Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: musicfixerupper on October 12, 2010, 10:02:12 am

Title: Capacitance showing on plate reading
Post by: musicfixerupper on October 12, 2010, 10:02:12 am

Hello all, while measuring plate voltage on an EL84 I can't get a reading and capacitance "uf" appears on the DMM screen. would this just be a bad solder joint?

Title: Re: Capacitance showing on plate reading
Post by: FYL on October 12, 2010, 10:11:07 am

Is your DMM set for voltage measurements? On which scale? Do you use the right sockets for your test leads?

Title: Re: Capacitance showing on plate reading
Post by: musicfixerupper on October 12, 2010, 10:19:33 am

Yes everything is hooked up properly. One of the power tubes read good but the other get the wonky reading.

Title: Re: Capacitance showing on plate reading
Post by: sluckey on October 12, 2010, 10:22:56 am

Which meter? Got fresh batteries? Can you try another meter?

Title: Re: Capacitance showing on plate reading
Post by: musicfixerupper on October 12, 2010, 10:42:29 am

Again, I'm still getting a good reading on one power tube.

Title: Re: Capacitance showing on plate reading
Post by: sluckey on October 12, 2010, 10:48:17 am

OK, I got that. What about my ???s

Title: Re: Capacitance showing on plate reading
Post by: musicfixerupper on October 12, 2010, 11:04:46 am

oops sorry, Wavetek meterman 35xl, batteries are fresh and its the only meter i have.

Title: Re: Capacitance showing on plate reading
Post by: sluckey on October 12, 2010, 11:23:11 am

I occasionally get bad readings on pa tube plates too using a Fluke 87V. Many times the tube will start oscillating. It is usually caused by a bad probe connection to the tube pin. Sometimes you can see a small blue arc at the probe tip. I now use meter leads with needle point probes that seem to work better. Try connecting the probe to the socket pin with a gator clip lead BEFORE turning on the plate voltage. Does that help?

Are you having any problems with the amp other than being able to measure the plate voltage?

Title: Re: Capacitance showing on plate reading
Post by: musicfixerupper on October 12, 2010, 12:41:14 pm

Nope same result. The amp also starts a hum when the probe is on the plate.

Title: Re: Capacitance showing on plate reading
Post by: musicfixerupper on October 12, 2010, 01:13:27 pm

I removed the phase inverter tube and i was able to get a reading on the power tube. Things get haywire again after i plug the phase inverter tube in. Different PA tubes didn't help

Title: Re: Capacitance showing on plate reading
Post by: sluckey on October 12, 2010, 01:44:46 pm

Tell us about the amp. New build? Does it work? Got a schematic?

Title: Re: Capacitance showing on plate reading
Post by: musicfixerupper on October 12, 2010, 02:43:15 pm

Its actually a fender blues JR that i am working on. I am going to clean up the lead dress and see if that makes a dfference.