Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!usc!zaphod.mps.ohio-state.edu!uwm.edu!csd4.csd.uwm.edu!info-high-audio-request
From: warner@ecs.umass.edu
Newsgroups: rec.audio.high-end
Subject: Strange tube amp problem
Message-ID: <8806@uwm.edu>
Date: 10 Jan 91 14:19:52 GMT
Sender: news@uwm.edu
Lines: 36
Approved: tjk@csd4.csd.uwm.edu
Originator: tjk@csd4.csd.uwm.edu

Some bizarre things have been happening to me with a Dyna Mark IV tube amp. I
was wondering if anyone of the forum members could give me some advice or 
help with the matter. There is only one amp that is consistently giving me
difficulty fro the pair.

First of all, I replaced all of the passive components with modern devices and
purchased matched pairs of 6CA7 output tubes. The amps worked wonderfully for
two months until I began to notice the 6CA7 in the left hand socket of the
problem amp was showing a glowing red spot on one side of the plate. I opened
it up and found the bias to be a bit high (1.67 V rather than recommended 1.56)
and the plate/screen voltage to be 300/225V respectively. This was alarming
because the tube on the opposite socket was reading a healthy 446/445V. So,
I checked all of the components, cleaned the tube sockets and bought another
pair of EL34s. I plugged it in, the plate and screen voltages measured within
+/- 2V on both tubes, the bias voltage held steady and the line to the grid from
the driver was holding at a nice, steady -33V. 

One day, I turned the amps on and was listening while preparing some work for
a class. Everything was fine. At one point, I got up to go to the bathroom. 
Upon return, I noticed that the sound had changed significantly during my 
absence. I glanced at the amps and to my shock I saw the entire plate assembly
of the left tube in the troublesome amp to be glowing bright orange. I opened
the amp up again and this time found that there was a break in the winding of 
the output xformer (ohm meter). Fortunately, neither my speaker not preamp
were damaged.

Now that I have put a new output transformer in and installed another set of
new tubes, checked the wiring, socket connectors and components, still I am
noticing that the darned left tube has an ever so slightly noticeable glow
spot on one side of the plate! I have unplugged the amps and am now using my
trusty (but not as nice sounding) Hafler XL280 until I figure what demon has
possessed the amplifier. If anyone is willing to help a troubled caretaker of
a pair of demonically inhabited vacuum-tube amplifiers, any advice, counselling
or suggested ritualistic exorcism would be greatly appreciated.

.........lee warner