Cap-Lifted Filament Center Tap
Posted: Fri 01/22/21 2:30 pm
Greetings 18Watters,
I was a regular here about ten years ago and then life got busy and I disappeared for a while.
A friend's Ampeg Reverberocket got me back to the electronics bench recently and I'm reaching out with a question: The filament center tap is isolated from ground by a .1uF film capacitor, which I've seen in other Ampeg models from around this time, and I'm really not sure why.
The model is a GS-12r from late 1965 (12AX7, 6U10, 7199, 7591x2). It's cathode biased and the cathodyne phase inverter has the cathode running at 50v (probably a little higher at modern mains voltages). The only reasons I can think of for such a cap would be either to filter out high frequencies from the heater circuit (doubtful that's why it's there) or to allow the heater DC reference to float with the cathodes - an odd choice but plausible.
As of now, I haven't even fired the amp up yet. It's in terrible condition, sat for many years and was worked on previously by someone who shouldn't have been in there at all - ever. I'm currently going through it and cleaning up everything that's obviously wrong before I apply power. I'm undecided as to whether I leave the cap in the circuit (replaced with a nice, new one, of course), remove it and ground the center tap directly, or lift it with a voltage divider but I'm curious to know what the group's thoughts are regarding why the cap would be there in the first place. Any takers?
Thanks,
Ethan
I was a regular here about ten years ago and then life got busy and I disappeared for a while.
A friend's Ampeg Reverberocket got me back to the electronics bench recently and I'm reaching out with a question: The filament center tap is isolated from ground by a .1uF film capacitor, which I've seen in other Ampeg models from around this time, and I'm really not sure why.
The model is a GS-12r from late 1965 (12AX7, 6U10, 7199, 7591x2). It's cathode biased and the cathodyne phase inverter has the cathode running at 50v (probably a little higher at modern mains voltages). The only reasons I can think of for such a cap would be either to filter out high frequencies from the heater circuit (doubtful that's why it's there) or to allow the heater DC reference to float with the cathodes - an odd choice but plausible.
As of now, I haven't even fired the amp up yet. It's in terrible condition, sat for many years and was worked on previously by someone who shouldn't have been in there at all - ever. I'm currently going through it and cleaning up everything that's obviously wrong before I apply power. I'm undecided as to whether I leave the cap in the circuit (replaced with a nice, new one, of course), remove it and ground the center tap directly, or lift it with a voltage divider but I'm curious to know what the group's thoughts are regarding why the cap would be there in the first place. Any takers?
Thanks,
Ethan