Maco Super 75

[238]

The sch is for a Maco/ Brute 75, 1 M2057, so would the 180 ohm be correct for the Super 75 with 2 M2057? 238

 

[Dmans]

Replacement of the under chassis coax shown in your tube socket picture as well as grounding the shield at both ends of the coax run would be a "next best bet" to making this unit work correctly.

73
David

 

[238]

The pic is a before pic. The coax has been replaced with 316 coax & grounded at both ends. The 2 resisters going to pin 3&11 have been changed too 100 ohm x 2 watt. The sch, from nomad, is for a Maco/ Brute 75, 1 M2057, so would the 180 ohm be correct for the Super 75 with 2 M2057? As Chris said you find different ohm in these little Maco. I have 4 an they all have different values Thanks all answers are welcome. 238

 

[nomadradio]

Don't have any separate notes about the 2-tube version. The 180-ohm value is all I show in any notes here.

Maco would "tweak" the circuit values from one batch to the next. Makes it tough to pin down what's "best" when they were still trying to figure it out.

73

 

[238]

The one in the Super 75, 2 M2057, appears to be 47 ohm it checks at 58 ohm. How would you know if you need to change the value? 238

 

[238]

That was KJ4CYH's amp, on another post that had the tube turning red.It was a Brute 2 tuber. 238

 

[238]

I got my Maco Super 75 back together with new caps. Bias set at -103 vdc. Works goodbut seem it keys to high 80 watts swings to 150 on my Swan meter. Had 5 ohm x 5 watt grid resister changed to 3.9k. Has 357 volts at the tube keyed. Changed the input resister to a 180 ohm, with a 270pf across it. If you try to lower the dead key with the tune, you also lose swing. 238

 

[238]

Well changed the grid resistor back to the 3 ohm resistor. Dead key 80 to 100, swings to 150 to 180. The other 2 tuber i got has a 22 ohm x 2w for the input resistor. I put a 180 ohm in this one, thing i will put a 22 ohm in it an see if that changes the dead key. 238

 

[238]

Put a 22 ohm resistor in the input an every thing seems to be working ok. I must be looking at this backwards. The 22 ohm let the key go down as the 180 ohm made it go up. 238

 

[nomadradio]

All resistors are not created equal.

Some types have more inductance in them than other types.

This can make the way they affect RF current hard to predict. Maco used a carbon-composition resistor in this spot. They tend to have very little inductance. Other types will have more of that going on in them. A wirewound resistor will have a lot of inductance, since the inductance wire inside is usually wound into a coil to save space.

Adding inductance to that input circuit would most likely cut the drive level.

73

 

[238]

So it is best to use a carbon- composition or a carbon film in the input like Maco did? Thanks 238