Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: whoops on July 10, 2019, 11:13:17 pm
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Hello,
had a strange problem on stage in a soundcheck today.
The AC30 from the guitar player, a chinese AC30, was doing a really strange distorted sound, it was like ugly distortion mixed with quick cutting in and out the sound. When we looked at the back of the amp there's was a spark/arcing between the speaker terminals of one of the speakers each time the problem occured.
We had to continue soundcheck, so as an easy way to try to fix it at least for the gig I put isolation tape around the speaker terminals and the problem stopped.
We were able to do the continue the soundcheck and do the gig with no problems, but as I know this is not a permanent solution it was just a temporary fix, I would like to ask you if you ever seen this kind of behaviour?
And also if you think the problem might be from the amp circuit or from a defective speaker?
Thank you so much
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It can't actually be arcing across the terminals. That needs >300V. Speakers don't get 30V. A mere AC30 can't do even that much, it takes big power to make voltage at speaker impedance.
More likely a bad connection. A loose crimp? With varying voltage (signal) you get small sparks when the contact makes/breaks. Tape may have just held it in a better position (or just hid the spark.
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Thanks PRR,
The sparks seem to be between point A (the speaker wire of the + terminal) and B (the Rivet that holds the terminals)
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If there was a short in the OT primary there could be HT on the senondary?
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I would think any voltage high enough to cause a spark or arc would fry the speaker instantly.
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I would think any voltage high enough to cause a spark or arc would fry the speaker instantly.
The Spark seems to cut on and off the speaker sound but it didn't fry them yet, I'm afraid it may fry the speakers of the OT transformer if it's not fixed.
For the gig, isolation tape was used around the terminals the coil wire (wire that comes from the coils to the terminals) and the rivet area, I didnt know what was the reason for that, maybe the speaker needs to be replaced.
I have to ask the musician/owner to bring the amp to my workshop and I should do an overhaul and make sure all other parts in the circuit are working properly like the tubes, both power transformers, Filter Caps, wiring, and ground connections.