I don't have the patience to produce a one-size-fits-all procedure for the Maco 750. Kinda like "Ford F100". It evolved numerous design revisions.
First thing to fix is the High/Low switch. If your amplifier was built with two "Foster's Lager" can size HV filter caps, this needs to get changed. And if yours was built with three filter caps in series, your High/Low switch is ALREADY wired properly. Maybe someday I'll sketch up the switch conversion for this.
If yours has the single "PRE"-driver tube, that comes out now, along with the plate choke that feeds to it. We'll be driving the two driver tubes directly.
We'll assume your amplifier has the High/Low switch wired between the transformer and the rectifier bridge. The original bridge looked like this.
I have no patience for burned rectifier bridges, so we beef it up to 6-Amp rated rectifier diodes.
These filter caps are held in place with a 3M product called "VHB". A double-sided foam strip with an andhesive that gets stronger from heat, not weaker. These pics are from 2014, before we started selling a pc board to do this job.
One detail we found to be absolutely necessary is to shield the driver tubes from the final tubes. It will want to oscillate without this.
Each tube should have its own separate parasitic choke. Some of these would combine multiple tubes on one parasitic choke. Not recommended.
There is a lot packed into this pic.
First of all you have a 3 Amp diode leading from the driver cathode choke to the center pole of the relay. A 6-Amp diode comes from the cathode choke on the six final tubes, carrying the cathode current to ground when the relay is keyed.
We had a problem with two-stage Maco amplifiers that would FLASH! a purple arc inside a driver tube when you unkeyed. Only happened when the relay was released back to receive mode. Figured out the insulation between cathode and heater was breaking down. Like any other kind of insulation, there is a limit to the voltage it can hold back. When it's exceeded, an arc forms between the heater wires and the cathode metal wrapped around it.
Bad juju, that.
A diode separates each of the two cathode circuits so they can't cross-feed from driver to final stage, or vice-versa. I'll admit I'm not completely sure I understand why this works, but it cured the habit of popping a tube when you drop transmit.
Shameless plug for the keying circuit we use. And sell.
The driver cathodes are fed from this input matcher. It probably came with this one, but rolling your own could be a challenge these days.
Yeah, it had variable cutoff bias to control the carrier. Control-grid bias on the driver tubes had a max of 60-odd Volts. Note the diode that blocks grid-leak voltage on the tube grids from reaching the control's center lug.
The higher bias voltage came from a full-wave voltage doubler and this small transformer tucked underneath.
The higher negative grid voltage made it seem wise to use a 10-Watt part from the driver grids to ground.
The final tubes share a single resistor return to ground. Violet wires are the control-grid circuit for the final tubes also.
The finals have about 16 Volts negative bias. One diode has its cathode on the 12-Volt heater pin of this tube. The filter cap's negative side goes to the anode of this diode to filter the DC. The second diode feeds from the filter cap to the control-grid pin of this tube, and through the violet wires to the grid pin of the other five finals. The second resistor blocks any high DC grid-leak voltage from charging the 1000uf filter cap.
So this just hits the highlights. Still reminds me of a frame-off rebuild of a 1978 riding mower. Sure you could, but should you?
73
First thing to fix is the High/Low switch. If your amplifier was built with two "Foster's Lager" can size HV filter caps, this needs to get changed. And if yours was built with three filter caps in series, your High/Low switch is ALREADY wired properly. Maybe someday I'll sketch up the switch conversion for this.
If yours has the single "PRE"-driver tube, that comes out now, along with the plate choke that feeds to it. We'll be driving the two driver tubes directly.
We'll assume your amplifier has the High/Low switch wired between the transformer and the rectifier bridge. The original bridge looked like this.
I have no patience for burned rectifier bridges, so we beef it up to 6-Amp rated rectifier diodes.
These filter caps are held in place with a 3M product called "VHB". A double-sided foam strip with an andhesive that gets stronger from heat, not weaker. These pics are from 2014, before we started selling a pc board to do this job.
One detail we found to be absolutely necessary is to shield the driver tubes from the final tubes. It will want to oscillate without this.
Each tube should have its own separate parasitic choke. Some of these would combine multiple tubes on one parasitic choke. Not recommended.
There is a lot packed into this pic.
First of all you have a 3 Amp diode leading from the driver cathode choke to the center pole of the relay. A 6-Amp diode comes from the cathode choke on the six final tubes, carrying the cathode current to ground when the relay is keyed.
We had a problem with two-stage Maco amplifiers that would FLASH! a purple arc inside a driver tube when you unkeyed. Only happened when the relay was released back to receive mode. Figured out the insulation between cathode and heater was breaking down. Like any other kind of insulation, there is a limit to the voltage it can hold back. When it's exceeded, an arc forms between the heater wires and the cathode metal wrapped around it.
Bad juju, that.
A diode separates each of the two cathode circuits so they can't cross-feed from driver to final stage, or vice-versa. I'll admit I'm not completely sure I understand why this works, but it cured the habit of popping a tube when you drop transmit.
Shameless plug for the keying circuit we use. And sell.
The driver cathodes are fed from this input matcher. It probably came with this one, but rolling your own could be a challenge these days.
Yeah, it had variable cutoff bias to control the carrier. Control-grid bias on the driver tubes had a max of 60-odd Volts. Note the diode that blocks grid-leak voltage on the tube grids from reaching the control's center lug.
The higher bias voltage came from a full-wave voltage doubler and this small transformer tucked underneath.
The higher negative grid voltage made it seem wise to use a 10-Watt part from the driver grids to ground.
The final tubes share a single resistor return to ground. Violet wires are the control-grid circuit for the final tubes also.
The finals have about 16 Volts negative bias. One diode has its cathode on the 12-Volt heater pin of this tube. The filter cap's negative side goes to the anode of this diode to filter the DC. The second diode feeds from the filter cap to the control-grid pin of this tube, and through the violet wires to the grid pin of the other five finals. The second resistor blocks any high DC grid-leak voltage from charging the 1000uf filter cap.
So this just hits the highlights. Still reminds me of a frame-off rebuild of a 1978 riding mower. Sure you could, but should you?
73
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