Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Oddvar on October 24, 2016, 09:23:36 am

Title: AC30 Hoffman works for a while, the the hum comes
Post by: Oddvar on October 24, 2016, 09:23:36 am

HI.  AC30 Hoffman works for a while, then the hum comes and the guitaraudio fades to almost zero.  I was wondering what that could be?  Overheating?

Title: Re: AC30 Hoffman works for a while, the the hum comes
Post by: Oddvar on October 24, 2016, 10:52:36 am

It seems like V4 and V5 get very hot at one point.  Glowing much more than V6/V7. 

Title: Re: AC30 Hoffman works for a while, the the hum comes
Post by: tubenit on October 24, 2016, 11:08:51 am

When I have had something similar happen, it was the power tubes starting to red plate & oscillating.  Putting in Doug's Sovtek EL84's resolved the issue for me.  Ironically, the tubes that were oscillating were brand new but not Doug's.

I had tone, an obnoxious hum and then close to silence. 

With respect, Tubenit

Title: Re: AC30 Hoffman works for a while, the the hum comes
Post by: Oddvar on October 24, 2016, 11:16:35 am

The powertubes are all new, but I'll try to change them.  Thanks.

Title: Re: AC30 Hoffman works for a while, the the hum comes
Post by: tubenit on October 24, 2016, 11:35:55 am

Yeah, when it happened to me,  I had brand new JJ EL84's.  They red plated and oscillated.  The Sovteks didn't.

I am not saying one brand is better then another, this was simply my experience.

With respect, Tubenit

Title: Re: AC30 Hoffman works for a while, the the hum comes
Post by: Oddvar on October 24, 2016, 01:50:13 pm

I tried Shuguang for the first time, never again.