Re: Music Man HD130 tube problems

Mike Schway wrote:
In article <1146933149.780923.112750@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
kremer307@xxxxxxxxx wrote:


So, to finally answer these questions, the chassis number of the amp is
2275-130, and the diagram on the inside calls for 4 6CA7's. I was
unable to find a bad connection or resistor around the tube socket, but
I didn't look too thoroughly, because I'm still planning on taking the
amp in.


Check the value on the two large 1.5K/10W resistors. One may have overheated. This wouldn't be a *cause* of a loss of bias, but could be a result of a shorted tube.

Also, there's a small fiberboard adjacent to the tube sockets with some resistors and caps. If one of those caps is leaky, or open resistors, or cracked solder, you've lost bias to two tubes. Touch up all the solder joints on this small board (MAKE SURE THE AMP IS DISCHARGED).

Unless you can remove the caps and do a leakage test, you'll have to measure voltages while the amp is energized to see if they're leaky. This amp works at 700VDC. Needless to say, BIG DANGER!!!! This job is best left to a pro. Look to see if there are any signs of arcing between pins 2 and 3 on any of the output tube sockets (on the solder side of the socket).

So, to confirm a couple of things with you all, I'm better off leaving
the cabinet wired at 3.6 ohms and using the 4 ohms setting, or wiring
it back up to 16 ohms? Seems like 3.6 ohms is the best bet, but it's
still 10% under the expected load. Not enough to worry about? Would
finding a totally new 8 ohm cabinet to use be far superior to using the
3.6 ohm (or 16 ohm if I wire it up again) cabinet I've got now?


<nitpicking>
3.6 ohms is actually the DC resistance, not the impedance. Because impedance is the vector sum of inductive reactance (a frequency-dependent AC property not measured wihh your ohmmeter) and resistance, impedance is always more than resistance. Usually something like a factor of 1.4 more or less. 3.6 ohm resistance would be considered a nominal 4 ohm impedance. </nitpicking>

DON'T run this amp at 16 ohms!!!! 8 ohms is probably a little more robust than 4 since you're using the entire secondary of the OPT, and not half of it, but it really should run OK at 4 ohms with the correct toggle setting. 4 new speakers will set you back nearly $300...more than the cost of replacing your output tranny. Is this worth it to you?

And I should look for some high-pot tested electroharmonix 6CA7's if
I'm not planning on hunting down the original sylvanias? These EH's
seemed to be the favorite throughout the post.


Yeah, they work. I'm skeptical if *any* vendors routinely match these tubes at the high voltages of a Music Man. I'm not sure if you'll see any difference between a normal matched pair and one which is "hi pot" tested. FWIW, most vendors will match at around 450VDC. Some vendors spot-check batches for high voltages, but not routinely. New Sensor, the main distributor of ElectroHarmonix and Sovtek, matches at only 350V. My advice is to buy some from your favorite reputable vendor and hope for the best.

--Mike

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Mike Schway | [Picture your favorite quote here] mschway@xxxxxxx |
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