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Hello
I have a Vox AC4 knock-off amp with minor tone adjustment changes to it, but the basic architecture is unchanged. I'm looking at voltages and have some fundamental (read: noobie) questions about them. I should note that the amp appears to operate fine and doesn't exhibit any audio issues. So I'm not reporting a behavioral issue.
The places I've taken measurements are shown in this diagram, locations A, B, C, and D:
What I'm confused about are the following measurements. Note that the input voltage to the transformer is ~115VAC (from the wall socket) and the output of the power transformer is measured at approximately as marked (within a volt or two) on the schematic (250-0-250). All the measurements below are with the amp at idle.
At "A", I'm seeing ~250VDC (with the expected post-rectifier ripple). I have a couple of EZ80 rectifier tubes I've tried and they both show roughly this voltage. The schematics I've been able to find don't show what the voltage should be at "A". They only show the B+ voltage after filtering and resistors (voltage at "B").
At "B", I'm seeing ~220VDC (reduced by the 1K resistor). This deviates quite a bit from the schematic, which shows 270VDC. Since the schematic shows 270VDC, I'm assuming this implies the voltage at "A" (to get 270VDC at "B") would need to be much higher than what I measured at "A" (more than 300VDC I would expect). I noticed that if I remove the EL84 tube, the B+ sits at about 365VDC. So the EL84 is drawing it down quite a bit. I did this experiment with the other tubes (EF86, 12AX7) and they didn't have nearly as dramatic effect on the B+. I tried another EL84 I had around, and it had a similar draw-down on the B+ (pulling it down to 210VDC).
My question at this point is: Is this amount of draw down on the B+ by the EL84 normal?
On a totally different note, I looked at the idle signals at "C" and "D" with an oscilloscope. The following are what I captured (I apologize for the poor quality of the images, but you get the idea):
Scope measurement at "C":
Scope measurement at "D":
The signal at "D" is directly the B+ voltage. It shows the expected ripple, which amounts to about 80mV peak to peak. The plate scope image is quite jaggy and is about 200mV peak to peak. My question on this one: Does the scope image from the plate look like what would be expected (i.e., does it look "normal")?
I have a Vox AC4 knock-off amp with minor tone adjustment changes to it, but the basic architecture is unchanged. I'm looking at voltages and have some fundamental (read: noobie) questions about them. I should note that the amp appears to operate fine and doesn't exhibit any audio issues. So I'm not reporting a behavioral issue.
The places I've taken measurements are shown in this diagram, locations A, B, C, and D:
What I'm confused about are the following measurements. Note that the input voltage to the transformer is ~115VAC (from the wall socket) and the output of the power transformer is measured at approximately as marked (within a volt or two) on the schematic (250-0-250). All the measurements below are with the amp at idle.
At "A", I'm seeing ~250VDC (with the expected post-rectifier ripple). I have a couple of EZ80 rectifier tubes I've tried and they both show roughly this voltage. The schematics I've been able to find don't show what the voltage should be at "A". They only show the B+ voltage after filtering and resistors (voltage at "B").
At "B", I'm seeing ~220VDC (reduced by the 1K resistor). This deviates quite a bit from the schematic, which shows 270VDC. Since the schematic shows 270VDC, I'm assuming this implies the voltage at "A" (to get 270VDC at "B") would need to be much higher than what I measured at "A" (more than 300VDC I would expect). I noticed that if I remove the EL84 tube, the B+ sits at about 365VDC. So the EL84 is drawing it down quite a bit. I did this experiment with the other tubes (EF86, 12AX7) and they didn't have nearly as dramatic effect on the B+. I tried another EL84 I had around, and it had a similar draw-down on the B+ (pulling it down to 210VDC).
My question at this point is: Is this amount of draw down on the B+ by the EL84 normal?
On a totally different note, I looked at the idle signals at "C" and "D" with an oscilloscope. The following are what I captured (I apologize for the poor quality of the images, but you get the idea):
Scope measurement at "C":
Scope measurement at "D":
The signal at "D" is directly the B+ voltage. It shows the expected ripple, which amounts to about 80mV peak to peak. The plate scope image is quite jaggy and is about 200mV peak to peak. My question on this one: Does the scope image from the plate look like what would be expected (i.e., does it look "normal")?
It's about 200VDC at the plate. So a little over 8 Watts dissipation at the plate?
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