The AM Forum

Yes...this old topic is entered as "new topic" here.

I've read a LOT here on comments on modulating a tetrode.
And...lot's of other places as well!

It seems to me...that the 'issue' is getting max ( 100 ) modulation...by ALSO modulating the screen...along with the plate.

Here's my situation:
I have this exact homebrew circa 1959 pair of 807's in parallel... attached.
I have an Eico 730 modulator.

Frankly, I'm not looking for the 'best' audio...just want to be heard = have QSO's.

Just about ready to fire these up...with just plate mod...but...wish for your generous replies, first.

Tnx,
Steve

Almost, but not quite; you're missing an important part of the equation. Modulating the screen along with the plate isn't done simply to increase modulation percentage; it's necessary to maintain linearity. In a tetrode without modulation applied to the screen, the plate current won't follow plate voltage linearly.

PS
This 730 schematic may be helpful.

I would tie them together and modulate in triode connected mode. I run my 813s that way.

You can triode strap those 807s ..........they work very good that way..simple, there is an RCA bulletin on how to do it.  I have a copy out in the shack if you guys can't find it online...

Gosh...joining the grids...I think I could do that...
Please provide me the details.
Tnx

Looked at World Radio History...RCA published a load of things...can't find the 'bulletin'

cheesy cell phone pictures...  I strapped the 807s in a viking 2 and drive them with an external PP 6L6 speech amp using a  stancor A4761...Good Luck... Steve

https://ia801504.us.archive.org/4/items/RCA_Ham_Tips_v07n02/RCA_Ham_Tips_v07n02_text.pdf


page 3

Hello Steve

Excuse me...it looks like the 807's ARE the mod tubes.
Mine are the RF output that I want to modulate.

If you are looking for something different, Collins used 3 807's in the final and modulated them with a quad of 6L6G's. I will add the schematic for the modulator and final here. But you will notice that Collins like many other manufacturers at that time used a dropping resistor on the modulated B+ for the screens. I've been told the best way is to add a choke to modulate the screens that way.

Sorry......Never mind.......(:

 That was my impression from the start—that your intent was to use those 807's in the RF PA. Triode-connected tetrodes in modulator use, yes, but in the RF PA—not sure, really. Has anyone tried it? If it's not workable, then why? I'm thinking problems with input impedance and capacitance at RF with both grids tied together.

break the wire to the screens of the 807 above C4 and connect the screens to the plate voltage below the plate RFC.

load the transmitter to an input of twice your modulator’s watt rating.

start talking :-)

807 screens are only rated for 300V (some say 400V). Might be a problem.

Well Gents...spent the whole day on this...and..yesterday...and before that...etc..
Read a LOT...might have learned some...maybe "catching" up to you??

Plate ONLY modulation is for triodes...as you know.

The screen in a tetrode needs to be modulated...as you know.

Per Mr. Orr's "Radio Handbook"...there a 4 ways to do this.

Here are the choices: attached.

yeah, sorry, I forgot the “with a dropping resistor” part.

egg on my face!

Orr is awesome. You're reading the right stuff!

The West Coast Handbook is available, for download, for free.



http://www.tubebooks.org/technical_books_online.htm

Lots 'o stuff there.

KLC