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Mystery tube amp problem - can YOU solve it?

Posted on January 19, 2015 at 15:17:15

Posts: 361
Location: Wyoming
Joined: April 17, 2002
VTL IT-85 integrated amp. Manufacturer had it for three months, but couldn't reproduce the problem, so they didn't fix it. Local repair guy couldn't reproduce problem after 6 hours of listening to it. He's returning it unfixed, too.

Left channel cuts out. Happens more often when the unit is cold. It stays off until I do something to it. Turning the volume up to between 10:00 and 2:00 brings the left channel back in suddenly, and the volume can be turned down again for normal listening with both channels. If the unit has been warming up for a long time (overnight), but with no music playing, the volume trick will resolve the problem and the problem will not return until cooled down again. If the unit is cold (first one or two hours), the problem may or may not return (during that cold time), even after doing the volume trick.

Additional diagnostic info: when the unit is the sole source of amplification, the problem causes the left channel to drop out entirely. When I was biamping with the gain-matched ST-85 (IT-85 driving the higher frequencies, ST-85 driving the bass unit), the problem would result in dropping out the left channel high frequencies, but the left channel low frequencies would be boosted and distorted (tubbier, like the notes were made up of loud, low-frequency hum). If I was in a different room, that's how I would notice the problem (the bass got way loud and distorted). Connecting a separate preamp through the home theater bypass of the IT-85 allowed me to get the unit to a point where the problem was present (no left channel sound) when the IT-85's preamp section was part of the system, and not present (left channel operating fine) when the bypass switch was engaged (other preamp driving the IT-85's amp section). Sometimes engaging and disengaging the mute switch would get the left channel to come back, but that trick is not as reliable as the volume trick (volume trick always works). Also, I was able to get the left channel to return by hitting the unit with a rubber mallet, and to drop out by hitting it again. The repair person that suggested this diagnostic trick was unable to get the hammer to make it fail.

In case you have some simple suggestions to make, please note the following: it is indeed the amp, and not something else in the system. When auditioning my new speakers, I brought in the unit to the dealer's room, and the problem occurred (totally different system, upstream and downstream of the amp). Also, the amp is plugged in the wall socket, not a power conditioner or other filter. The unit was totally re-tubed by VTL when they had it, so that's probably not the problem. The tubes bias just fine, and the unit is not blowing fuses.

I'm having a hard time understanding why reproducing the problem is necessary for repair of the unit. Why can't my detailed description of the failure suffice to open up the amp and look for the problem? Reproducing the problem would only confirm that I'm not lying or delusional (remember: the problem occurred when the unit was dropped into a totally different system).

Thanks for any suggestions you might have. This has been a problem for 3.5 years now, and I feel like I'm out of options.

May the bridges we burn light our way....


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RE: Mystery tube amp problem - can YOU solve it?, posted on January 19, 2015 at 18:03:13

Posts: 9340
Location: northern Virginia
Joined: August 23, 2000
Replace the volume pot assembly. Just a hunch.


RE: Mystery tube amp problem - can YOU solve it?, posted on January 20, 2015 at 11:16:56

Posts: 6851
Joined: July 4, 2002
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.
Danley Sound Labs


And remember, "wedding ring" is merely a code phrase for "grounding buss"! Take 'em off! , posted on January 20, 2015 at 13:25:45

Posts: 7188
Location: Kansas
Joined: January 28, 2001
Speaking from a nearly fatal experience in my teenage years involving my left hand and an Ampeg SVT tube head...

Analog Junkie


RE: Mystery tube amp problem - can YOU solve it?, posted on January 20, 2015 at 14:13:31

Posts: 4652
Location: San Antonio, Texas
Joined: January 24, 2003
January 14, 2010
I was going to say that. Sounds like a bad volumn control. Try spraying it with a contact cleaner before replacing


I say a bad solder joint, posted on March 6, 2015 at 16:06:50

Posts: 11600
Location: No. California
Joined: December 26, 2003
I have an integrated SS amp that does that once in a while. Sometimes, I just tightened the speaker connections to fix it, but increasing the volume would fix it as well. So, you are definitely dealing with a bad connection somewhere.

I have a tube amplifier that for the longest time was blowing a particular output tube quite regularly. What a pain. Eventually, I examined the socket and saw the BAD tube socket pin that wasn't well soldered. Don't know why I couldn't figure that out sooner.

Open the unit (after it's been cold for a while). Get a good light and, maybe, a loupe would help to see better. Examine every solder joint and make a note if one seems subpar, maybe use a marker. If after this exercise, you only have one or two marked at most, then go ahead and reflow the solder on these joints. Usually, there was insufficient solder applied, so you will have to add some. If you can't solder, but can see, find the bad connections yourself and take it to someone who can solder. Usually a 25-watt iron will do the trick and buy some solder for electronics assembly.

Most likely, the problem solder joint is in the signal path on the channel with the problem.


Another possibility, posted on February 28, 2016 at 15:01:18

Posts: 104
Location: Midwest
Joined: September 1, 2015
This is similar to a bad solder joint, but sometimes even less obvious -- a hairline crack in the foil on a circuit board. Try examining the board tracings with a magnifying glass. If you find a break or questionable spot, the fix is to scrape off any coating on each side of the break in the foil and solder over the spot.


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