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I'm in the process of learning electronics and amps. Slow process due to time available, but it's enjoyable. I have a problem that's cropped up and am hoping someone can help me understand what's happened.
I bought a 1974 Fender silver face Champ at a guitar show a couple months ago. It worked fine for a few hours, then started malfunctioning.
At the time I was told about three capacitors that where replaced (and given the originals). These turn out to be the three cathode bias caps. I've since found out the 1Meg volume pot has also been replaced. That's where the trouble started, I think.
For the first couple hours, the amp worked fine. At some point I started to get volume drops, it started making some popping and cracking noises, and I shut it down because I could tell something was wrong.
So, I pulled the chassis and had a look. This is when I could see the volume pot had been replaced. Whomever did it was pretty sloppy. They'd used a (I assume) non stock shielded single conductor wire to run from pin 7 of the 12AX7 to the volume pot's center lug. They did not connect the sheild at the tube end, but soldered it to the ground lug at the volume pot end. There was such poor lead dress that the sheild wire effectively shorted the input and the ground lugs of the pot.
I fixed this and put it all back together and fired it up once more. The popping and crackling was still there, then I heard one loud pop and saw a bit of smoke coming from the unused input jack. This was a "oh shit" moment for sure. I was getting no signal shortly afterwards.
So, for reasons I can't explain other than idle stupidity, I installed a new rectifier tube and fired it up. The rectifier and the pre-amp tube both glowed (heater's working), but not the power tube. I replaced the power tube with a known good and now the amp worked. It did not sound it's best, but there was no noise that shouldn't have been there (like popping and cracking). I played through it for a bit and noticed the power tube was red plating, especially when I wasn't playing, or playing at low volumes. When I cranked it up and wailed on some barre chords, it wasn't redplating.
Hmm, so I got out my Weber biasrite and plugged it in between the 6V6 and the tube socket. It read 272 volts and 140mA! Geez, no wonder it's red plating and the previous (new) power tube had failed.
So I took the chassis back out and started inspecting. It was then I noticed the 22µF/25volt cathode bias cap had literally blown up and shot it's stuffing out one end. The plastic wrapper was also somewhat melted on the same end. I removed this and the 1 watt 470ohm resistor and measured them both. The resistor measures 516 oms and shows no evidence of being overheated. The capacitor actually measures 0.5 ohms. So effectively, my cathode bias went from around 500 ohms to 0.5 ohms.
I think I can now understand why my Weber was showing 140mA. But, did that cap blow because of the shorted volume pot, or should I be on the lookout for other issues as well?
I bought a 1974 Fender silver face Champ at a guitar show a couple months ago. It worked fine for a few hours, then started malfunctioning.
At the time I was told about three capacitors that where replaced (and given the originals). These turn out to be the three cathode bias caps. I've since found out the 1Meg volume pot has also been replaced. That's where the trouble started, I think.
For the first couple hours, the amp worked fine. At some point I started to get volume drops, it started making some popping and cracking noises, and I shut it down because I could tell something was wrong.
So, I pulled the chassis and had a look. This is when I could see the volume pot had been replaced. Whomever did it was pretty sloppy. They'd used a (I assume) non stock shielded single conductor wire to run from pin 7 of the 12AX7 to the volume pot's center lug. They did not connect the sheild at the tube end, but soldered it to the ground lug at the volume pot end. There was such poor lead dress that the sheild wire effectively shorted the input and the ground lugs of the pot.
I fixed this and put it all back together and fired it up once more. The popping and crackling was still there, then I heard one loud pop and saw a bit of smoke coming from the unused input jack. This was a "oh shit" moment for sure. I was getting no signal shortly afterwards.
So, for reasons I can't explain other than idle stupidity, I installed a new rectifier tube and fired it up. The rectifier and the pre-amp tube both glowed (heater's working), but not the power tube. I replaced the power tube with a known good and now the amp worked. It did not sound it's best, but there was no noise that shouldn't have been there (like popping and cracking). I played through it for a bit and noticed the power tube was red plating, especially when I wasn't playing, or playing at low volumes. When I cranked it up and wailed on some barre chords, it wasn't redplating.
Hmm, so I got out my Weber biasrite and plugged it in between the 6V6 and the tube socket. It read 272 volts and 140mA! Geez, no wonder it's red plating and the previous (new) power tube had failed.
So I took the chassis back out and started inspecting. It was then I noticed the 22µF/25volt cathode bias cap had literally blown up and shot it's stuffing out one end. The plastic wrapper was also somewhat melted on the same end. I removed this and the 1 watt 470ohm resistor and measured them both. The resistor measures 516 oms and shows no evidence of being overheated. The capacitor actually measures 0.5 ohms. So effectively, my cathode bias went from around 500 ohms to 0.5 ohms.
I think I can now understand why my Weber was showing 140mA. But, did that cap blow because of the shorted volume pot, or should I be on the lookout for other issues as well?
When I got the voltage readings I intended to only play for about 5 minutes and then shut her down and see what you guys had to say....that 5 minutes turned into well over an hour!
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