Main fuse blows when trying to measure plate voltage

[DGMcCloud]

Hi, hope someone here can help me solve this.

I’ve just installed a matched quad of JJ EL34IIs in my Marshall 6100. The amp fires up fine, seems to work ok. I now want to bias these tubes, and to do so, I first want to measure the plate voltage.

DMM (AstroAI M4KOR) on DC, ground wire clipped to the chassis, red wire to pin 3 of one of the powertube sockets. All controls to 0, cabinet connected via a THD HotPlate. Fire up the amp, wait a couple of minutes, and the moment I flip the Standby switch, the main fuse blows.

I’ve tried different wires on the DMM, tried clipping ground at different places in the chassis, but the fuse constantly blows (I’ve gone through 4 already).

Does someone have an idea what I’m doing wrong here? Or whether there’s something wrong with the amp?

As said, the amp seems to work fine. Sounds OK, it’s just that I can’t measure the plate voltage atm …

Thanks

 

[Kuga]

Try pin 1 on power valve socket as ground point.

 

[BlueX]

Maybe you connected the red wire to the 10A socket on the DMM (instead of the V socket)?

 

[Purgasound]

Sounds like the meter is set to amperes and not voltage.

 

[DGMcCloud]

Man, I’m so stupid. New multimeter, and just read the manual again …. and I think I indeed had lead in the 10A instead of Input …. djeezzzz …..

 

[dtier]

If your meter is on DCV and your blowing HT fuse, try reading a power tube pin from the other side of the chassis. A DMM can induce occillation when connected to a power tube plate, sometimes just on one side of the push/pull circuit.

 

[PelliX]

new MF'er!

Maybe you connected the red wire to the 10A socket on the DMM (instead of the V socket)?

This would be my first guess, too. If you have a lightbulb limiter, now might be a good time to hook it up. Don't bias the amp based on its operation when running on the limiter, but it will generally save you from swapping fuses while determining the cause of the fault condition.