Powerboard question

[Oddvar R]

Hi. I have just finished my 50w build and trying to turn it on. I was wondering what the reason for a drastic voltage drop I have on the 50w Dumble in blue circle on layout. Everything else seems fine, even the lamp doesn't glow more than it should, bias works, heater works, lamp works, all the cabling seems to be fine at the moment. Any idea?

[erwin_ve]

Hi. I have just finished my 50w build and trying to turn it on. I was wondering what the reason for a drastic voltage drop I have on the 50w Dumble in blue circle on layout. Everything else seems fine, even the lamp doesn't glow more than it should, bias works, heater works, lamp works, all the cabling seems to be fine at the moment. Any idea?

What voltage did you measure?

[martin manning]

I see a green circle... What voltage do you have there? There should be about 140V across the 27k resistor, roughly 5mA flowing through it for the preamp tubes and FET.

[Oddvar R]

On the mainboard side of the 27k resistor there is only 15mv, but on the other side it's roughly 360v and the other side of the 22k resistor I have 390v. That is with the lamp limiter in the loop. I have changed the 27k resistor and the adjacent electrolytic.

[martin manning]

Looks like there is a short to ground on the right (main board) side of the 27k. It's a 2.2k, not 22k, right?

[Oddvar R]

Yes, sorry, 2.2k.

[martin manning]

Power down, drain the caps, and measure resistance to ground from that point. If it's ~0, that confirms a short.

[Oddvar R]

It's 0. (With no power on)

The resistor gets very hot by the way, can't touch it for long, if that means anything.


As far as I can see, there are only to grounding points, or?

[martin manning]

Must be some wiring error or accidental grounding of the supply rail. You'll just have to find it.

[erwin_ve]

It's 0. (With no power on)

The resistor gets very hot by the way, can't touch it for long, if that means anything.


As far as I can see, there are only to grounding points, or?

You now have to look for unintended grounding causing the short.
Cross post Martin

[Oddvar R]

Hi and thanks so far. Since I won't do the fetboard I am sending the lead from the preampboard that was meant for that board to ground, is that ok or should it be taken somewhere else?

[Charlie Wilson]

Were is the fourth(clean side) electrolytic in your amp?
CW

[martin manning]

The short to ground is somewhere along the heavy red path I have traced in the snip below (from the 27k dropping resistor to the 2k2 dropping resistor). If you are not including the FET, ground the heavy yellow lead through a 150k-180k resistor.

[Charlie Wilson]

Hi and thanks so far. Since I won't do the fetboard I am sending the lead from the preampboard that was meant for that board to ground, is that ok or should it be taken somewhere else?

Yes I get that but that is where the wire leaves the board and there may be some issue with the wiring of the final electrolytic. However I am wondering about the above post. Whatever is being sent to ground, I hope a 150k resistor comes first.
CW

[Oddvar R]

The short to ground is somewhere along the heavy red path I have traced in the snip below (from the 27k dropping resistor to the 2k2 dropping resistor). If you are not including the FET, ground the heavy yellow lead through a 150k-180k resistor.

That's amazing, thank you.