JazzGuitarGimp wrote:If you're measuring 715V between the reds, but nothing from either red to chassis, I would suspect the center tap is not connected to the chassis.
This is true. It appears that someone before me cut off the center taps. I'm guessing this was done when the power cord was changed to 3 prong.
Seriously, thank you all! I'm learning a lot from this!
JazzGuitarGimp wrote:If you're measuring 715V between the reds, but nothing from either red to chassis, I would suspect the center tap is not connected to the chassis.
This is true. It appears that someone before me cut off the center taps. I'm guessing this was done when the power cord was changed to 3 prong.
Seriously, thank you all! I'm learning a lot from this!
Well that pretty much explains everything except why anyone would cut the center taps! With the center tap disconnected, the bias tap becomes the center tap (actually an "off-center" tap ) but with the resistance to ground from the bias set network, the "center tap" will have a pretty significant negative voltage on it. This will also cause your B+ voltage to drop. The good news is that if the proper center tap was not grounded, misconnecting your bias tap to ground, as you did briefly, wouldn't likely hurt anything.
Firestorm wrote:Well that pretty much explains everything except why anyone would cut the center taps! With the center tap disconnected, the bias tap becomes the center tap (actually an "off-center" tap )
Yes! Same scenario as a fuse on the CT going open.
Well, it was working without the CT before, so I'm not sure.
I understand if you all want to shoot me by now, but if you feel like it, I'm attaching some audio of the bad noise and what happens when power is switched off.
BTW I just finished my vibrolux clone build, and it's amazing. I think I have to solidify my ground scheme, but otherwise it sounds great!
Well that doesn't sound good at all, and I'm not so sure that the explanation above is the correct one, as the bias supply components would have been severely stressed, probably to the point of burning up. I can't see where the HV center tap is grounded, so I still don't know what you have there. Did you make any changes? What voltages do you have now?
I'd like to see a couple of shots of the PT end of the amp showing all the PT leads and where they go. The bias supply looks odd: where there should be a reversed diode, there appears to be a 1/2W resistor (and a toasted one at that). The PT wire going to the 1W 470R (I think) looks red/yellow to me, rather than red/blue. If red/yellow, that is the center tap. Can you make out the colors?
Could this have been deliberate? If that is the center tap and the bias diode was eliminated, this might have been an attempt at back biasing.
I see something scary. There's a metal washer on the left-most screw for the circuit board that looks like it's about 1mm away from the HT connection. That's going to want to arc.
Firestorm wrote:I'd like to see a couple of shots of the PT end of the amp showing all the PT leads and where they go. The bias supply looks odd: where there should be a reversed diode, there appears to be a 1/2W resistor (and a toasted one at that).
It's a banded diode. An easy fix to this exercise is to unbolt that PT , throw it in the trash and install a stock equivalent.
) but with the resistance to ground from the bias set network, the "center tap" will have a pretty significant negative voltage on it. This will also cause your B+ voltage to drop. The good news is that if the proper center tap was not grounded, misconnecting your bias tap to ground, as you did briefly, wouldn't likely hurt anything.
