PW Series Modulated Transmitter

[DMOD]

Hi Phil,

How stout would the PS supply need to be here for the HV version. Say 1800V at 350ma be enough? I have an 8000 laying around that would be perfect for this I think (after the 813 rig is done, of course...have to stay on track).

John

One needs good PS regulation for any transmitter design.

I would think something like this should suffice.

In a CM system the average plate-to-cathode voltage across the final is about 0.70 X PSV to 0.8 X PSV depending on the tube gain.

The grid sees the same variable DC as does the cathode, minus the grid-leak bias. In a CM system one also needs some way to supply a protective bias in case RF drive fails which is why I included a zener.


Phil - AC0OB

[N4LTA]

I am a Hammond distributor, and am thinking a Hammond 284 X - same that I am using in the Nico Amplifier. It is CCS rated at 1200 VCT at 230ma  and I should get 600 volts at 430 mA or so with a two diode FW circuit. It also has two 6.3 V at 4 A  filament windings and a 5 volt winding that I won't use. I'll probably use another small transformer for the screens with the same screen regulator that I have used in the past. I think this transformer is enough for two 4D32s. Probably used a 10H 500 mA choke in the 600 volt supply to help regulate the voltage.

BTW - If you  need Hammond transformer , chokes,  or chassis email me with your best price and I will try to beat it with the items dropped shipped from Hammond to you.

Pat
N4LTA

[DMOD]

I am a Hammond distributor, and am thinking a Hammond 284 X - same that I am using in the Nico Amplifier. It is CCS rated at 1200 VCT at 230ma  and I should get 600 volts at 430 mA or so with a two diode FW circuit. It also has two 6.3 V at 4 A  filament windings and a 5 volt winding that I won't use. I'll probably use another small transformer for the screens with the same screen regulator that I have used in the past. I think this transformer is enough for two 4D32s. Probably used a 10H 500 mA choke in the 600 volt supply to help regulate the voltage.

BTW - If you need Hammond transformer , chokes,  or chassis email me with your best price and I will try to beat it with the items dropped shipped from Hammond to you.

Pat
N4LTA

The plate current required is about 440 mA@600V for a dual 4D32 and with the screen current of 50 mA that's 490 mA total. You need 7.5A@6.3V for two 4D32 filaments in parallel.


Phil - AC0OB

[N4LTA]

I see the with tetrodes the screen is fed with a large dropping resistor as in Plate modulation. Can this be done with a choke and a seperate power supply?


Pat
N4LTA

[DMOD]

I see the with tetrodes the screen is fed with a large dropping resistor as in Plate modulation. Can this be done with a choke and a seperate power supply?

Pat
N4LTA

As I stated earlier, "One thing about cathode modulated transmitters using tetrodes is they need solid screen voltages, low modulation stage series resistances, and plenty of RF drive."

RSG1 is not a large dropping resistor. It is a resistor to meet the screen voltage and the required screen current needed for changing plate current per the Raytheon tube specs. The electrolytics are there to smooth out the voltage variations, to try and keep the Vsg constant.

That is, we don't want the screen to self-modulate.


Phil - AC0OB




 

[N4LTA]

Then using the Hammond 372KX transformer  (600 volt CT at 575 mA and 6.3V CT at 9 amp) with a full wave bridge should work?

The old "economy style " connection by using the center tap as a full wave half voltage (300 volts) connection can work as a screen source eliminating dropping resistor heat has  worked well for me.

Thanks

Pat
N4LTA

[ka1tdq]

I'm still waiting on some other parts to arrive, but here's what I've got so far.

I'll have another meter to measure grid current. Audio gain and power control knobs will be below the meters.

The grid tank capacitor will be mounted about in the middle of the chassis and the 1/4" shaft will run up to the front panel as well.

Not being picky, but I did note that the screen dropping resistor for the KT-88s is shown at 1 watt. It should be at 5 watts or so.

Jon

[DMOD]

I'm still waiting on some other parts to arrive, but here's what I've got so far.

I'll have another meter to measure grid current. Audio gain and power control knobs will be below the meters.

The grid tank capacitor will be mounted about in the middle of the chassis and the 1/4" shaft will run up to the front panel as well.

Not being picky, but I did note that the screen dropping resistor for the KT-88s is shown at 1 watt. It should be at 5 watts or so.

Jon

Looking like it's coming together well.

Yes good catch, the dissipation is 2.97 Watts so a 5 Watt 3.3k will be needed. Bad holdover from another design.


Phil

[DMOD]

Then using the Hammond 372KX transformer  (600 volt CT at 575 mA and 6.3V CT at 9 amp) with a full wave bridge should work?

The old "economy style " connection by using the center tap as a full wave half voltage (300 volts) connection can work as a screen source eliminating dropping resistor heat has  worked well for me.

Thanks

Pat
N4LTA

Oh yea that should give you at least 100 mA of headroom. I am trying to envision your economy power supply. Have a drawing or a link?

I had plans one time for a dual 4D32 but never punched the metal for it. Should be good for at least 175 watts out.


Phil - AC0OB

[N4LTA]

I have used this connection many times. It was once called and "economy" connection.

Pat
N4LTA

[DMOD]

I have used this connection many times. It was once called and "economy" connection.

Pat
N4LTA

ok, my foggy brain now remembers that one. Should work well.

Here was the PS design for the transmitter that I never built. Sorry, it was to use a competitor's transformer.  


Phil - AC0OB

[ka1tdq]

I'm still waiting on the variable cap for the VXO to come in, and some knobs, but other than that the metal work is done.

I have distinct separate sections for RF, audio and power. I installed a shield to protect the audio section from the RF. I also floated the 12 volt transformer for the 7984 filament using a piece of teflon. It seemed like a good idea.

I added a separate switch to lift the ground connection on the full wave rectifier bridge so that the tubes could warm up before applying HV.

Jon

[ka1tdq]

There was enough room to put another wall in between the grid tank and the pi output network.

I tell you what, for PW and a rig that requires very little iron, real estate is still at a premium even with a large chassis.

Jon

[ka1tdq]

All the drilling and blasting is done. All that's left to do is the wiring.

I haven't bought the audio tubes yet.

Jon

[ka1tdq]

Most of the left side is done.

I used a large grommet hole to run the solid wire through in the center to attach to the plate/parasitic suppressor.

The more I build this rig, the more I'm excited about it and how it would perform paired with a big linear. I could build another triple 3-500 linear to be cost effective, but an Alpha or Acom would look really pretty.

Jon

[KD6VXI]

Be careful with regards to grommets.

I purchased some from harbor freight for feedthroughs on the posts of a 70A 12v power supply.

Plug it in, turn it in and it's under load.

Take resistance readings from each post to each other and to chassis.

I have resistance! And not much of it.

Using an ohm meter I find a hundred or so ohms.

Using my VNA I find it's conductive to the a couple kHz to 1.4 ghz.

I don't use those grommets for anything but shielded wire now.

--Shane
KD6VXI

[ka1tdq]

Thanks for the tip!

Yeah, Harbor Freight anything is bad news. Look at my holes for the larger tubes on the underside of the chassis. That was a Harbor Freight step bit. At a certain point the bit lost cutting ability and I finished the hole with brute force and the metal folded in from heating up. Anyway, it’s through... right?

Large washers on the top of the chassis made it clean up nicely.

Jon

[DMOD]

It will be interesting to see what the DC voltage division across the Final and Mod tubes are after you get it set up.


Phil

[N3GTE]

Don't know if any one mentioned it but you need some B+ for the osc.

Terry N3GTE

[ka1tdq]

Yes, and actually I had to make a few resistor changes in the original provided schematic since my lower B+ is 375 vdc and not 350. I hand drew a complete schematic yesterday. The oscillator supply is from the 375 vdc center tap and uses a series dropping resistor.

Jon

[K8DI]

Yes, and actually I had to make a few resistor changes in the original provided schematic since my lower B+ is 375 vdc and not 350. I hand drew a complete schematic yesterday. The oscillator supply is from the 375 vdc center tap and uses a series dropping resistor.

Jon

Hi Jon,
I am reading your schematic over coffee this morning, and I have a couple questions.

First, why no audio bypass caps for the last audio stage cathodes (and maybe why such a small bypass on the first stage)?

Second, the word around here seems to be that the screen bypass on finals should be smaller, like 0.001uF, to avoid bypassing the higher audio frequencies along with the RF. I’m not sure what the reactance target is supposed to be or how to determine the AC impedance at the screen so that one could choose the bypass to be small at RF but big at AF compared to the screen’s reactance in a given circuit, though.

I’m building my knowledge here....not questioning your decisions. Please explain so I can learn.

Ed

[ka1tdq]

I'm building my knowledge too. To be honest, I was relying on Phil's schematic for those components. I'm not sure what changing their values would have on the circuit.

Jon

[ka1tdq]

Modification to the schematic:

I'm skipping the voltage regulation on the 7984 screen and just using the series dripping resistor.

Jon

[ka1tdq]

I just finished.

Now I just need to wait for the audio tubes to arrive and test.

Jon

[DMOD]

Yes, and actually I had to make a few resistor changes in the original provided schematic since my lower B+ is 375 vdc and not 350. I hand drew a complete schematic yesterday. The oscillator supply is from the 375 vdc center tap and uses a series dropping resistor.

Jon

Hi Jon,
I am reading your schematic over coffee this morning, and I have a couple questions.

First, why no audio bypass caps for the last audio stage cathodes (and maybe why such a small bypass on the first stage)?

Second, the word around here seems to be that the screen bypass on finals should be smaller, like 0.001uF, to avoid bypassing the higher audio frequencies along with the RF. I’m not sure what the reactance target is supposed to be or how to determine the AC impedance at the screen so that one could choose the bypass to be small at RF but big at AF compared to the screen’s reactance in a given circuit, though.

I’m building my knowledge here....not questioning your decisions. Please explain so I can learn.

Ed

In post #10 I explained the reason:

One thing about cathode modulated transmitters using tetrodes is they need solid screen voltages, low modulation stage series resistances, and plenty of RF drive.

CM transmitters have great audio.

And in post #29 I stated:

As I stated earlier, "One thing about cathode modulated transmitters using tetrodes is they need solid screen voltages, low modulation stage series resistances, and plenty of RF drive."

RSG1 is not a large dropping resistor. It is a resistor to meet the screen voltage and the required screen current needed for changing plate current per the Raytheon tube specs. The electrolytics are there to smooth out the voltage variations, to try and keep the Vsg constant.

That is, we don't want the screen to self-modulate.

Phil - AC0OB