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Author Topic: "Rookie" question on modulating...  (Read 1532 times)
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wa2tak
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« on: November 25, 2023, 07:34:37 AM »

I've read a lot about modulating an RF transmitter.
Alllllllllllll those details are mind boggling to me.

My simple question:
Can I use my UNALTERED Eico 730 50 w modulator
with my homebrew 2 x 807's with input of 60 watts...40 watts out?

If "not really"...what must be done?
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K8DI
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« Reply #1 on: November 25, 2023, 11:27:10 AM »

The follow up question is a “yes, but”…

Yes, but how well will it work? You will have to modify your transmitter at least a little bit. You may find you have a poor impedance match between the two which may mean you can’t get 100% modulation, but that’s not critical to it “working”.  A schematic of your HB transmitter would let readers help with suggestions quite a bit.

Aside…2x 807s being 60w in 40w out seems like only one tube is running, or has a relatively low B+ voltage. Not meaning to bust your chops, but there may be quite a lot left on the table there.  For comparison,  my HB AM single 807 transmitter is 67w input, 44w output, and I think I could do better if I started over…learned a lot since then…
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Ed, K8DI, warming the air with RF, and working on lighting the shack with thoriated tungsten and mercury vapor...
W7TFO
WTF-OVER in 7 land Dennis
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IN A TRIODE NO ONE CAN HEAR YOUR SCREEN


WWW
« Reply #2 on: November 25, 2023, 11:48:47 AM »

You'll want to mod-u-tate the screens a little in your TX for linearity... Wink

That is, unless you're running them triode connected.

73DG
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Just pacing the Farady cage...
wa2tak
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« Reply #3 on: November 25, 2023, 03:41:46 PM »

Ed...you're right!

And I made a calculator mistake before, too.

Took new 807 readings:
In Operate:
Plate 600 v
Screen 297
Grid -48
Cathode 183 ma

comes to 110 watts in!!!

Now...EACH 807 in the rig by itself = 40 watts into dummy
Both in = 40 watts!!!!

The soldering job is most questionable...found a terminal 'soldered' that was literally loose.

And...BOTH output variables had NO ground...due to 'chassis aging' surface.

At this point I need to check the output coils.

Really appreciate your chop busting...certainly helps!!!
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KD1SH
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« Reply #4 on: November 25, 2023, 04:26:24 PM »

Heads up—cathode current includes screen current; you've got to subtract the screen current before you can arrive at the true DC input. Control grid current, too, but it's comparatively small.

Ed...you're right!

And I made a calculator mistake before, too.

Took new 807 readings:
In Operate:
Plate 600 v
Screen 297
Grid -48
Cathode 183 ma

comes to 110 watts in!!!

Now...EACH 807 in the rig by itself = 40 watts into dummy
Both in = 40 watts!!!!

The soldering job is most questionable...found a terminal 'soldered' that was literally loose.

And...BOTH output variables had NO ground...due to 'chassis aging' surface.

At this point I need to check the output coils.

Really appreciate your chop busting...certainly helps!!!
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"Gosh, Batman, I never knew there were no punctuation marks in alphabet soup!"
—Robin, in the 1960's Batman TV series.
W4AMV
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« Reply #5 on: November 26, 2023, 12:27:37 PM »

Perhaps another reference point on 807's... I am running 4 || 807 (RCA)
at 1 kV on the plates. Screen at 300 V with very heavy grid swamping. I probably over did it! Anyway, 10 dB power gain and run CW at 120 W output. Main point, the old 807's from way back readily handle the plate V with no issue. I was concerned with flashover, but the AM folks here said not a problem. As the 807 at 750 V with proper modulation see peaks well in excess of their published ratings.
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KD1SH
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« Reply #6 on: November 26, 2023, 12:32:06 PM »

  As far as getting 40 watts out with both tubes or a single tube, it appears that you've proven that both tubes are good, but have you proven that both sockets are supplying everything needed? I would imagine that you checked each tube in each socket, but sometimes it's easy to overlook the obvious in the heat of battle (ask me how I know). Another obvious dart-toss: did you re-tune—both plate and load—when you went from both tubes to the single?
  You might also want to take a closer look at the supply; it could be that it's maxed out. Still, it's weird; 183ma, even with the control grid and screen currents included, seems like a lot for a single 807. Like K8DI said, a schematic would be helpful. Where are you getting your screen voltage from? If you're using a zener shunt regulator to bring the 600v down to 297v, whatever value of current you've established as your zener's quiescent current (I've always preferred "stiff" regulators myself; to heck with the waste) is part of the total load on the supply, of course, even though it's not part of the cathode circuit.

Ed...you're right!

And I made a calculator mistake before, too.

Took new 807 readings:
In Operate:
Plate 600 v
Screen 297
Grid -48
Cathode 183 ma

comes to 110 watts in!!!

Now...EACH 807 in the rig by itself = 40 watts into dummy
Both in = 40 watts!!!!

The soldering job is most questionable...found a terminal 'soldered' that was literally loose.

And...BOTH output variables had NO ground...due to 'chassis aging' surface.

At this point I need to check the output coils.

Really appreciate your chop busting...certainly helps!!!
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"Gosh, Batman, I never knew there were no punctuation marks in alphabet soup!"
—Robin, in the 1960's Batman TV series.
wa2tak
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Posts: 110


« Reply #7 on: November 26, 2023, 03:39:18 PM »

Well Gents, here's the final tally...( for the time being )?

Gone from 40 watts out to 60...on 40...don't have xtals for other bands...that could be the 'time being'.
"Logic" told me that  - most likely - something AFTER the 807's 'is' questionable.

Put my nice Eico GDM ( with full set of coils ) to the loading coil.
Dipped at around 8-9ish....hmmmmmmmm
Moved the tap...found sweet spot 3 turns away...Walla.

Did some research:
The rig by Mr. McCoy QST Sept 1959 is - except for the keying method - identical
to the Globe Chief...on market in 1958-9
Who came first...or is he on second base  Smiley

Found 2 items on the Internet:
D-Lab got 60 watts out on his Globe - 40M
A forum gent got 48 watts from his - 40M

Modulation
The Eico 730
Homebrew the Eico SM 90 screen modulator

Permit me, please, to present my views on plate vs plate/screen vs screen modulation.
As you know, they ALL work!
THE matter is how WELL!

LOTS...repeat...LOTS of info/data/testing...opinions.

Tnx for all the help.

HAPPY HOLIDAYS TO YOU & YOURS!!!!!!!!!!

73
Steve
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