Grounding a Bogen PH10 PA amp

[Pixel]

I was about to put a three wire plug on this nice Bogen PH10 when I noticed there was 0.6VAC measured between the chassis and either of the neutral or the ground in the wall socket. After running for a half hour it went up to 1.6VAC and stayed there, neither the power or output transformer ever got more than a little warm. I have not connected the chassis to the ground on the wall socket yet. I have already removed the death cap and fixed it so the hot wire is switched and fused.

I checked the primary and I don't find any leak to ground. I have not started pulling the rest of the leads to look for leaks there.

How should I proceed with troubleshooting this?

Thanks, Jeff

[Enzo]

By putting the three wire grounded power cord on it like you intended in the first place. Now the chassis will be grounded instead of floating up to whatever it feels like.

[Tom Phillips]

And that small AC voltage you measure is usually caused by parasitic capacive coupling. Your meter registers it because the meter's input impedance is high. (1 Meg Ohm typically)

[Pixel]

By putting the three wire grounded power cord on it like you intended in the first place. Now the chassis will be grounded instead of floating up to whatever it feels like.

That is a big relief. I have a lot of hours into this amp learning "tube stuff" and it would have been sad to have to replace the old iron.

Thanks! Jeff

[Pixel]

And that small AC voltage you measure is usually caused by parasitic capacive coupling. Your meter registers it because the meter's input impedance is high. (1 Meg Ohm typically)

Darn it, now I am sad again.

I had all the tubes out anyway so I hooked up a speaker and measured between chassis and earth ground again and found 0.6VAC. Then I put a 3 ohm power resistor between chassis and earth ground and found 0.8VAC across the resistor. There is no voltage showing between ground and neutral at the wall socket.

[TimmyP1955]

... There is no voltage showing between ground and neutral at the wall socket.

When the amp is on, off, or unplugged from the wall socket?

[Pixel]

When the amp is on, off, or unplugged from the wall socket?

There is no voltage between ground and neutral in all of the above situations.

[Tom Phillips]

...I had all the tubes out anyway so I hooked up a speaker and measured between chassis and earth ground again and found 0.6VAC. Then I put a 3 ohm power resistor between chassis and earth ground and found 0.8VAC across the resistor. There is no voltage showing between ground and neutral at the wall socket.

This is significant new information.
1) I'm assuming that you have not done the 3 wire grounded power cord conversion yet.
2) If there is 0.8VAC across a 3 ohm resistance connected between chassis and earth ground then there is 267mA flowing through that resistor which indicates that there is unacceptable leakage. That could be caused by a defective transformer or a failed component such as a line grounding capacitor affectionately known as a "death cap."

We have not seen a schematic for your Bogen PH10 and we do not know if it is stock or has been messed with over it's life. Can you post detailed photos of the circuitry?

Note about the "death cap." This can be present in amps that do not have a ground switch so you need to look and trace out the circuitry between the line power input and the power transformer primary. Can you do that? If that cap is found then we will suggest that you remove it and redo your leakage measurements.

[Jazz P Bass]

Here is the schematic.

Bogen_PH10.pdf

[Tom Phillips]

Here is the schematic....

Thanks for that schematic Jazz.
The cap I was talking about is C12 on the schematic.
Remove that cap and repeat your voltage measurements Pixel.

[Pixel]

Thanks for that schematic Jazz.
The cap I was talking about is C12 on the schematic.
Remove that cap and repeat your voltage measurements Pixel.

Hi Tom,

The measurements were all taken without the C12 capacitor.

When I got the amp there was no evidence that it had ever had anything replaced other than perhaps tubes. The amp was different from the schematic only in that R19 is 140 ohms instead of 250 ohms but I am pretty sure it came from the factory like that. I went over the amp pretty carefully and it seems to be wired like the schematic. No solder whiskers that I could see. Voltages in the power supply section are pretty much in agreement with what is in the schematic.

I replaced all the electrolytics and wax paper capacitors. I changed C1A, C1B and C2 to 22uF 450V. Left the mica caps alone. Replaced resistors that were more than 10% out of spec with metal film resistors of 2-3 watts. The thing sounds quite nice and I had the Rolling Stones going through it for a couple hours on the bench. Output and power transformers never got more than a little warm.

I am starting to take the power transformer out of circuit to look for leaks.

Thanks, Jeff