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|3/25/2005 6:48 PM|
|Floyd||Please help, wierd Jacobs Ladder sound|
Hi, thank you so much for checking me out
here, I am loosing my hair on this amp.
I build a high octane amp but made my own
layout so I could use less expensive solder
terminals (less expensive when you already
have a gaggle of em anyway).
I am having a frustrating intermittent "Boris
Carlof movie, Jacobs Ladder" sound from the amp,
it happens when I turn the standby off, lasts
for a minute or so then abrubtly goes away like
flipping a switch and the amp plays great and
is dead quiet for up to an hour (the longest time)
or as soon as a few minutes or untill I flip the standby on and off again.
I have thoroughly checked all the grounds, replaced
the standby switch to a heavy duty 10 amp unit,
switched all tubes with known good, moved and re-routed all the preamp wires to try and tame it, all
to no avail. Jacobs Ladder is the only way to
describe the sputtering humming buzzing sound
the amp is making. When I finished building
the amp I turned it on and it was making the same
noise but had another problem, I had grounded
the first preamp B+ at pin 6 to ground, I have
since replaced all components between that point
all the way back to the filter caps but left the
original filter caps in, could the filter caps be bad or is my layout/lead dress causing the problem?
Here is a link to the layout I used, the amp in
question is wired up almost identical to it
so looking at the layout is just like looking
at my amp.
|3/26/2005 7:27 AM|
Floyd, you may have a problem with one of the dropping resistors in the filter cap string.
These resistors should be at least 1 or 2 watt style, especially the first ones in the chain. They see the most current drawn through them.
Resistors can arc inside, where you can't see them! Especially those hissy old carbon comps!
Any hidden arc will sound like a Jacobs Ladder if it's on a line feeding a gain stage. You can also get such an arc in a bad solder joint, where the heated flux made a gas "bubble" that prevented the tin/lead from actually sticking to the wires. This last is not that common and mostly happens with printed circuit boards but I mention it as a long shot, for what it's worth.
I'd check the filter resistors first and then start replacing preamp plate resistors. Check back if the problem hangs in but I'll bet a beer it's one of those resistors...
|3/26/2005 8:10 AM|
First question is does the volume or character of the noise change with any adjustment of the pots ?
|3/26/2005 12:59 PM|
Yes, the volume control can shut the noise off,
and thank you so much for your help.
|3/26/2005 12:57 PM|
Thank you Wild Bill, replacing those
resistors right now.
|3/26/2005 2:27 PM|
Good luck, Floyd!
Let me know if it fixes your problem. It makes me feel better to know someone else is having some luck when I'm cursing over my own amps!
|3/26/2005 6:12 PM|
Ok, once again, thank you all for giving me
a reason to keep on looking, I almost gave up
on this one since I thought if I am going to
replace every component why not just start over?
When I had tried everything I could you guys gave me some more options to try and I finaly found
the problem. The "diode array" I have in the amp
to lower B+ is also in harms way when one has a short to the HV, one diode (the middle of six)
was reading infinity ohms both ways, I don't know how it was still conducting and keeping the B+
at the right voltage but it was, now the amp is
all fixed up and rockin. I think this one is
much more loud and angry sounding than the first
one I built due to the shorter signal path, yes
my layout works great, no oscillation or wierd
noises, very quiet amp. Not bad for a newb huh?
Going to keep that one in my notes, I have had
a bad zener before but it didn't make noise like that and why is anyones guess.
needless to say I would not be ready to play right now if were not for all the great people here.
Seriously, thank you again for all your help
you guys are the best.
Wild bill, special thanks to you, if I didn't
change all the resistors I would never thought
to check the diodes, actualy I was thinking it was my layout causing oscillation since it is my first
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