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I've got a Park 150 here, came in working , but with a hum. It has the UL OT and has around 600VDC on plates, loaded with 4xKT88. The filter caps had some bulges, so I replaced all the original capcans, and also moved the giant 27K 9W dropping resistor in the power supply off board(where it had burnt) and onto the chassis with a chassis mount 10W. The amp sounds fantastic, but has a 60cyc hum that is not affected by the volume.
This OT is ultra linear, and the screens are fed by the screen taps. The screen resistors are these 4 large cylindrical 1K things, that also sit on top of two 40r resistors of the same size, and wired in series for 80r, that I believe were originally wired to the cathodes. There is a fixed bias arrangement also, which I restored with the cap job, and I'm getting a good range fromm -100VDC to -50VDC from it.
As it is now(when I got it), the cathode resistors were disconnected from the cathodes, which were grounded, so this amp is solely running in fixed bias, as opposed to the dual bias method it had previously.
I can't find a schematic for the power supply, and I've reached out to the tech from Psionic that has the video series on youtube, but haven't heard back. His amp had the same hum, but it was not an UL, so he just moved the screen supply to after the choke.
This power section is nearly impossible to trace out, as every wire is sheathed in tubing, and neatly bundled in groups. I would literally have to disconnect everything and start doing tests to figure out what is what. I'd like to avoid that.
I'm having to look at the Marshall Major schematics for clues. Something that I don't understand is that in this bridge rectifier setup, w/a CT PT, the CT is run to the junction of the first set of stacked filter caps, for balancing. It is a black wire. I understand that, and that also lines up with the color code for the Major. However, there is another black wire that also connects to the junction of the stacked caps, and I can't figure out what that is. That may neither be here nor there, but I'm trying to consider if my bias circuit or the heaters could be the culprit, and trying to get the big picture here first to get a plan together. There is another ground tab close by the first caps' ground, that has 2 wires grounded, that I believe to be the heater and bias supply center taps.
Any thoughts are helpful, thanks! Here are 2 schematics, one from Marshall and another redrawn
Marshall-Major-200W-Schematic.pdf
This OT is ultra linear, and the screens are fed by the screen taps. The screen resistors are these 4 large cylindrical 1K things, that also sit on top of two 40r resistors of the same size, and wired in series for 80r, that I believe were originally wired to the cathodes. There is a fixed bias arrangement also, which I restored with the cap job, and I'm getting a good range fromm -100VDC to -50VDC from it.
As it is now(when I got it), the cathode resistors were disconnected from the cathodes, which were grounded, so this amp is solely running in fixed bias, as opposed to the dual bias method it had previously.
I can't find a schematic for the power supply, and I've reached out to the tech from Psionic that has the video series on youtube, but haven't heard back. His amp had the same hum, but it was not an UL, so he just moved the screen supply to after the choke.
This power section is nearly impossible to trace out, as every wire is sheathed in tubing, and neatly bundled in groups. I would literally have to disconnect everything and start doing tests to figure out what is what. I'd like to avoid that.
I'm having to look at the Marshall Major schematics for clues. Something that I don't understand is that in this bridge rectifier setup, w/a CT PT, the CT is run to the junction of the first set of stacked filter caps, for balancing. It is a black wire. I understand that, and that also lines up with the color code for the Major. However, there is another black wire that also connects to the junction of the stacked caps, and I can't figure out what that is. That may neither be here nor there, but I'm trying to consider if my bias circuit or the heaters could be the culprit, and trying to get the big picture here first to get a plan together. There is another ground tab close by the first caps' ground, that has 2 wires grounded, that I believe to be the heater and bias supply center taps.
Any thoughts are helpful, thanks! Here are 2 schematics, one from Marshall and another redrawn
Marshall-Major-200W-Schematic.pdf
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