Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Oddvar on October 21, 2016, 02:52:03 am

Title: Hoffman AC30 electrolytic blew up
Post by: Oddvar on October 21, 2016, 02:52:03 am

HI again. I was just finishing my Hoffman AC30 build, plugged it in, and after a few seconds the electrolytic capacitor besides the 50 ohm 10 w resistor blew up, creating a lot of smoke.  It was a 220 uf 100 volt cap.  What could possibly cause this I wonder?  The fuse didn't blow. V3 went all white at the top.

Title: Re: Hoffman AC30 electrolytic blew up
Post by: tubenit on October 21, 2016, 05:14:11 am

Installed backwards?   Exceeded voltage rating?  Faulty cap? 

Title: Re: Hoffman AC30 electrolytic blew up
Post by: Willabe on October 21, 2016, 09:26:20 am

Quote from: Oddvar on October 21, 2016, 02:52:03 am

V3 went all white at the top.

That tube lost it's vacuum, it's dead. The white is/was the silver colored getter.

Title: Re: Hoffman AC30 electrolytic blew up
Post by: 2deaf on October 21, 2016, 09:53:45 am

If the 50 ohm resister is not presenting any resistance in the circuit, the electrolytic will charge up until it explodes.  Exciting.  Check to be sure the cathodes have around 50 ohms to ground.

Title: Re: Hoffman AC30 electrolytic blew up
Post by: bnwitt on October 21, 2016, 10:00:06 am

I never start up a new build without having it on a light bulb current limiter.  This can save a lot of destroyed components when something is faulty improperly valued or wired wrong:

https://www.youtube.com/watch?v=wRFRwOnLsZI (https://www.youtube.com/watch?v=wRFRwOnLsZI)

Barry

Title: Re: Hoffman AC30 electrolytic blew up
Post by: PRR on October 21, 2016, 10:45:48 am

Bad EL84.

Title: Re: Hoffman AC30 electrolytic blew up
Post by: shooter on October 21, 2016, 11:41:07 am

Quote

Bad EL84.

but as the cause, or another effect

Title: Re: Hoffman AC30 electrolytic blew up
Post by: Oddvar on October 21, 2016, 01:28:42 pm

Didn't see a an unsoldered ground from the board to the cpacitor that blew, bad work.  Now I have sound, but accompanied with a bad hum, the sound is low.  Gues I have som investigating to do...