2. amps-preamps
  3. 1051901390

Any electronics wizards? Tube problem with RM-9

I have an early Music Refernce RM-9 Mk.I amplifier that sounds absolutely wonderful - when it works.

I bought it used about 8 or 9 years ago and have had a reoccurring problem with it since day one: after some time of operation (days/weeks/months) one pair of output tubes (not necessarily the same set each time) will go into saturation, lighting up and eventually blowing the output fuse.

I sent it back to Modjeski twice and they kept telling me it was mismatched output tubes, and techs at two other shops couldn't find any problem with the electronics of the amp. I went through a multitude of "matched" tubes from a variety of sources and eventually ran into the same problem with each new set.

The tech at Music Reference suggested I was pushing the EL-34s too hard and suggested switching to 6990s, so I bought a set of RAM 6990s and they went about 3 months before starting to do the same thing.

The amp has been sitting in my closet for about 3 years now, and since I just purchased a pair of ProAc Response 2.5s I'd like to bring it out of hibernation.

The only outside influence I can think of which may contribute to the problem is I've always run the output through a Stax SRD-7Pro box (this amp is really sweet thru 'phones), but other amps (tube and SS) haven't had a problem with this load at all.

Why are my output tubes saturating???
jdillion

Showing 1 response by herman

herman
2,215 posts
 
There is a negative DC bias voltage applied to the grids of the tubes. If you lose this voltage the tubes will run wide open and blow the fuses. That is a likely explanation. A cold solder joint in the bias circuits could be the culprit or some part that is intermittent. These can take a while to track down and many times the people at the factory don't want to put in the time necessary to find it.

I think the mismatched tube theory is bunch of balogna. It might cause some tubes to run hotter than others but won't intermittently light them up and blow the fuse. This is the kind of crap you get from techs who can't find the problem and just want you to go away.

Not being familiar with the layout of your amp limits the ability to troubleshoot. Do you have a schematic?

When you say not the same set each time, do you you mean that sometimes it is the left channel and sometimes the right?

After it blows the fuse, can you just replace the fuse and it works again for a while?
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