MaCo 300 amplifier

[Gman]

Why do some MaCo 300 amps have 2 power transformers? The 300M schematic, Im guessing is the mobile version (12v), showing 2 transformers, well the schematic symbol for 2 transformers, but I see other chassis' with 2 transformers yet a 120 vac ac cord. The 300B schematic, I can only assume is the base model, and shows just 1 power transformer. Would like some clarification before I buy one. Thanks.

 

[nomadradio]

The Maco 300 isn't a product name, it's a product line. It would get redesigned every production run, seemed like.

Five, tubes, six or seven. One power transformer or two, two fat Foster's Lager-size filter caps or three skinny ones.

Grid-driven driver tubes (grounded cathode) or cathode drive (grounded grid).

Like a box of chocolates.

73

 

[Amperex]

The Maco 300 isn't a product name, it's a product line. It would get redesigned every production run, seemed like.

Five, tubes, six or seven. One power transformer or two, two fat Foster's Lager-size filter caps or three skinny ones.

Grid-driven driver tubes (grounded cathode) or cathode drive (grounded grid).

Like a box of chocolates.

73

Maco only had one or two transformers at beginning. They used two for higher power amps than the Maco 200.

 

[Gman]

Ive not found a schematic showing this unfortunately. So is 1 transformer feeding 1 set of tubes and the other feeding the rest of the power tubes? Thats the only thing that makes sense to not overload just 1 transformer for all tubes.

 

[nomadradio]

If one transformer is big and the other one is small, this is the grid-drive version that requires a high negative DC voltage to control the driver tubes' control grids. The big one provides B+ and heater/relay voltages. The small one is for the negative grid bias supply. If it has two identical transformer they will be wired in parallel.

It's time I took my pocket folder of Maco diagrams and got them scanned to upload. Fedex Kinkos will do that. Their copiers will scan to a file. The diagrams I have are a random kind of mess, different revisions of different models, but they're probably better than nothing.

The circuits in that kind of box explain themselves once you've seen a hundred or more of them. Haven't cracked open that folder in a while. Just the same it might add some detail to what's in the CB Tricks collection.

73

 

[Gman]

Thank you Nomad.

 

[DelR]

Hey, just the subject I need. I come up with a Maco 300, and could use a schematic that matched. All I have is the one from cbtricks. This one has 5 tubes and 1 trans.

 

[nomadradio]

Here's the rundown of a 7-tube version we converted to six. Your specimen has a grounded-grid (cathode-driven) driver stage. Much better than the grid-driven version. You'll need a small radio to match that single driver tube. A Cobra 2000 busting a gut to swing 15 Watts would be about perfect. A single driver tube just isn't compatible with a modern radio that has two finals. Was no such thing for the designer to worry about in 1976.


https://www.worldwidedx.com/threads/maco-300-gets-47-year-tuneup.268043/

Still haven't made it to Kinko's, er Fedex.

73