Maco Super 75

[238]

Trying to find the valve of the 2 resistors that go to pin 2&11 and the one that goes to pin 5 & 9 on both tubes. 5&9 are wired straight across the 2 tubes, so you only have 1 resistor. Also i can only turn the -vdc down to -157 vdc . I put a NTE5156a zener 100v x 5w. The pot someone put between the zener an ground will turn down too 3 ohms. 238

 

[kopcicle]

@nomadradio may be along to help. Better him than me.
The CBtricks schematic is the only one I've ever seen.

 

[238]

On the parasitic choke i have read either a 36 or 47 ohm 2 watt. I have 30 ohm & 68 ohm would one of them be close enough? 238

 

[kopcicle]

just being anal, all parasitics the same, 30 to 68 ohm is fine. 47 ohm just happens to be more common.

 

[238]

ok. thanks. It is funny how you never have the one it calls for. 238

 

[238]

I have some metal fim 68 ohm @ 3watt will they be ok? 238

 

[238]

Have found out the resister that go too pin 5&9 is a choke. I believe the 2 resisters that go too pin 3&11 are 100 ohm, not sure. 238

 

[238]

I have tried to buy some carbon resisters, but i see NOS. Do they make new carbon resisters? Also i have some that are 20 years, do they go bad with age? 238

 

[kopcicle]

They go bad with heat (use) and humidity

 

[238]

These have been in my shop. Never been used. Most are 1/4 watt. Do they still make new carbon resisters? I have carbon film/ metal oxide + others. I am just not sure what kind to use. Thanks 238

 

[238]

Trying to find out the size of the resistor going from the input too the tube. Can not tell by the color bands. On Nomads sch not sure if it 18 ohm or 180. The one in mine reads 77 ohm, but guessing at the band colors it looks like 47. 238

 

[nomadradio]

The 180 ohm resistor from the drive coil goes to pins 5 and 9. No telling what you might find in that spot, this is what we found worked best. The disc capacitor mounted in parallel to it may or may not be the 270 pf value in the schemo. The note in the upper left-hand corner suggests that 120 pf works better for the 8950 tube.

The schematic I posted has a big boo-boo. The tube's grids are shown in the wrong order.

Very wrong.

Ought to patch that somehow.

If it showed the tube's pin numbers, it wouldn't matter that the order of the three grids in the tube symbol are scrambled.

Pins 4 and 10 should be grounded.

Pins 3 and 11 are the screen grid. This is where the positive voltage shown as "5" or "6" will go. The diagram doesn't show a resistor value for this one. Oughta take the bottom off the one in the boneyard and look.

73

 

[238]

This one has 2 M2057 in it. The disc cap is 270pf. I put a 100 ohm resisters too pin 3 & 11 on each tube.

 

[238]

r
This was supposed to go on the other post. 238

 

[nomadradio]

Both sockets really appear to be wired exactly the same.

Makes no sense that a tube would behave differently in one socket than it did in the other.

Unless one socket is bad.

Weird!

73