Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: LooseChange on August 07, 2011, 06:40:18 am
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I am curious about how a DVM (A decent one) is effecting the circuit when I probe...
Sometimes I have an amp with a certain amount of noise (Hum, Buzz, etc.). When I probe around for voltages sometimes the noise will go away but the amp still runs fine.
What effect is the meter having on the amp?
Could this be duplicated with a component?
Thanks!
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? I usually have the amp hooked up to a dummyload when I'm probing.
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I don't use a load when diagnosing. I like to use all my senses including ears, eyes and nose... Never mouth!!!
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Just a guess, but maybe the meter is providing a clean path to ground where you have a bad solder joint?
When diagnosing problems - typically in my own builds - I use a Weber Mini-Mass as my dummy load so that I can listen to the speaker if I want to.
Cheers,
Chip
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I have a bunch of speaker motors from warped frames or the like where I cut away the frames and most of the paper surround leaving the cone and tinsel wires intact. This is my dummy load and it actually produces sound too. Plus it doesn't take much room on the bench w/ the other stuff and when I'm done it "sticks" to a nearby metal rack. Easy to stow away for another day.
LC, being a repair guy you've had to see this a whole bunch of times where maybe it's part of Murphy's Law but doesn't it seem like many times a problem doesn't want to "express itself" in front of the repair guy?!? Over my years I couldn't say how many times this happened and I had to try to get it to "re-enact the crime".
Ya know, the meter puts a tiny bit of a load on things and in your situation it's possible some stray capacitance or something is "shorted away" or like Chip's suggestion is a possibility too? Another thing especially in Fender amps is moving wires around causes some things to occur AND to disappear. Just be careful that YOU don't disappear too! Check this out - Kendrick Amp guy electrocutes himself in demo video (http://www.youtube.com/watch?v=1lozkT1zrIU#)
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That is funny!... Yes, we all need to be careful.
I use a dummy load every once in a while.
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:laugh: He has a better sense of humor about it than I do anyway. I swear a lot. :laugh:
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? I usually have the amp hooked up to a dummyload when I'm probing.
Not ideal. Besides Gerald Weber, also see: "El. Guitar Amp. Handbook", by Jack Darr. Noises made, or not made, by an amp under test are important and useful clues to identify an issue in the amp. Some noises should not be made; if you hear such a noise, the specific noise is a clue to a specific issue. Other noises, like clicks and pops in the speaker when you measure certain voltage points, tend to confirm that the circuit is working.
OTOH, some tests, like max output in watts, are better made with a dummy load. :m11
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Dan,
I've had a similar experience trying to troubleshoot a 5F2A, Several points I would probe for DC voltage would change the symptom. It would almost go away and so I would think it (the issue) was where I was probing.
Ultimately the problem was discovered to be a bad OT (used ebay) and the symptom was a fizzy decay of a note with an abrupt end of the note.
Ray
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Probing with a multi-meter...........
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I'm still very curious about Dan's original post, having experienced it myself.
Anyone have any ideas what is going on?
Dan, can you describe where you're probing and in what circuit and what symptom is disappearing?
Ray
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I don't know any better, but I'd hazard this: if you're checking to ground then that must mean you've made a connection through some resistance to ground. A pretty big resistance I'm guessing. Maybe you could simulate it by jumpering to ground through a large resistance. Maybe at times in certain parts of the circuit with different builds there's a ground loop or some ground bug of some kind. You would think that introducing another ground connection would exacerbate the problem, but who knows? Certainly I don't.
You could also try measuring across the offending component to see if that has the same effect.
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In this case it was the power tube grids. One grid was buzzing until I probed it. (Then it let out a yell. kidding)
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Do you think you could permanitly hook up your MM to the amp and problem solved! Make a nice little doghouse for it outside the chassis. ;>/
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Ha!