Music Man Frequently Asked Questions
Amp F.A.Q.
Q: Can you tell me if there are any authorized service centers that will still work on the old Music Man amplifiers?
A: When Ernie Ball acquired the Music Man Company in 1984, the decision was made at the time not to manufacture the amplifiers.We did get a limited number of parts, (which we have since sold to Mojo Musical Supply, (800) 927-6656) and most of the circuit schematics for them, many of which are available on our web site:http://www.ernieball.com/mmonline/techinfo/old_amps/.
There are no authorized service centers. For repair purposes, it would be best to find a qualified amp repair technician, preferably one familiar with these amplifiers.
Q: Can you tell me if my HD 130 amp needs a bias adjustment when the power tubes (EL34's) are replaced? I looked inside and I do see two blue dials on the pre amp board, but those are the only things that looked adjustable to me.
A: The early version has the 12ax7 tube phase inverter, which does have the adjustable bias, and used EL34 or 6CA7 tubes. The later versions have a solid state phase inverter, which does not have adjustable bias. The first amps made with the solid state phase inverter used the EL34/ 6CA7 power tubes. These were changed to 6L6 power tubes in in the latest versions. Refer to the tube chart inside the amp cabinet if there is any question which power tubes should be used. It is important to use the proper output tubes, as the phase inverter is different, depending on which tube it was designed to drive. Amps made for 6CA7 should not be fitted with 6L6's, and those made for 6L6's should not be fitted with 6CA7's.Q: Chassis number 2475-130, Music Man HD-130. The Vibrato Channel is getting weak and I believe it is the power tube going bad.
A: Try adjusting the internal trim pot to adjust the intensity of the tremolo circuit. It is possible there is a defective Op Amp. (LM307H)Q: I am the proud owner of a 70s Music Man HD-130, chassis # 2475-130. I'd like to replace the tubes and bias the amp myself, but I don't know much about the amp except it has 4 x EL-34s.
A: It is an early version. It has a completely solid state pre amp and a Tube power amp design which has one 12AX7A used as a phase inverter / driver tube for the output section of four EL34 output tubes.The bias is adjustable on early versions later versions are not (see above).