screen modulation again...

[N2DTS]

I have been switching things around a lot, screen modulating things, plate modulating things, using different modulators both screen and plate modulators.

The screen modulated stuff sounds much better to me and has no phase shift, and I notice the EQ needs to be changed between plate and screen modulation because the screen modulators are flat and the mod transformer changes the response some (I guess).
It likely does not mean anything, but I also like the peak power that is easy to get out of the screen modulated rigs.

So, I think a very nice rig would be a pair of 4-400's screen modulated.
Should do a nice 300 watts carrier and 1500 watts pep without working very hard.
I just need to get a pair of 4-400's.

[N2DTS]

Maybe three 4-400's would be good, or four 4-250's which might be cheaper.
As a test, I may be able to pull the 4-125's out of the 4x rig and get two 4-400's to fit (as a test).

One compact good sounding high power rig.

[N2DTS]

Got two 4-400's in the mail today and put them in the 4-4-125 deck.
Got 300 watts carrier out no problem at 3000 volts, not showing much color.
More 4-400's on the way.
Not sure if I should use two or three of them, two seemed to work well..

[N2DTS]

Put my new plate choke in to get more room, plugged the 4-400's in and tested it.
Peak power was down because I think the 4-400's want more plate voltage, at 3000 volts, they made about the same power as the four 4-125's but with little color showing.

Something also started smelling like toast (bread).
The tube sockets are too close together and to other things to run 4-400's for anything other then a test.
I also need to see if my power supply will be safe to run 3500 to 4000 volts, it was intended to be for the 813 rig with 2000 volts and 400 ma with headroom, not 4000 volts...

[N2DTS]

Looked at my power supplies and they have 3000 volt caps.
Guess I should not run them at 3500 volts any more!

I increased the air flow to the 4x150 pair and tested it at 200 watts out ok.

[N2DTS]

I ordered another filament transformer and I am going to scrap the quad of 4-125's and use the parts for a trio of 4-400's.
1200 watts of plate dis, can run 350 watts carrier easy at 3000 volts.

I suppose it would also make a nice amp for a flex or something.

I could also plate modulate it with the quad of 100th's at 3000 volts.

Its nice to have choices...

[Opcom]

Variac on the primary gives lots of choices!

[N2DTS]

I have variacs on everything, bias, screen and plate.
I tore apart the quad of 4-125's which was a hack job and found a feed through insulator had cracked and arced without me knowing about it.
Likely when I plate modulated it.

I ordered a 4X17X17 chassis so I could reuse the front panel, and to get three 4-400's to fit will move the loading cap over and use those CV joint type shaft couplings.
4-400's are quite wide!

I am going to make it with enough space to cool well and have a decent layout.
It should do 300 watts carrier under screen modulation without pushing things at all, which is more then enough for me.
Who knows what it would do with plate modulation...


 

[N2DTS]

Started on the deck, progress is slow in summer, too many other things to do...

http://n2dts.smugmug.com/Ham-radio/i-bZMpBhD/A

[PD0RTT]

Nice cabinets/chassis where do you buy these?

[N2DTS]

Hammond makes them, and maybe Bud, and Mouser or Allied sell tjhem, www.tubes and more carries some of the smaller ones I think.

[N2DTS]

Got the new setup done, three 4-400's with the screen modulator.
Does 300 watts carrier without pushing the tubes.
I could go up to 400 watts but that seems to exceed the power limits feeding the rack (120 volts 15 amps).
That would be 3000 volts at 400 ma on the tubes, 1200 watts just to the tubes, not counting bleeder power, filament power, screen power.

2500 volts and 400 ma seems to be the happy point, 300 watts carrier, dull color on the plates, 1200 watts pep.

[N2DTS]

At 300 watts out:

[Opcom]

That's very nice to see!

[VE3LYX]

Nice. Interesting watching you experiment and work your way through. Impressive to say the least in the day of "look what I bought" look what you made! congrats. Where do you hang out? I would like to hear it even if just reading the mail.
don

[VE3ELQ]

At 300 watts out:

Very nice work.  One question though.   What are those glass things with light bulbs in them?? 
73  Nigel

[N2DTS]

I went through the rig and tuned things up on 80 and 40 meters and noted the settings on the chart on the front panel.
I noticed the rig seemed to work much better on 80 meters, more power out with less color, and WAY more peak power output.
80 was 300 watts carrier and almost 1500 watts pep, and 40 was 300 watts and 1200 watts pep.

So I bypassed the wire wound 10 ohm resistors in the cathode circuits (filament trans center tap) and the 100 ohm wire wound resistors in the screen circuits, plus lowered the inductance of the parasitic chokes in the screen circuit (at the tubes).
I also took 2 turns off the 40 meter grid coil to center the tuning cap in its range, and spaced the windings out.

A retest had 40 meters do 300 watts carrier easy and 1500 watts pep.
Looks like it will run ok at 400 watts carrier and something like 1800 watts pep with the tubes running dull red on the plates.

Very happy with the way this deck turned out.
Distortion is low, and the only thing that limits or changes the frequency response is the audio chain equipment.
I have that set on the narrow side, not much past 5000 Hz I think.


I operate mostly 40 meters on weekends, mostly in the winter.
Summer I am outside on the motorcycle, or working in the yard, or doing some other chore, but operate from time to time on rainy days.

[VE3LYX]

In my twim 811A amp HB deal which I realize is a pipsqueak compared to yours I had trouble with the wirewound ten ohms in the cathode circuit. Sometimes depending on what I was driving it with I would find at the next use the amp would not work and would very quickly find out the 10 ohm wirewounds despite being 10W were popped. They never popped when in use . So after replacing them a few times I sat down and made a check list of what I was doing and why they had popped. I concluded there was no real good reason EXCEPT for the far off possibilty that the wirewound was introducing some HF level activity of an undesired nature into the circuit. I knew it was perhaps a long shot but could see any other reason for these resistor to pop in such a strange manner and they were never burnt, just open. Now perhaps they were made in the land of rice and the quality was questionable. i will give you that. However I bought several 2 watt carbons of the appropriate value and paralleled them to get at least the 10 watt rating and a similar ohm value. I have not had  problem of any kind since. It has now been awhile and I have done everything and more I did before with it so I am convinced my hunch was correct. As I suspect is yours.
don

[N2DTS]

Wire wounds have their issues, and likely are a bad idea in anything around RF circuits.
I just eliminated them and everything seems fine, all the tubes show even color...

[PA4WM]

So I bypassed the wire wound 10 ohm resistors in the cathode circuits (filament trans center tap) and the 100 ohm wire wound resistors in the screen circuits, plus lowered the inductance of the parasitic chokes in the screen circuit (at the tubes).


Summer I am outside on the motorcycle, or working in the yard, or doing some other chore, but operate from time to time on rainy days.

What bypass did you use?
Isn't a cathode always bypassed with say 10nf for a short HF path to ground?

I think we can omit the 10ohm/10 watt "center tap" resistors and replace them with two diodes from cathode to ground. In that case there is no concern about wirewound issues.


And I love the combination of hamradio and motocycles....  

[N2DTS]

Bypassed as in shorted them out with wire.
Next time I have the thing out of the rack I will remove the resistors and put something like a 1 ohm 2 watt carbon in the cathodes and a 10 ohm 2 watt carbon in the screens.

[w4bfs]

Bypassed as in shorted them out with wire.

Now thats STRAPPING

[N2DTS]

Not really.
It runs at about 300 watts out (dull red plates), could do more if the voltage was 3500 to 4000 volts maybe.
Interesting because it takes line level audio in and has no transformers in line, no mod transformer.



[/quote]

Now thats STRAPPING
[/quote]

[N2DTS]

Its coming up on radio season again and I was testing the equipment since its not been on for a while.
I hooked the QIX dx60 screen modulator circuit to the pair of 4x150's and gave it a tune up on 40 meters.
I can not find the diagram here, but its around someplace.
That design really works well, line input or D104, but I am pushing the tube (6de7) with screen current into a pair of 4x150 tubes at 200 to 250 watts out.
So I was thinking of changing the last Triode out with a 6080 triode section.
Or is there a better triode in a smaller package?
 

[w4bfs]

hi Brett .... had enough of hot wx ? .... the audio people have snatched up the 15W triodes with big $ .... maybe a triode connected tetrode ... in cathode follower service most anything will work fb