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Please help, wierd Jacobs Ladder sound

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.  
Thank you,  
3/26/2005 7:27 AM
Wild Bill

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...  
---Wild Bill
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
Wild Bill

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! :)  
---Wild Bill
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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