Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: RoadShow on July 20, 2023, 06:38:09 pm

Title: Hoffman_AB763_1 channel build, output transformer feedback issue
Post by: RoadShow on July 20, 2023, 06:38:09 pm
Hello,

I'm wrapping up the Hoffman_AB763_1 channel build and "almost" everything is working great; inputs, volume, tone control, reverb, tremolo...

The issue I'm having is when I hook up the output transformer feedback.  When I do, I get a god awful howl/squeal.  When I pull the feedback wire it goes away and the amp sounds great.  In the attached pictures I have a jumper in place instead of a wire so that I can disconnect it quickly.

Any idea what I messed up?  I've included the AB763 pdf below with the pictures, and I've looked it over several time but am stuck and hoping someone can help me out.

Thanks in advance...

Bill
Title: Re: Hoffman_AB763_1 channel build, output transformer feedback issue
Post by: sluckey on July 20, 2023, 06:42:44 pm
Swap the OT primary plate leads (blue and brown).
Title: Re: Hoffman_AB763_1 channel build, output transformer feedback issue
Post by: sluckey on July 20, 2023, 06:48:49 pm
If OT wires are too short to swap, then swap the two brown grid wires on the board. (see pic)
Title: Re: Hoffman_AB763_1 channel build, output transformer feedback issue
Post by: RoadShow on July 20, 2023, 06:49:41 pm
Quote from: sluckey on July 20, 2023, 06:42:44 pm
Swap the OT primary plate leads (blue and brown).

Thanks, I kept looking and I was just reading that in your post here, bout halfway down:

https://el34world.com/Forum/index.php?topic=20167.msg211693#msg211693
Title: Re: Hoffman_AB763_1 channel build, output transformer feedback issue
Post by: dogburn on July 20, 2023, 07:01:12 pm
With both amps I have built, I had to switch the OT wires because of the squeal. Fortunately, I was already aware this could happen (I think I read about it at Rob Robinette's site. The lesson should be to get ready to shut the amp off the first time it's all hooked up with a speaker.
Title: Re: Hoffman_AB763_1 channel build, output transformer feedback issue
Post by: RoadShow on July 20, 2023, 07:01:57 pm
Quote from: sluckey on July 20, 2023, 06:42:44 pm
Swap the OT primary plate leads (blue and brown).

BINGO, thx!!
Title: Re: Hoffman_AB763_1 channel build, output transformer feedback issue
Post by: sluckey on July 20, 2023, 08:08:55 pm
Quote from: RoadShow on July 20, 2023, 07:01:57 pm
Quote from: sluckey on July 20, 2023, 06:42:44 pm
Swap the OT primary plate leads (blue and brown).

BINGO, thx!!
And the take home lesson for this is "DON'T TRIM THE OT PLATE LEADS UNTIL YOU KNOW THE NFB IS CORRECT".
Title: Re: Hoffman_AB763_1 channel build, output transformer feedback issue
Post by: bmccowan on July 20, 2023, 10:11:48 pm
I just paint the brown lead blue and vice-versa - it's easier.
Title: Re: Hoffman_AB763_1 channel build, output transformer feedback issue
Post by: nandrewjackson on July 21, 2023, 03:30:27 am
Nice looking build


😃✝️☮️🏆😎🥸
Title: Re: Hoffman_AB763_1 channel build, output transformer feedback issue
Post by: bmccowan on July 21, 2023, 06:05:50 am
Wow - that is a very clean build. Well done Roadshow.
And, I like the looks of that jumper - I need to get me some - sometimes alligators just don't make it.
Title: Re: Hoffman_AB763_1 channel build, output transformer feedback issue
Post by: RoadShow on July 21, 2023, 07:46:19 am
Quote from: bmccowan on July 21, 2023, 06:05:50 am
Wow - that is a very clean build. Well done Roadshow.
And, I like the looks of that jumper - I need to get me some - sometimes alligators just don't make it.

Thx,

Those jumpers are a godsend, great for discharging the caps when I power it off to work on it.
Title: Re: Hoffman_AB763_1 channel build, output transformer feedback issue
Post by: RoadShow on July 21, 2023, 07:47:33 am
Quote from: sluckey on July 20, 2023, 08:08:55 pm
Quote from: RoadShow on July 20, 2023, 07:01:57 pm
Quote from: sluckey on July 20, 2023, 06:42:44 pm
Swap the OT primary plate leads (blue and brown).

BINGO, thx!!
And the take home lesson for this is "DON'T TRIM THE OT PLATE LEADS UNTIL YOU KNOW THE NFB IS CORRECT".

Yep, 2 lessons learned here.