new project for winter...

[N2DTS]

I think I am going to prototype the KS3K circuit and try it on my 4D32 deck, and if it works well, build the rest of the transmitter and put it in a cabinet with my first homebrew receiver for an all homebrew station in a box.
6AG7 xtal osc into a 4D32 with as much voltage as I can get on it, screen modulated by a 6DE7 with a 12AX7 d104 mic preamp.

[fg5fc]

Hello N2DTS, i guess the time is now to try that      http://amfone.net/Amforum/index.php?topic=26603.0

[N2DTS]

Yes, that looks to be the ticket!
I need to order some 12 volt zeners.

Hello N2DTS, i guess the time is now to try that      http://amfone.net/Amforum/index.php?topic=26603.0

[Opcom]

and don't skimp on wattage. It's only a few cents and they might survive a crapout better. just a suggestion.

[N2DTS]

I would just get the 5 watt jobs.
That is my problem when building, everything gets made crap out proof which makes everything huge.
Looks like a great circuit.

and don't skimp on wattage. It's only a few cents and they might survive a crapout better. just a suggestion.

[N2DTS]

Two or three ways to go, large, small, and/or with a vfo:

Big
http://n2dts.smugmug.com/Ham-radio/i-4r753qd/A

Small with outboard supply:
http://n2dts.smugmug.com/Ham-radio/i-hN3Gj2D/A

Supply:
http://n2dts.smugmug.com/Ham-radio/i-dng5VJ2/A

vfo?
http://n2dts.smugmug.com/Ham-radio/i-Q9bMNFb/A

Small with vfo would be tough.

[KC2ZFA]

I'm wondering what sort of output one will get from screen modulating a Class F stage (with 3f traps on both the tube's plate and cathode).

[KA2DZT]

Nice start on the new rig.  Wondering where you get the chassis?  Those HP 4 gang caps usually require some sort of reduction unit on the shaft otherwise the cap won't stay put.  I used two of them in my xmtr, one for the 6146 final and another for the 813 final.

[N2DTS]

I have no idea what class F is.

I use those 2 inch drives:
http://www.ebay.com/itm/2-Pieces-of-Calibrated-Vernier-Dial-for-Amplifier-Builder-/370770555222

Prices have really gone up.

Hammond makes chassis, and www.tubesandmore sells them.

[N2DTS]

$12.00 to $14.00 each!
I ordered three.

Also, I suppose no one makes crystals anymore, but that is the easy way to do a 40 meter AM rig.

ICM still supplies them:
http://www.icmfg.com/hamradio-manufacturers.html

Pete

I've used this guy with good results

http://www.af4k.com/crystals.htm

[PD0RTT]

I'm wondering what sort of output one will get from screen modulating a Class F stage (with 3f traps on both the tube's plate and cathode).

In a grid modulated amplifier the efficiency at carrier conditions are the half of that during maximum output level, that is during class C.
So when you use a class F amplifier the efficiency is about 88%, a class F amplifier equipped with grid modulation is about 44% under ideal circumstances.

Martin

[N2DTS]

Four 6de7's came in the mail yesterday, little tubes for modulators!
I was playing around with building another vfo as a test.
Its just an experiment to see what happens if I build it for 40 meters and key the power to it instead of running it on 80 and on all the time and doubling it to 40.

I wonder what the limit is for the 6de7 as a screen modulator.
Could I drive two 4D32's at 50 watts output?

[Opcom]

well it'll take a pretty high peak voltage on the power section and up to 50mA. Looks like plate dissipation is the only thing to watch. What screen current do you consider for the 4D32? Do you have a schematic drawn up?

[N2DTS]

Schematic?
What's that?

The 4D32 is like two 6146's, but the screen voltage and current is very low in screen modulated rigs as the carrier level is what it runs at with only peaks of audio getting up to normal levels, with the other peaks in the negative voltage and current zone.

The actual screen voltage and current with a 4D32 will be around 200 volts and almost no current.
Peak current will be whatever the screen current is at full output (100 watts).

It seems to me like what you modulate with the tube would not have much difference between them, more so the voltage input to the modulator and the voltage out to the tube.
A 6146, or a 4D32, or a number of tubes are all going to run about 700 volts on the plates, and 700 volts input to the modulator, with about 200 volts out to the screens, with little average current, but more current on positive audio peaks the more/bigger tubes you use.
Average current is going to be very low, peak current will be the normal screen current.
A 6de7 was used in the knight kit t150 to modulate a pair of 6146's...
A pair of 4D32's would be 50 watts carrier which is a nice power level...

[KA2DZT]

Four 6de7's came in the mail yesterday, little tubes for modulators!
I was playing around with building another vfo as a test.
Its just an experiment to see what happens if I build it for 40 meters and key the power to it instead of running it on 80 and on all the time and doubling it to 40.

I wonder what the limit is for the 6de7 as a screen modulator.
Could I drive two 4D32's at 50 watts output?

First rule in building a VFO if the VFO is going to be built in the xmtr.  The base frequency should be half of the lowest freq of operation.  Having the VFO oscillate on the same freq of operation may cause pulling or FMing to occur.  This problem is eliminated by doubling up to the operating freq.  OTOH, if the VFO is going to be some sort of solid state or digital VFO  you may not have this problem.

[N2DTS]

Thanks for that.
The last vfo/exciter I built runs on 80 and is on all the time, and when keyed is doubled to 40 meters.
It works very well.
I built it that way because I wanted it to run all the time to be stable, but not foul the receiver with its signal.
So I guess on frequency is out and I would need to double again.


I think I am going to build a standalone version of the 6DE7 screen modulator to try on various setups before I start building the standalone 40 meter rig.

I have tried and used the solid state modulator on various setups, and can try the 6DE7 on one, two and three 4D32's as well as one or two or four 4x150's to see what happens.

It would be VERY wild to modulate four 4x150's at 350 watts out with a little 9 pin tube!
D104 in, a 12AX7 and the 6DE7 would be quite crazy.

[N2DTS]

I got sidetracked, I received the 2nd socket for a 4x150 and changed the 2X 4X150 rf deck from the loctal sockets to the air system sockets.
The loctals worked ok for a year or more at 300 watts out but I want to push things a bit more.
It ended up being a real hack job because there was not quite enough room.
At some point I will have to re do the chassis.

Next is to build the 6DE7 standalone screen modulator and try it on various RF decks and see what happens.

[N2DTS]

New standalone tube type screen modulator so far:

[fg5fc]

New standalone tube type screen modulator so far:

i'm very interested by this project!

[W2VW]

New standalone tube type screen modulator so far:

i'm very interested by this project!

Me too. Brett's signal sounds FB also.

[N2DTS]

Got everything painted, parts mounted, and started the wiring.

[N2DTS]

Got the standalone screen modulator done and tested it.
Its a total failure (so far).
I hooked it up to the 2x4x150 rf deck and gave it a shot.
Lots of hum in the microphone input, plenty of distortion, the carrier level adjustment works fine, nothing burnt out/up.

With the line level input it sounds a bit better, still lots of distortion.
A pair of 4x150's may be a bit much to ask the modulator to deal with.
The mic input needs work to get the hum out.

I need to put the 4D32 rf deck in place of the 4x150 and put only one 4D32 in and see what happens.

[N2DTS]

Grounded one side of the 6.3 volt filament at the 12AX7 tube socket and got rid of 99% of the hum.
What hum is left is ok with me.

Doing tests, if I reduce the grid drive to the 4x150's to almost nothing, the audio gets very nice indeed!
Why is the grid drive so critical?
It was not with the solid state modulator, it did not seem to matter at all.
The adjustable grid leak resistor did not seem to make any difference in how it was set.

I adjusted the voltage input to the modulator, no real change in anything, the carrier level control works very well, I can get plenty of audio on the carrier, 50 watts carrier and over 200 watts pep, or run it much higher.
As the circuit is, the negative peak limiter does not work, a single 6146 in a dx60 is different then a pair of 4x150's, that may be why it does not work.

I actually like the way the D104 sounds in the modulator, but I need to reduce the low end a bit, it sounds a little bit too good for a low power rig.

So why would the grid drive be so critical and need to be so low?

[W3RSW]

My first thought is that it isn't too low.
Your whole audio and 6de7 circuit was designed to work into a 2 or 3 ma 6146 grid drive circuit. A la Dx60.

I wonder how clean was the audio back then in the Knight with a pair of 6146's?

Your single 4-150 or the 4d32 might be the edge of your circuit's capability.

Well, probably just mirrored or repeated your thoughts.

Perhaps you can parallel a couple of 6de7's with a low mu driver, say a 6c4 or half a 12au7.

[N2DTS]

But what does the grid drive got to do with it?
The modulator drives the screen grid, that sits at say 200 volts, I had 1000 volts on the plates of the 4x150's, and it sounds bad unless the grid drive is reduced to a very low value.
With my other screen modulator, the grid drive does not seem to make much difference in how things work.
Low or high grid drive, the screen is still going to be at 200 volts and modulated from there.
There was no problem getting 100% modulation positive or negative  with high grid drive, but lots of distortion.