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PostPosted: Thu May 21, 2020 10:28 pm 
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Location: Shawnee, OK
I have an old AX84 P1 amp that I am trying to get running and learn more about diagnosing amp problems. I built this amp around 2010, worked fine for awhile and then started redplating both pre and power tubes, also had an oscillating sound. After that I kinda lost interest until I retired.

So checking the caps today, I found the cathode cap for the power tube to be out of spec. Using MM cap function showed the 100uF cap to be 69.5uF. The pre-amp cathode cap was good at 1uF. The 47 uF caps were low and going to replace them as well. They are Rubicon brand and noted for shady practices with their caps at the time. Am I correct from reading the tube theory docs that the cathode cap on power tube can affect the bias on both the power and pre-amp tubes?

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Last edited by Hankules on Sat May 23, 2020 8:18 pm, edited 1 time in total.

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PostPosted: Sat May 23, 2020 7:58 pm 
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Going through a rebuild/rehab of this amp and want to attach a separate star ground for the power section. Is this a good idea? However, looking at the layout, it has ground plane that runs the entire length of the board and terminates at one point. I'm thinking that I should separate power side and the amp side to prevent a ground loop. So, should I separate it between the the 3rd eyelet (C2 ground point) and the 4th eyelet (shield ground for VR4 control)? Also, is it correct that C6 is connected to both pin 3 and 4 on V2? I am attaching the P1 docs.
Attachment:
AX84_P1_100616.pdf [313.1 KiB]
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AX84_P1_080708.pdf [300.3 KiB]
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PostPosted: Sun May 24, 2020 3:52 pm 
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I don't see anything in the layout drawing that would create a ground loop. Unless you use an input jack that isn't insulated from the chassis AND you also connect the jack to the ground lug on the chassis. If you use an uninsulated jack just connect the board's ground to the jack and don't use the ground lug on the chassis.

You could split the grounds as you propose, though, and run the power grounds from the board and the power transformer to a separate lug on the chassis. That would provide a slightly lower resistance path to ground for them.

Yes, on the drawings C6 connects to both pins 3 and 4 on V2. It's for "heater elevation." The difference between the two heater wires will always be around 6 volts AC, but without being referenced to something the voltage is free to float to whatever level it wants with respect to ground.

Usually the heater winding on a transformer is a split one with the center tap connected to ground for a reference. In this case the winding is not split and the heaters are referenced to a positive voltage. The theory is the positive reference induces less hum. And for convenience, they chose to use the fixed cathode bias voltage for the output tube.

I built a version of this amp a few years ago. It started as a P1 but I eventually put in the rest of the parts and upgraded it to an SEL. I experimented with several references for the heaters, including using ground and a separate voltage divider connected to B+ for elevation. I ended up making an artificial center tap for the heater winding using a couple of 100-ohm resistors and connecting it to the output tube's cathode for a reference. I found this to be the quietest in terms of hum. Basically just connect one 100-ohm resistor between pins 3 and 4, and another 100-ohm resistor between pins 3 and 5.

I haven't heard that the cathode capacitor on a power tube can affect the bias on preamp tubes. If it's bad it will definitely affect the power tube, of course. Also if the power supply isn't working right because of bad capacitors, the whole amp won't work right.


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PostPosted: Sun May 24, 2020 4:09 pm 
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Thank for your insights. I will check the PT. Rebuilding from the pretty much the ground up :thumbsup:

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