Hi David,
Getting back to square one, let me take a moment to try and
answer your question(s) as asked. First off, the hum is not
good news. The factory supplied tubes were made by Phillips,
GE or Sylvania and were tested for operation at the higher
anode/plate voltage used/supplied by many of the Music Man
Amplifier tube models. So any tube you place into a Music
Man amplifier should be tested for operation at the much higher
anode/plate voltage.
Testing tubes for breakdown at higher anode voltages is called
high-pot testing. It's not normally done by most dealers but should
be requested for tubes installed in amplifiers operated well
above the typical anode voltages found in many/most guitar
amplifiers with similar tubes. A few of the better or "high-end"
tube dealers can and do test tubes before shipping. Some mfgrs
like Groove Tubes make and sell tubes specified and tested for
operation in circuits like the Music Man Amplifiers. I could
also direct you to a web site with much information about making
your own basic high-pot tester.
Otherwise you take a gamble any non high- pottested tubes might
flash over, arc or short when placed into the Music Man Amplifier...
very bad news indeed.
Regarding your yellow wire, the real answer to where it goes is
found by tracing where the other end where it's attached and reversing
the wire function out using the location and the circuit diagram.
Otherwise someone with the exact same amplifer open and visible
might be able to tell you where they think the wire goes.
Excessive current draw could be caused by a number of problems,
so you should use the proper diagram, tools and test equipment
to troubleshoot the circuit through. If you have excessive
final/output tube idle current, something in the bias supply
or cathode drive circuit might be out of whack/failed. Depends
on which chassis version and circuit you have. If the tubes you
have in the amplifier are/were sold as hot or cold range type
units, they might not be able to bias up properly at the anode
voltages found in the Music Man Amplifiers.
Typical faults in the cathode drive hybrid circuit are shorted
transistors, the actual bias pot failed or mis-adjusted... when
you have excessive idle current. If your bias pot is the white
thin-flat-square trim ceramic type pot, they are well known for
being problematic after decades of service. Many people spray a
bit of Caig Labs DeOxit onto/into the pot and the circuit again
works fine. Other people replace the pot when they find it to be
the problem source. The Caig Labs DeOxit Spray will restore some
of the ceramic thin bias pots to regular service, it's just a
question of your trust in using the reconditioned original pot
or replacing it (if found to be the source of the problem).
If the cathode drive transistors are shorted, someone probably caused
an unwanted "glitch" event by putting generic replacement tubes
into the amplifier, which were not able to handle the higher anode/plate
voltage. One or more of the tubes might have flashed over - arced
or shorted. You can test and replace the transistors with a cross
referenced device. Also be sure to measure and check the transistor
emitter lead resistors as they can also open upon tube-glitch events.
There are a number of Music Man Amplifier diagrams on the web. I have
a fair number of them from my Music Man Service days and from Ernie
Ball after they bought out Music Man. If you email me direct, I can
help you find a diagram of an amplifier with the same final/output
section to trouble shoot your amplifier.
Once you get the right type of tubes in your working Music Man
Amplifier, it should last for years of playing, no problama'
cheers,
skipp
skipp025 at yahoo.com
: David Farber wrote:
: I received this Music Man HD-130 amplifier with the complaint that it hums.
: The four output tubes are 5881WXT. The owner replaced the two main filter
: caps himself and said it didn't correct the problem. When I opened the unit
: and found one of the yellow wires hanging loose from one of the filter caps,
: I soldered it to the other yellow wire which was connected to the filter
: cap.
: See photos he
:
http://www.pbase.com/image/57610175.jpg
:
http://www.pbase.com/image/57610176.jpg
:
http://www.pbase.com/image/57610312.jpg
:
http://www.pbase.com/image/57610174.jpg
: Now the amp is drawing too much current at idle and I definitely need to
: check the wiring to the filter caps. I don't service many tube amps like
: this. Does anyone have a schematic of this set? I tried the ernieball.com
: website but it's not listed there.
: Thanks for your reply.
: --
: David Farber
: David Farber's Service Center
: L.A., CA