warner@ecs.umass.edu (01/10/91)
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
jhess@orion.oac.uci.edu (James Hess) (01/14/91)
In article <8806@uwm.edu> warner@ecs.umass.edu writes: >Some bizarre things have been happening to me with a Dyna Mark IV tube amp. > >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. >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. >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,... >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. Red spots mean excessive current flow, which is why the plate voltage is low. I remember the one time I got to work on an Audio Research preamp. It was acting funny, but I couldn't find any defective parts. Finally, with the help of factory techs, I found out that it had been modified--one of those changes that was supposed to increase the "effortless sound - improves the 'air'". It involved some cap changes that destabilized the circut and caused ultrasonic oscillation, sometimes motorboating. It left the circut in a critical state in which slight deviations from nominal in the values of the components would upset it. Tweeks not familiar with the requirements of design for the real world failed to consider these problems. Later I ran into a $5,000 esoteric solid state amp designed that way--let the bias adjustment drift 20% and poof! There go a dozen output transistors! I think your problem is in those new passive components -- a capacitor that is mislabled, out-of-spec, or not properly soldered, etc. Start by redoing your solder joints. Connect a scope to the output and look for oscillations. Tap and spray-cool components to locate sensitive ones. (Many components are temperature sensitive, so look for unusual sensitivity and compare the results between the amps.) Switch suspect components from one amp to the other. (A lot of work!) But first -- check your connections, particularly the inputs. It could be a poor ground -- clean the plugs and jacks, replace the cables, etc. Happy Hunting! Jim Hess