wa2tak
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« on: November 13, 2023, 08:53:00 AM » |
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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
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KD1SH
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« Reply #1 on: November 13, 2023, 09:13:33 AM » |
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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.
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It seems to me...that the 'issue' is getting max ( 100 ) modulation...by ALSO modulating the screen...along with the plate.
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Tnx,
Steve
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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.
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wa2tak
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« Reply #2 on: November 13, 2023, 09:17:29 AM » |
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PS
This 730 schematic may be helpful.
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w9jsw
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« Reply #3 on: November 13, 2023, 09:24:49 AM » |
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I would tie them together and modulate in triode connected mode. I run my 813s that way.
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KL7OF
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« Reply #4 on: November 13, 2023, 09:25:58 AM » |
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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...
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wa2tak
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« Reply #5 on: November 13, 2023, 09:46:57 AM » |
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Gosh...joining the grids...I think I could do that...
Please provide me the details.
Tnx
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wa2tak
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« Reply #6 on: November 13, 2023, 10:26:42 AM » |
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Looked at World Radio History...RCA published a load of things...can't find the 'bulletin'
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KL7OF
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« Reply #7 on: November 13, 2023, 10:49:57 AM » |
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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
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KL7OF
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« Reply #8 on: November 13, 2023, 10:56:49 AM » |
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wa2tak
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« Reply #9 on: November 13, 2023, 11:06:42 AM » |
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Hello Steve
Excuse me...it looks like the 807's ARE the mod tubes.
Mine are the RF output that I want to modulate.
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W3SLK
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« Reply #10 on: November 13, 2023, 11:44:22 AM » |
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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.
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Mike(y)/W3SLK
Invisible airwaves crackle with life, bright antenna bristle with the energy. Emotional feedback, on timeless wavelength, bearing a gift beyond lights, almost free.... Spirit of Radio/Rush
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KL7OF
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« Reply #11 on: November 13, 2023, 12:12:45 PM » |
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Hello Steve
Excuse me...it looks like the 807's ARE the mod tubes.
Mine are the RF output that I want to modulate.
Sorry......Never mind.......(: |
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KD1SH
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« Reply #12 on: November 13, 2023, 12:26:27 PM » |
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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.
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"Gosh, Batman, I never knew there were no punctuation marks in alphabet soup!"
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KC2ZFA
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« Reply #13 on: November 13, 2023, 01:12:41 PM » |
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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 :-)
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KD1SH
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« Reply #14 on: November 13, 2023, 01:55:28 PM » |
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807 screens are only rated for 300V (some say 400V). Might be a problem. 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 :-)
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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.
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wa2tak
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« Reply #15 on: November 13, 2023, 02:43:58 PM » |
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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.
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KC2ZFA
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« Reply #16 on: November 13, 2023, 03:10:14 PM » |
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yeah, sorry, I forgot the “with a dropping resistor” part. egg on my face! 807 screens are only rated for 300V (some say 400V). Might be a problem. 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 :-)
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KD1SH
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« Reply #17 on: November 13, 2023, 03:19:05 PM » |
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Orr is awesome. You're reading the right stuff! 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 |
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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.
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