Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: fingersoccer on December 18, 2016, 12:00:34 pm
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I finished putting together a BF Champ-style amp and it sounds pretty great—except for a crackling sound that happens when playing with the volume above 3; the louder I play, the more likely the crackling sound is. Playing quietly produces no irregular artifacts.
I assumed this was due to a bad/missed solder joint, so I opened the amp and reflowed all the connections, double-checking my work with a bright light from a few angles to make sure I didn't miss anything. Still, when I tap on the side of the chassis (no guitar plugged in), I get the same crackling sound. I prodded every wire in the amp using a chopstick and the only place I'm getting any response is from the power transformer 320V secondaries. Additionally, if I tap ever-so-lightly on the chassis near the PT with the chopstick, the sound comes back. So I'm pretty sure that something in/near the PT is causing the crackling, but I'm fairly confident it's not the joint the secondaries make to the rectifier tube.
Here's a YouTube video showing how I'm able to reproduce the crackling sound: https://youtu.be/pWq2yjFahHE (https://youtu.be/pWq2yjFahHE)
I'm hoping this doesn't mean a bad PT—it's a brand new ClassicTone and I don't relish the idea of purchasing a new one. Any ideas on what this could be or further troubleshooting steps I could take to help narrow this down? Thanks!
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Long shot... Disconnect the negative feedback wire from the speaker jack. Still staticky?
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Another couple long shot ideas. Swap rect. tube. Tap on the first couple resistors & filter caps.
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Just watched your video. Forget the restistor/cap idea. That sounds nasty!
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sluckey, I have a switch on the back panel that engages or turns off the negative feedback—nothing changes, crackle-wise, in either position. Are you thinking that physically disconnecting the wire would a difference here?
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If you can, check your voltages (PT) while the "noise" is happening. Doesn't sound like the crackle of a cold solder joint to me.
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Can you get your sniffer close without electrocution, do you smell ozone, mild odd electrical? I can't smell my wife's cooking, but I can smell minor arcs from 100yrds :icon_biggrin:
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upnorth2, I'll check the PT voltages today after work and report back. I'm guessing that if the voltage goes haywire while the crackling happens, that means something foul is afoot.
shooter, I haven't smelled anything out of the ordinary as yet, but I'll keep my nostrils open.
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Alright, I checked the PT voltages using the Geofex power transformer page (http://www.geofex.com/ampdbug/pwrtrans.htm) as a guide. The numbers all like they should, as far as I can tell. Voltages didn't change at all when probing 'round with a chopstick.
| Lead | Expected | Actual |
| Primaries (BLK) | 120V | 121V |
| Filaments (GRN) | 6.3V | 7.13V |
| Rectifier (YEL) | 5V | 5.66V |
| HT Secondaries (RED) | 320V | 352V |
There were no tubes in place when these voltages were taken—do you think installing them and taking more readings would give any additional insight?
I've also swapped out the 6V6 and 12AX7 tubes with ones known to be good to no effect.
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no tubes in place when these voltages were taken
guessing the crackling still present without tubes and the volts stayed constant while it crackled ?
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shooter, yep, the voltages stayed constant even when tapping with the chopstick. I didn't try hooking anything up to the speaker w/o the tubes though—I'll try that now.
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No sound at all when I turn on the amp and tap without the tubes in. I guess that's expected though, right, since there's no amplified signal getting to the speakers?
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I guess that's expected though
ya, I was thinking you may have been able to hear "arcing" without the amplified sound, which in itself is probably a good thing you can't.