Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ernie_jr on August 18, 2013, 08:26:48 pm

Title: amp start up help
Post by: ernie_jr on August 18, 2013, 08:26:48 pm

Just finished a redo of an amp. It is basically an Ampeg B15, tubes are 12ax7 channel one, 6sl7 channel 2
12ax7 PI and TAD 6l6's. The issue I have is at med to top volume when I hit a note hard (Using it for bass)
I get a momentary hi pitch squeal. Not the constant squeal when you have the O/T leads backwards, but just a
the begining of the note. Any advice would be helpful.
Thanks,
Ernie

Title: Re: amp start up help
Post by: Willabe on August 18, 2013, 08:42:30 pm

Is it in a combo cab? You could start by swapping out the tubes 1 at a time to see if 1 of them is slightly microphonic.

Bass energy might be shaking the guts of a tube.


                    Brad      :icon_biggrin:

Title: Re: amp start up help
Post by: ernie_jr on August 18, 2013, 08:44:17 pm

squeal is in both channels and the amp is on a carpeted floor to minimize and induced rattling from the speakers.
Ernie

Title: Re: amp start up help
Post by: sluckey on August 18, 2013, 09:33:35 pm

Quote

Not the constant squeal when you have the O/T leads backwards

Reversed phase does not always produce a constant squeal or howl. This may not be your problem, but it could be. It needs to be eliminated as a possibility. Easy to do.

Title: Re: amp start up help
Post by: ernie_jr on September 08, 2013, 08:43:30 am

Reversed the grid wires (was easier than the O/P leads on this set up). amp sounds great now, very quiet and plenty load thru 2 x 12 in a ported cab for bass. Thanks for the help, will post pics shortly.
Ernie