Modulator in a PP HB Rig

[Carl WA1KPD]

Hi,
I have completed the restoration of a really cool hb CW rig I picked up at Nearfest several years ago. It uses a pair of T-20s in PP with plug in coils. Very nicely done piece of equipment.

Anyway I want to modulate it and have a nice Stancor modulator to use with it, now that has to be recapped etc.

But before I I take the xmtr of the bench I want to provide for the ability to break in to the B+ for plate modulation, I am attacheing a copy of the schematic.

1. I plan to make the break at the red x on the right side of the schematic.
2. I am hoping the voltage switch just downstream will allow it to be dropped enough for comfortable AM.
3. There is an extra set of contacts on that switch and I am thinking of using them to short across the "X" in cw to protect the modulator xfrmr. Thoughts.

Appreciate your feedback

Carl
/KPD

[W7TFO]

1,2,& 3 all sound like a good plan to me.

Obviously keeping modulator proper as a separate unit gives you the ability to tailor the B+ used for best ratio.

I really like the Taylor (a ten on steroids) T-20.  A sort of "In your face" to the RCA dominance .

How about a photo or two of this rig?

73DG

[KA2DZT]

Your X looks to be correct to add in plate modulation.  The only thing you should change is the .01 cap at the base of the plate RFC. It's OK for CW but way too large for modulation.  Replace it with a .001 ufd ceramic disc at a voltage rating of 3-4 times the plate voltage.

Fred

[KA2DZT]

Looking further at the schematic,  you could do away with the switched circuit.  The switch changes the power supply from choke input to cap input to raise the HV B+.  The other contacts adds in more parallel resistance to the dropping resistors to bring up the B+ to the early stages when using the choke input filter.

Plate modulation puts a heavier duty cycle on the plate supply than CW.  Using the Stancor modulator the lower B+ is probably better and it will allow the plate supply to run a lot cooler.

My 2 cents, with inflation, it's now 3.85

Fred

[Tom WA3KLR]

Hi Carl, Fred, and Dennis,

I don't see any r.f. bypass condensers at the T-20 filaments to circuit common/ground.  I guess it worked o.k. without them, but the r.f. return path for the directly heated cathodes is through the filament transformer,  plate meter leads, then to common.  I think many designs ran this way but I wonder; perhaps o.k. at 160 and 80 but I have to question the 20 meters operation this way.

It would be interesting to get it going as is and then add the caps to ground at the point where the air-variable tank capacitor connects to ground.

My 3.85 cents worth.

[KA2DZT]

Tom,

A closer look and you're right, no by-pass on the filaments of the T-20s.  Probably went unnoticed with CW, with AM, I think there would be with some hum on the audio.

I just finished my HB AM xmtr.  I think I have about 30 pages of schematics of all the power supplies, control relay circuits, audio stages, etc.

Not one schematic of any of the RF stages, including non for the built-in VFO.

I figured, down the road, any good ham should be able to follow the RF circuits just by looking at them.  The rest of the xmtr isn't going to be that easy, even with the schematics.

Fred