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#1
08-04-2009, 02:41 PM
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Troubleshooting Head/Cab Problem
I recently bought a Jackson 4x12 cab from my neighbor, from the early 90s. It's in pretty good shape, it was just really dusty when I got it.
Anyway the main reason I bought it was for something to plug my Crate Blue Voodoo head into, but ever since I've been playing through the cab I've noticed not only a volume drop but also severely reduced bass response compared to the Carvin 2x12 combo I used to plug the Crate into. This seems wrong, especially since the 4x12 is way bigger. Incidentally, around the same time I got the new cab, I replaced V2 in the Blue Voodoo with a 12AT7 and the PI slot with a 12AU7. I'm pretty sure that's not the source of the problem... So my big question is: is there a problem with the cab or the head? Which is more likely? I'm thinking it's probably power tubes since the head is about 2 years old and I've never replaced them. I suppose it could be the cab, but I have no way to check since the BV is my only head... Any ideas? Any advice? Thanks! 
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#2
08-04-2009, 03:16 PM
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Dont be so sure about the 12AU7 in the PI-thats a pretty hefty reduction in gain. (100mu for a 12AX7 and only 20 mu for a 12AU7.) Swap it back out with a 12AX7. Also make sure the 12AT7 is good too. Bob
__________________
"Reality is an illusion albeit a very persistant one " Albert Einstein Was I crazy? I began to wonder, for such an appalling lack of perspective seemed near being just that.
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#4
08-04-2009, 04:23 PM
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Quote:
5751-70% 12AT7-60% 12AY7-45% 12AV7-40% and lastly 12AU7-20%
__________________
"Reality is an illusion albeit a very persistant one " Albert Einstein Was I crazy? I began to wonder, for such an appalling lack of perspective seemed near being just that.
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#6
08-04-2009, 09:08 PM
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Quote:
Lemme make sure on that - the Jackson cab says "16 ohms stereo" on the back, so does that mean I flip the switch on the head to 8 ohms or 16? I kind of assumed it was supposed to be at 16 ohms, but I may be wrong... Also, I might consider swapping that 12AU7 in the PI slot to an AT7 or maybe even back to an AX7. Would that help things? Thanks for the help! I don't know a whole lot about servicing amps/figuring out what's wrong with them. :P
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#7
08-05-2009, 03:22 AM
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He means put the type of tubes it calls for back in. In the case of the Blue Voodoo Im pretty sure thats all 12AX7's. I already stated the swap to a 12AU7 in the PI is probably the culprit. Its simple enough to go back to what its supposed to take and find out. FWIW-If your not sure about tube substitutions dont try them. With 12--7's its usually harmless but with power tubes it might cause damage so if your not sure its best you dont try things.
Not sure about the cab impedance. From the sound of it it seems like it would be 8 ohms mono and 16 ohms stereo(or per side). If you flip it to 8 you can check it with a multimeter. The resistance should read about 6-7 ohms for an 8 ohm impedance and 14-15 for a 16 ohm impedance. Match the amp selector to which ever youve got. Bob
__________________
"Reality is an illusion albeit a very persistant one " Albert Einstein Was I crazy? I began to wonder, for such an appalling lack of perspective seemed near being just that.
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#8
08-05-2009, 06:35 AM
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Quote:
I'll have to check on the cab. I don't have a multimeter, but IIRC it can't hurt the amp (or the cab) if I switch the impedance on the amp to 8 ohms and that's the wrong impedance... right? Last edited by nufan947; 08-05-2009 at 07:37 AM.
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#9
08-06-2009, 09:21 AM
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I swapped the 12AU7 I had in the PI slot back out with a 12AX7 last night, and it made some improvement, but not much... I think I'll try the head with my old cab again and see what I actually remember being different about it. It's been a while, so I could just be remembering wrong...
Is it at all possible that it'd be power tubes? And how could I check?
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