On Thu, 23 Mar 2006 10:50:54 -0700, Gilbert Bates
wrote:

On Wed, 22 Mar 2006 11:30:35 -0800, "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.

Hey David,

I'm not going to analyze what you've done already, but I'll give you
another path for troubleshooting.

The Music Man amps had a tube phase inverter (12AX7) and regular
output tube biasing, and solid state inverter/driver (no 12AX7) WITH
TRANSISTOR biasing design in the cathodes of the output tubes.
Unfortunately, this amp has the transistor drivers as shown in your
last picture.

MM amps have very high plate voltages at around 700V+.

These amps use 6CA7/EL34's normally, not 6L6 (5881 Sovtek) types. They
must be a good quality tube also. Note the voltage rating on both
types.

I imagine one of the pussy Sovtek 5881wxt's has given out on you under
the high voltages. If a tube shorts, the transistor in the cathode
gets the full plate voltage and goes bye-bye on you. This is common on
the solid state driver MM amps, Google it. So a shorted tube along
with a shorted transistor may be badly loading your power supply with
the resultant hum in the speaker.

Looking at your last picture, It looks like the transistors may have
been replace at some point before? The screws look a little bunged up
and I don't think the TIP41C was the original device that came in
these???

I have worked on a half dozen MM amps, fortunatley they were the early
ones with a tube phase inverter :-)

I'd make sure you check the condition of both the output tubes and
these drive/bias transitors. Look around some, I'm sure you'll find
some information on this.


Ok, so this is a transition model to 6L6 that I overlooked.

I use to do some work for a music store in town here. Marshall uses
these Sovteks in like the model 4100, dual high gain reverb head.
Marshall runs them fairly hot, but nothing out of the ordinary. A
shorted 5881 was a common problem that I saw 1-6 months down the road
on a new Marshall amp that uses these tubes. Backing the bias down a
little seemed to take care of the failures.

Sorry, but I'm leery of the Sovtek 5881's and probably wouldn't want
to use them it this voltage and with the drive setup on these amps.
While nothing to rave about, they work reasonably well at more
moderate conditions, but at 700V I'd probably look for something else
to use in this amp.