanted distortion using the existing low level modulator after modification.

every watt counts

[AC0FA]

Roger that;
Increasing the screen voltage and plate voltage is where it is at.
I get the feeling Johnson kind of cut the balls off of it with the lower than normal screen and plate voltage. I thought there might be some un-tapped potential in the 800w AM Mode. Additional 50w-100w carrier would give another 400w pep.
It actually does have a lot of guts for a 2000w input linear on SSb It will do 1200-1300 watts pep.   

It does make sense 1000w input to the finals was the law back then on AM but now days I can do better Just want the invader 2000 to be as good as it can.
thanks Slab and all.
John AC0FA
       

[W2VW]

Rip out the HV supply and stick a modern low resistance secondary transformer into a voltage  doubler like just about every modern linear amp has. More power out the pipe.

Roger that;
Increasing the screen voltage and plate voltage is where it is at.
I get the feeling Johnson kind of cut the balls off of it with the lower than normal screen and plate voltage. I thought there might be some un-tapped potential in the 800w AM Mode. Additional 50w-100w carrier would give another 400w pep.
It actually does have a lot of guts for a 2000w input linear on SSb It will do 1200-1300 watts pep.   

It does make sense 1000w input to the finals was the law back then on AM but now days I can do better Just want the invader 2000 to be as good as it can.
thanks Slab and all.
John AC0FA
       

[The Slab Bacon]

If you can get 1200w PEP out of it, that should support 300w AM @ 100% mod.

Sounds like you need to hook it to a dummy load and an Oscope. Keep bringing your carrier level up and increasing the audio till you run out of headroom and see some flat topping. Then look at your inline wattmeter and see what your resting carrier is.
You might just be surprised, or..............

If you cant get more than 200w of carrier out of it, but still have a ton of audio left to go, you may have another problem hiding somewhere. Like I said on many occasions, I2Ks can be hateful pieces of szht to get right.

[flintstone mop]

HELL 200 watts carrier fully modultated is hard to beat..
I had 250Watt b'cast rigs and no problem getting through the QRM.It wasn't the extra 50 watts that did it..........................it's the antenna.
And Frank is right about the power increase. You would have to double it twice (6dB) to move most carrier signal meters.



It's gonna be the antenna that does the magic. Full sized dipole, 50 feet high, commercially available shortened vertical, with elevated radials has done wonders for me.
The Bacon has a working, effective, shortened antenna that does well

Fred

[The Slab Bacon]

John,
        Does your I2k have PL-175s or 4-400s for finals?? If you have 4-400s (drop in replacements) your output will be down a little from the PL-175s.

I wonder also, could your finals be a little soft? Mine makes 250-300 with ease and has plenty of positive peaks. Keep in mind that even with 33% plate efficiency, 800w DC inpoot should give 266w outpoot. With just an additional 100w of inpoot you should get 300w outpoot. That is only an additional 50mA of inpoot plate current. Also, you should be able to get a little bit better than 33% efficiency with that configuration.

If your 175s are getting a bit soft, you may have to settle for the 400s as 175s are getting to be made of unobtanium these days but 4-400s are still quite plentiful.

[AC0FA]

You make some good points Slab;
I should hook it to the signal generator and the scope to get a good base line first before I re-wire the screens.
The I2K has some low miles PL-175a's.  The specs I was quoting 1200w pep SSB and 800w pep AM was just modulated with my voice. I am confident that with 1200hz sine wave modualtion the carrier would go up. I didn't realize the figures you were quoting in your article of 250-300w was with 100% sine wave modualtion. It says 300w AM output in the Johnson manual also maybe they were refering to 100% sine wave modualtion. That would make a big difference. If I can get 300w carrier out of the I2K with a sine wave. She is running 100% Top condition. 800w dc input should give about 266w carrier out. The line voltage alone is higher these days and I have measured 250v AC line voltage in my shack. I will take a closer look at things this week end and establish some good starting figures. Thanks all.
John AC0FA

P.S. You are right 200w carrier cooks all the frost off the antenna, and hits like a ball peen hammer. When one is expecting 300w carrier out, it falls a little short. It makes one wonder what is wrong with the rig.         
 

[The Slab Bacon]

You dont even need to use an audio generator. Just dump the outpoot into a dummy load, use your voice and see how much carrier you can run and still make something close to 100% without flat topping.

If 200w carrier is all the quiescent carrier you can get out of it period (no modulation) you may have a problem somewhere. Or you are not loading it properly. You have to establish a benchmark with a scope and then you'll know how load it. These things can be a little peculiar to load until you figure it out. You need to look at it with a scope on the output. After tuning it up with a scope, you may be pleasently surprised. (or find a possible problem) The manual doesnt really tell the whole story when it comes to loading it up. You need to fiddle with it a little bit.

After experimenting with your voice then hit it with a sine wave if you want.

[AC0FA]

Fb;
Slab I hit the I2K with my voice today and it looked good on the scope removing the big resistor from the supply really helped out. The I2K pushed out 1100w pep with voice on both single side band and AM.

I have confirmed I am starting with a healthy I2K paitent. When I increase the screen voltage tomorrow I can see how much of an improvement it makes.   
The watt meter needle didin't bump much at all just hovered at 1100w . Turning the carrier over 300w didn't give any more peak power. It just filled in the valleys with carrier. Turning the carrier below 250w or so showed a significant drop in power. The I2K didn't take much audio to drive to full power. The audio pot on 3 drives the crytal by passed DSB AM quite nicely. The other modes like a setting of 5.

I normally just tune in cw for max smoke switch to AM drop the carrier to 1/4 of the max output and let her rip. I double checked this method with my scope and all seems fine.

Later
John AC0FA

[w3jn]

One point, doesn't the I2K keep the sideband filter in circuit when in AM mode, thereby providing carrier + one sideband?   I know Frank modified his to switch the filter in and out - we found out the hard way you gotta switch BOTH ends of the filter! 

Another trick when using SSB rigs on AM is getting the right balance offset in the balanced modulator so that the carrier vs audio ratio (ie modulation %) is correct.  If there's too much carrier, the lower power stages could be saturating on audio peaks causing flat-topping.  Only way to find out for sure is with a scope.

If you haven't modified yours, John, your PEP with one sideband will be less than with two. 

[AC0FA]

Hi, johnny;
Yes I did that mod also and switched both ends of the filter out of the circut.
The max plate curent for the 6146's is around 125ma  and the max plate voltage is around  750v.

That supply will work real nice for the 4-400 screens of the invader which only requires 25-50 ma of screen current and will take 750v.

The according to the tube data 4-400a's dont like more than 50ma average screen current.
Will they just draw what they need when i start to drive them?

The invader manual dosent deal with screen current at all.
John AC0FA     

[The Slab Bacon]

If it's working properly, the balanced modulator makes DSB with no carrier and if it is working properly with the filter totally switched out of the circuit, and the carrier is inserted a stage or 2 later. It doesnt unbalance the modulator to make AM.  That is why it is so critical to have the filter COMPLETELY switched out of the circuit.

In stock form, the I2K makes SSB-AM (carrier + 1 sideband.) Electrically removing the filter allows it to make true DSB AM.  

Most good wattmeters have a dampened movement, and you wont see a lot movement out of them. That is why Bird makes the "peal hold" peak reading kit.
In AM mode you'll just see some wiggle out of the meter needle, but most likely you wont see a ton of "forward swing" A scope is your best friend here!!    

1100w PEP = +275w carrier, thats quite respectable. I think I'd leave well enough alone at that point............................

[AC0FA]

Hi Slab;
I was thinking the same thing.
The old girl (I2K) is doing pretty good. Now that I got her all peaked and tweaked, scoped and doped. 275w carrier is better than 200w. 1100w voice on my Ameritron AWM-30 peak reading watt meter probably is a dampened fly wheel measurement.

I must have peaky squeaky spikes up to 1250W.
PL-175a's are getting red in the face, just like Doctor Slab said. 
Thanks for all the help.

I discovered I have a intermittent, wobbly, loose carrier insertion pot. (R51-52)
I will have to find re-build replace before I can operate. At this kind of power I want my carrier to stay where I put it.

With knowledge and age comes wisdom. The most important lesson I have learned from hot rodding these tube rigs is when to leave well enough alone. Especially with the I2k pushing 50 years old and being made out of un-obtanium.

Now, I will have to find something else to fix.  I might still borrow the I2K power supply for a nice plate modulated 4-1000 monster but that is a different thread .
Merry Christmas
John AC0FA

[The Slab Bacon]

<snip> "The old girl (I2K) is doing pretty good. Now that I got her all peaked and tweaked, scoped and doped. 275w carrier is better than 200w. 1100w voice on my Ameritron AWM-30 peak reading watt meter probably is a dampened fly wheel measurement.
I must have peaky squeaky spikes up to 1250W. " <snip>

But they airt real watts 'cause they aint Bird watts!! They mist be Dosy watts!!     


One thing one must realize when building or modifying things, whether they are Hot Rod cars or Hot Rod transmitters is when you've hit "the point of diminishing return".
It kinda sounds like you are pretty much there. If it is working good and most important RELIABLY any further mods may well be a big waste of your time.
Now, it sounds like it's time to use and enjoy it.

Have Fun with it!!