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National MB-40L vs MB-40SL




 
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Author Topic: National MB-40L vs MB-40SL  (Read 628 times)
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KI4YAN
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« on: June 16, 2020, 11:38:44 PM »

So going through a lot of my parts and I've come up with two MB-40-L, one with the original center-tapped link winding, and one with a modified 3-turn non-centertapped link winding. I've also got 95% of an MB-40-SL swinging link version, but parts of it are broken and it will need repair. I'm wondering just how much power can be run through one of these. All the data I can find suggests grid-tuning circuits of 20W or less, but I can't find any usage for being used as a link-coupled output. (specifically wondering if a push-pull 100W stage could use one of these in the tank circuit.)

I've also got a few small mod transformers, a UTC S-19 30W audio unit too-wondering if a 50W output unit would be OK to drive a medium sized amplifier after-this is all just for fun anyway.

If the MB-40-SL can be repaired, I'll use it. There seems to be more data on it. Does anyone have anything on the MB-40-L fixed-link version?
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WA4WAX
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« Reply #1 on: June 18, 2020, 12:40:16 AM »

Take a look at the short piece by author Nose in the February 1956 QST.

Good luck!
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Pete, WA2CWA
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« Reply #2 on: June 18, 2020, 12:55:18 AM »

Good info here:
http://amfone.net/Amforum/index.php?topic=18076.0
Including last post on the MB-40L.
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Pete, WA2CWA - "A Cluttered Desk is a Sign of Genius"
KI4YAN
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« Reply #3 on: June 23, 2020, 03:44:43 AM »

Who'd have thought, the 40 stands for how many watts...shoulda seen that comin.

Once I fix this -SL swinging link version, I think I might use it as the output network for a push-pull 2E26 stage! Now, to figure out how to get screen modulation for a push-pull output.

I'm thinking dropping resistor and screen inductors? I don't think the screens can be fed from the same source, so self-modulation would be the way to go I imagine.
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WA4WAX
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« Reply #4 on: June 23, 2020, 11:44:10 AM »

http://www.electronicsandbooks.com/eab3/manual/Magazine/Q/QST%20US/195602%20[156].pdf
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KA8WTK
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« Reply #5 on: June 23, 2020, 09:00:05 PM »

Web link above doesn't work.
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Bill KA8WTK
WA4WAX
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« Reply #6 on: June 23, 2020, 10:09:03 PM »

http://www.electronicsandbooks.com/eab3/manual/Magazine/Q/QST%20US/195602%20[156].pdf

Trying again.
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WA4WAX
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« Reply #7 on: June 23, 2020, 10:10:24 PM »

Here is what to do:

Copy the whole link, and paste it directly into your URL window.

Good luck.
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KI4YAN
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« Reply #8 on: June 24, 2020, 03:08:11 AM »

http://www.electronicsandbooks.com/eab3/manual/Magazine/Q/QST%20US/195602%20[156].pdf doesn't work as a hotlink because of the brackets around [156] in the url.

However, it is a link to a 1956 issue of QST magazine, and there is both a nice 6146 transmitter article and an article explaining and detailing the modifications to an MB-40-SL grid circuit, including mention of thermal damage occurring at 70W before the modification.

I'm still thinking a fun little 2x2E26 plate mod with the -SL as the output would be cool. 2 would do about 40W carrier, , and two more in the modulator could make 40W of audio easily.

40W of AM might seem silly, but it's enough to play with-and it'll take me a few years more to finish this anyway. Just something to occupy the brain.
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KD6VXI
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« Reply #9 on: June 24, 2020, 11:25:17 AM »

40 watts of carrier makes a darn fine exciter for two to three x 500Z tubes though!

--Shane
KD6VXI
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KI4YAN
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« Reply #10 on: June 24, 2020, 12:49:49 PM »

I have a pair of 4-125A's that I was going to plate modulate-the RF deck has been punched and channeled for airflow and the tubes mount in behind a screened window, would be trivial to wire it up as a grid-drive or even cathode-driven amp instead of plate mod, and probably wouldn't be held up on plate network parts like it has been for years.
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