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Maco 200 all finals

238

Active Member
May 20, 2019
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I bought this amp 7 years ago, ran it some. It had been converted to all finals. 6lb6 Decided i needed to overhaul it. The input never had any effect on it. I added a 100pf across the legs no different. I have put new caps & diodes in it. No sure how you figure out what to do with the input. 238 DSC00859.JPG DSC00860.JPG
 

" It had been converted to all finals" What do you mean by this ?

I think what he means is that all four tubes are tied in parallel, with only one stage, the 'final' stage. No driver.

The input-tuning capacitor on the rear panel is set by placing a SWR meter and a second coax jumper between the radio and the amplifier. The cap is adjusted for lowest SWR between the radio and amplifier.

Using the output-power reading on a wattmeter to set this adjustment almost never works quite right. It's an input-side circuit you're adjusting.

This one looks bone stock, as if it was built to use tubes with the 6.3-Volt heaters.

The one suggestion I always make for a Maco amplifier is to ground the coax braid on the coax that runs from the Load control to the relay. Stripping a quarter inch of the plastic sleeve from each end would let you wrap a thin wire around the exposed braid and solder it.

If you have a light touch.

Ground the other end of that wire at each end of the coax, or at least the relay end. This will improve the stability of the amplifier when it's being tuned.

73
 
Last edited:
It has 6lb6 tubes. The coax has been grounded on both ends, but it is old coax. I will change it to 316 coax like i did my others. Then i will try the swr meter in between the radio & amp. Thanks 238
 
Changed the coax out to 316 grounded at both ends. When they changed it to all finals they removed the driver tune. You have the input match still there. It is a 304m with the coil soldered across the legs. One wire, off the coil, goes to the relay the other to pin # 3 & 11. Pin 3 & 11 are tied together on all 4 tubes. On the other Macos i have there is a resistor inline with the wire before it is tied to Pin #3. I believe you can see it in the pic above. 238
 
This amp was changed from 1x3 to all 4 tubes are finals. They are 6lb6. Changed the coax out to 316 grounded at both ends. When they changed it to all finals they removed the driver tune. You have the input match still there. It has no effect on tuning. It is a 304m with the coil soldered across the legs. One wire, off the coil, goes to the relay the other to pin #2 & 6. Pin 2 & 6 are tied together on all 4 tubes. On the other Macos i have, which are 75s, there is a resistor inline with the wire. I believe you can see the gray wire in the pic above. 238
I had to change the pin # because i put the wrong # in the first post.
 
Well i put it back together with out a resistor. It does have some effect now. The only thing i changed was the coax, put 316 in it, grounded at both ends. Using a Drake W4 between radio & amp it shows 1 watt in reverse. Going try a resistor in line later after i run it a little. 238
 
238,
It is doubtful that adding a resistor between the input tune capacitor/coil to the tube(s) cathode will lower your input match. I don't think the resistor was there when initially built. (Maybe it was there on a single tube box???)
The diameter of the coil, number of turns and length of the coil and the value of the capacitor was "optimized" for a single 6LB6 driver input match most likely to a tube type driver (that was not particular of a 3:1 mismatch) as was common when this unit was built.
Now that your transistor driver(Cobra 200) is "seeing" (or driving) 4-6LB6 tubes, you will most likely have to "optimize" the diameter of the coil, number of turns, length of the coil and value of the capacitor for your situation.
Nomadradio has made several posts in the last year or so addressing this situation. A search of this forum may likely yield the answers for you.

73
David
 
I usually run a Sonar or a Browning, but i would have to run a driver. I might have a coil out of a 300/750 it might be different. The one tubers had a resistor between the little stand up input & the tube. 238
 

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