About three years ago I bought a pair of Minnesota Audio Labs MAL 6550
tube mono blocks, about 10-12 years old and cost me $400 used. New they
cost $2400(from what I was told). Apparently they are fairly well
built, quality units which I can verify, that really do sound nice-when
working. Each amp uses 2 6550's and a 6an8. They worked very well for 2
years and then one of the output transformers went bad(Chinese
transformers?). The repair place replaced the output transformers on
the 2 amps with quality Hammond transformers, parts and labor $450.
They worked great for 6 months and then both amps started blowing
fuses, the big Buss KTK-1/4 $12 each type of fuse. One of the amps
started smoking from the new output tranformer before the fuse blew.
Took the bottom plate off and one of the wires from the transformer is
toast.


Worth fixing or is there possibly some design flaw that would make
these just a money pit. Seems that my old Dynaco's that I use as a back
up are way more reliable, but don't sound as good. I really don't want
to take these back to the same repair place, I don't trust them. But I
have another repair place to go to.


Any suggestions?


Thanks, Tom

wrote in message
oups.com
About three years ago I bought a pair of Minnesota Audio
Labs MAL 6550 tube mono blocks, about 10-12 years old and
cost me $400 used. New they cost $2400(from what I was
told). Apparently they are fairly well built, quality
units which I can verify, that really do sound nice-when
working. Each amp uses 2 6550's and a 6an8. They worked
very well for 2 years and then one of the output
transformers went bad(Chinese transformers?). The repair
place replaced the output transformers on the 2 amps with
quality Hammond transformers, parts and labor $450. They
worked great for 6 months and then both amps started
blowing fuses, the big Buss KTK-1/4 $12 each type of
fuse. One of the amps started smoking from the new output
tranformer before the fuse blew. Took the bottom plate
off and one of the wires from the transformer is toast.
Any suggestions?

You don't seem to be alone:

http://www.allaudio.org/detail-9427979.html

"I have a mono block tube amp, Minnesota Audio Labs MAL 6550
that needs
an output transformer. Replacements have been
difficult(impossible) to
find according to the shop that is working on it so I am
looking for
someone to rebuild it. Anybody out there do this? Anybody
know anybody
in the Portland, Oregon area?"

Try Inner Sound or Echo Audio, both located in Portland. Yahoo yellow
pages has there number.

Later, Tom

wrote in message
oups.com...
About three years ago I bought a pair of Minnesota Audio Labs MAL 6550
tube mono blocks, about 10-12 years old and cost me $400 used. New they
cost $2400(from what I was told). Apparently they are fairly well
built, quality units which I can verify, that really do sound nice-when
working. Each amp uses 2 6550's and a 6an8. They worked very well for 2
years and then one of the output transformers went bad(Chinese
transformers?). The repair place replaced the output transformers on
the 2 amps with quality Hammond transformers, parts and labor $450.
They worked great for 6 months and then both amps started blowing
fuses, the big Buss KTK-1/4 $12 each type of fuse. One of the amps
started smoking from the new output tranformer before the fuse blew.
Took the bottom plate off and one of the wires from the transformer is
toast.


Worth fixing or is there possibly some design flaw that would make
these just a money pit. Seems that my old Dynaco's that I use as a back
up are way more reliable, but don't sound as good. I really don't want
to take these back to the same repair place, I don't trust them. But I
have another repair place to go to.

It sounds to me like they ARE Dynacos. That's the exact tube complement
used in the Dynaco 60 watt units. These are fixed bias circuits. It's
entirely possible that the output stage bias was improperly set (not set at
all.)

Norm Strong

On Tue, 02 Aug 2005 17:59:21 -0700, tomrp wrote:

snip

Worth fixing or is there possibly some design flaw that would make these
just a money pit. Seems that my old Dynaco's that I use as a back up are
way more reliable, but don't sound as good. I really don't want to take
these back to the same repair place, I don't trust them. But I have
another repair place to go to.


Any suggestions?


Yep - stick some more Hammonds in and fit fuses and 10R resistors in
the o/p valve cathodes. That's what I would do anyway. You can then easily
check the bias by measuring the voltage across the resistors and the fuses
will protect the output circuit against drifting bias. You absolutely
*must* also set up the bias correctly - and within the rating of your
chosen output transformer. The small cathode resistor will also help a
little in keeping the bias stable.

You could use this as an exercise in doing your own repairs - you would
learn a lot and the satisfaction in listening to music on stuff you have
built/modified yourself is well worth it. You will save lots of $$$ too!
Be *very* careful though if you decide to do this. Valve amps bite hard!

--
Mick
(no M$ software on here... :-) )
Web: http://www.nascom.info
Web: http://projectedsound.tk

These amps have cathode biasing and were set and have been very stable.
Perhaps the shop didn't set them to an amount adequate for the new
transformers, though. I will probably get them fixed-eventually.

Thanks for the help,
Tom

On Wed, 03 Aug 2005 15:22:51 -0700, tomrp wrote:

These amps have cathode biasing and were set and have been very stable.
Perhaps the shop didn't set them to an amount adequate for the new
transformers, though. I will probably get them fixed-eventually.


That's quite possible. They may have set them up for optimum operation of
the output stage but not checked the OPT max primary current rating. I
wonder if there is an HF stability problem? That can cause odd things to
happen to both speakers and OPTs.

--
Mick
(no M$ software on here... :-) )
Web: http://www.nascom.info
Web: http://projectedsound.tk