that "ah geez" feeling

I built a 2204 using Weber parts a couple years ago but I am now
surprised it worked at all. I don't have any real tech experience on
amps just built a lot of models and took apart stuff as a kid. I
designed a chassis and spent time on making everything look good but I
do remember being full of joy when I was finished.

I have had this Hum, really loud, 60 and 120 with a buzz too. It was
driving me crazy. For about 4 months its been opened up, on the
bench. I fricken checked every ground wire, Cap and solder point. I
replaced the input filter caps. pulled off wires to switches, moved
tubes around, each time leaving me dejected as nothing worked. I
started researching here and other forums for similar problems but
mine always had something different than these so I was pissed and
left it for a while. A couple weeks went by and I started to think
about it again. So I went back to the bench and looked around - WTF -
I found this cap in parallel with the 1 M ohm resister on the High
channel the solder was non existent on one side and cold soldered on
the other. It was like somebody else did it. Not the case
unfortunately.

Well I fixed that but still had the hum. I found an article about
grounding on el34 site and followed some of the suggestions. One of
them was not to use star grounding because the writers experience had
seen that in itself causing more problems than it fixed. I thought
about it. Then I decided to can the star ground scheme I had and
follow this guys. Sure enough the hum went away.

But, then I lost my high sense channel - sounded like a transistor
radio. The low channel was good as always. Hmm. So I pulled out the
schematic for my amp to study it, I was tired of leaning over this
amp.

Dare I take it to LV, my usual goto guy for amp issues and parts! The
thought of the humility was too much to bare. I am no tech after all
but I don't like not knowin' somethin obvious, and LV has a good habit
of pointing that kind of stuff out to ya, real quick.

"Well let me study this schematic some more" I thought. Just then
somethin started wiggin me out about the inputs. I have five or six
different schematics for this and similar amps so I started lookin at
all of them to cross reference information. Some had the Low input on
top, some had valve V1b first...huh - no000!

I ran back to the bench and found my inputs were miswired just enough
to let the amp limp along badly but not blow up. I was pissed and
happy, "Did I finally find the fricken issue?" was runnin through my
mind as pulled of the old wires and started to fix it.

Excelsior! "Adrienne....I did it!" "Cmon kid...move it along!" "ah
geez"

I had tied the middle lug of the high input to ground instead of the
middle lug of the Low input. That explained the loss of high input
after grounding the pots, and I guess I had a loop from floating the
ground before so the hum and the signal were the same strength. Now
the hum is comparable to the hum on my Marshall anniversary series
2204 and the other amp that I modified from a Marshall 2550 to a 2204.

Now maybe I can start looking at tweaking the remaining hum on the
heaters with a hum balance circuit and maybe a bias balance circuit as
well.

Its cool though because I persevered and finally overcame this issue
all on my own, so now I have that experience to draw from next time.
.

Relevant Pages

  • Re: that "ah geez" feeling
    ... I don't have any real tech experience on ... I have had this Hum, really loud, 60 and 120 with a buzz too. ... schematic for my amp to study it, I was tired of leaning over this ... I had tied the middle lug of the high input to ground instead of the ...
    (alt.guitar.amps)
  • Re: that "ah geez" feeling
    ... I don't have any real tech experience on ... I have had this Hum, really loud, 60 and 120 with a buzz too. ... schematic for my amp to study it, I was tired of leaning over this ... I had tied the middle lug of the high input to ground instead of the ...
    (alt.guitar.amps)
  • Re: that "ah geez" feeling
    ... I don't have any real tech experience on ... I have had this Hum, really loud, 60 and 120 with a buzz too. ... schematic for my amp to study it, I was tired of leaning over this ... I had tied the middle lug of the high input to ground instead of the ...
    (alt.guitar.amps)
  • Re: DC filament modding question
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  • Re: that "ah geez" feeling
    ... I don't have any real tech experience on ... I have had this Hum, really loud, 60 and 120 with a buzz too. ... schematic for my amp to study it, I was tired of leaning over this ... I had tied the middle lug of the high input to ground instead of the ...
    (alt.guitar.amps)
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