Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: acheld on February 05, 2021, 04:59:55 pm

Title: Troubleshooting AB763
Post by: acheld on February 05, 2021, 04:59:55 pm

Need some advice regarding a 1 channel AB763, with 6V6 output tubes.

While playing my g'tar, the amp started making a horrendous racket with popping and static.   

After getting the amp up on the test stand, turned it back on and saw:  arcing inside one of the 6V6s, and the some smoke on its associated 470 ohm plate resistor.  I wasn't able to start measuring voltages.  The resistor is intact, bless its soul.

Is it reasonable to assume that the tube has shorted internally, causing increased current through the plate resistor?

I'm thinking I should replace the tube, and recheck voltages.

Or, should I be following another procedure . . .

Title: Re: Troubleshooting AB763
Post by: sluckey on February 05, 2021, 05:39:13 pm

The 470Ω is a screen resistor. Smoking screen resistor and internal arcing are good reasons to change the tube.

Title: Re: Troubleshooting AB763
Post by: Latole on February 06, 2021, 03:13:22 am

And check bias.

Title: Re: Troubleshooting AB763
Post by: acheld on February 06, 2021, 09:10:04 am

Will do, and thank you.

I was surprised to see lightning (on a small scale admittedly) in that tube!  Just glad that I was able to shut down before burning a bunch of components.

Title: Re: Troubleshooting AB763
Post by: Latole on February 07, 2021, 01:38:38 pm

Be sure amp have the right fuse. Some people replace it with too much powerful fuse and amp ( Power Transf ) is not protected