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RE: Mystery tube amp problem - can YOU solve it?

Posted by tomservo on January 20, 2015 at 11:16:56:
In the world of trouble shooting, intermittent problems are the most annoying.
The rubber mallet diagnosis suggests a bad solder joint / bad connection or loose connector.

If you’re willing to trouble shoot further and have a soldering pen AND know how to use it;

Put a large sheet of cardboard on your table, place your unit on it unplugged and remove the screws so that you can see the insides.
If this were a power problem, restoring power would make a pop so lets assume it’s a signal level problem, an intermittent in the low level signal.

With the covers off, speaker and signal connected to your funky channel, plug in the power cord and power up.

Hopefully it’s dead when you turn it on but it might work just fine. Get a wood or plastic dowel about a foot long, not conductive, not a lead pencil.

This will let you press on things without accidental electrical contact with your body. In the tube era TV’s, that had very high voltage we were suppose to put your unused hand in your back pocket to reduce the chance you will get snapped and then “get all stiff and cold”.

Play a signal at normal level, start pushing lightly around the circuit board and signal connectors, see if you can zero in on where the intermittent is.

Your input pot might be the cause too, they can get dead spots, maybe try this first of all. With the unit off, turn the level control all the way back and forth a dozen or more times. Often that will expose a new path in the composite resistor pad.

If you find a connector or solder joint that causes the problem, since you have a tube unit, with capacitors that hold a charge, be safe, unplug it and wait a couple hours before soldering anything or fixing a connector. If you have a volt meter, clip it on pin 3 of one of your output tubes on a 500-600Volt scale.

I worked at a tube amplifier company called Grommes Precision in the early 70’s and spent half a day in the lab and half a day testing amplifiers that had just been built, brought to me at the bare chassis stage in batches of about 40 on a large wheeled cart.

As these were 100W amplifiers, it was easier to carry them back to the cart after testing in pairs. One day, one of these amplifiers had a lead which was not trimmed properly where it was soldered to the filter cap.

That extra wire stuck out right next to my knuckle where my middle finger wrapped around the chassis. About half way to the cart, my knuckle made contact with the wire of the just turned off filter cap which made a pronounced SNAP sound as well as an involuntary reaction on that arm.

Much later in life I saw a movie called A Christmas story (you’ll shoot your eye out kid) where they showed the car’s lug nuts floating in slow motion in the “Fudge” scene.

When i saw that, I immediately thought of the amplifier rotating in slow motion over and over as it cleared the row of 8 foot florescent lights and hovering in air spinning and then coming back down on top of the lights and even saying “oh fudge” loudly and like Ralphie, not actually saying fudge.

Two 8 foot florescent light fixtures hung by chains and the amplifier came down with a sound I wish I could have captured, all that glass breaking, all that metal crashing on concrete. The amplifier was bent in half, mangled, all the tubes broken, totaled.
What made that extra special was this was in a factory that was run by an extraordinarily strict lady who insisted on silence. I was glad many times that I didn’t’ work for her, as for me, there were zero repercussions. Just a little black hole in my knuckle which healed over long ago.

So, ah be careful, don't day dream and be in the moment so your amp doesn't give you black holes or try to fly on you.
Best
Tom
Danley Sound Labs