Here is an ugly one! A Musicman HD-130 head shoved into a Fender Super Twin cabinet with the original all too heavy Super Twin speakers! Easy repair: blown tube. Change it and re-bias. Customer had 3 original Sylvania 6CA7 tubes that were well used and....they tested great and still worked. This was a 'keep it cheap' repair so I put 2 of those in and left 2 of the Electro Harmonix ones in there. They actually were very close to matching. Anyway, amp is making music again! This is one with the solid state phase inverter. Biasing is easy but do not attempt this if you have no experience. These babies contain a lethal 700 volts DC. Stay out of that kitchen and let a pro take care of you.
Other work: I did clean up the cabinet. Re-glued the tolex etc. I get a little OCD about that. Lipstick on a pig? Sure. But the best rock n roll ain't pretty! I love me an ugly amp!
J
I need some enlightening about my musicman hd 130 2x12 combo. I bought it used from a friend of a friend. He replaced the factory spkrs w/ JBLs: 1-E 120 8ohm, 1 K120 16ohm. They are wired in series. The ohm selector switch in the back offers an 8 or 4ohm setting. It was set on 4ohms when I purchased it. I have gigged w/ it 3 times now. I am not so savvy electronically. Is the amp being damaged with these settings? Is it ok to have 2 differently ohmed speakers? Would the 8ohm setting be better? (seems to me it would). I certainly don't want to destroy this amp. The previous owner told me he would swap the old factory spkrs back to me if I preferred. Please help drive the clouds from my crowded ol' noggin sir. Thanks, LRW
ReplyDeleteHi there! Thanks for reaching out.
ReplyDeleteIf they are wired in series then the total load is 24 ohms!!! Not ideal by any standards. I would wire them in parallel which would give you an impedance of around 5-6 ohms and run it on 4. Ideally though you are best off by loading it with 2 speakers of the same impedance, like 2x8ohm wired in parallel for 4 ohms or 2x16 ohms parallel for 8 ohms.
As it is running right now you are driving the amp too hard and yes, theoretically can damage the output transformer and tubes. Myself? I'm not the biggest fan of JBL speakers and I think the originals are CTS which are okay. If it were mine I'd see it as an opportunity to load it with EMinence or Celestion speakers. Many warm choices! If you like JBL then find one 8 or 16 and do as instructed.
Thanks again! Jef
Thanks for the prompt dispensation of electronics wisdom my friend. I have since learned from the prior owner that the 16 ohm K 120 was modified to 8 ohms. I did notice that there was an attempt to scratch out the 16 ohm factory label so I'm assuming I was told the truth about the modification. So if this is true then the ohm switch should be set to 4 as opposed to 8? I apologize for the initial provision of bad info. Thanks again for your valuable time sir. Larry Wallace
ReplyDeleteNo problem! I would make sure they are wired parallel then set the switch to 4 ohms. That'll do the trick!
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