I built a 5C1 Champ using a pentode (6SJ7) for the pretube. Here is a link to the schemo:
http://www.schematicheaven.com//fenderamps/champ_5c1_schem.pdf
The only mods from the schematic are as follows:
1. The Power tube Cathode bypass resistor and cap changed to 1k 5w and 47uf/50v to better bias the output tube. As it was, the tube ran too hot.
2. The schematic doesnt list it this way but the 6.3v tap has a CT. At first, this was referenced to ground. I could hear this low frequency hum coming out of it so I moved the CT to the Cathode of the power stage. There was a free 28v so my 6.3v CT is now referenced to +28v. The hum did get better after this, but its still humming.
The problems Im having trouble solving.
1. When you turn the Volume up past 3 o'clock. the output tube gets these 2 pinpoint blue glowing beams and introduces a screeching sound. If you just nudge the volume past that same position (say, an RCH) I can hear it go into oscillation. The only thing I could figure was causing this was a lack of grid stopper on the power tube once the Volume pot's impedence got low enough. The only thing I could figure to do with it to prevent the oscillation was to put a 10K reisitor in series with the pot.
2. Im still getting a hum from the amp. It sounds 60Hz to me but I suppose it could be a multiple of it. I had a friend who has a scope check out the B+ for ripple. He said it was clean. *edit* Spectrum analyser said it was 120Hz*
All my grounds run to the same place (think buss grounding) The amp is wired Point to point and anything bare was heatshrinked. All my voltages seemed to be what I was expecting. The only thing I could think of over all was I was still biasing too hot. In which case, the idea I had for solving it was to put a 1k 5w pot in series with the cathode bypass on the power tube and revert it back to 500R, giving me a sweepable range of 500R to 1.5K, should be hotter than Ill ever need and colder than Ill ever need. My Plate voltage is 382v. My Cathode Current is 30.4ma, which to me still seems a touch high.
*edit* Being that the hum is 120Hz, its got to be something in the B+ causing the hum.
Am I on the right track to solving my oscillation problem? Im at a loss as to the low frequceny hum. Ill put it up on my spectrum analyer later today to find the exact frequency of the hum.
http://www.schematicheaven.com//fenderamps/champ_5c1_schem.pdf
The only mods from the schematic are as follows:
1. The Power tube Cathode bypass resistor and cap changed to 1k 5w and 47uf/50v to better bias the output tube. As it was, the tube ran too hot.
2. The schematic doesnt list it this way but the 6.3v tap has a CT. At first, this was referenced to ground. I could hear this low frequency hum coming out of it so I moved the CT to the Cathode of the power stage. There was a free 28v so my 6.3v CT is now referenced to +28v. The hum did get better after this, but its still humming.
The problems Im having trouble solving.
1. When you turn the Volume up past 3 o'clock. the output tube gets these 2 pinpoint blue glowing beams and introduces a screeching sound. If you just nudge the volume past that same position (say, an RCH) I can hear it go into oscillation. The only thing I could figure was causing this was a lack of grid stopper on the power tube once the Volume pot's impedence got low enough. The only thing I could figure to do with it to prevent the oscillation was to put a 10K reisitor in series with the pot.
2. Im still getting a hum from the amp. It sounds 60Hz to me but I suppose it could be a multiple of it. I had a friend who has a scope check out the B+ for ripple. He said it was clean. *edit* Spectrum analyser said it was 120Hz*
All my grounds run to the same place (think buss grounding) The amp is wired Point to point and anything bare was heatshrinked. All my voltages seemed to be what I was expecting. The only thing I could think of over all was I was still biasing too hot. In which case, the idea I had for solving it was to put a 1k 5w pot in series with the cathode bypass on the power tube and revert it back to 500R, giving me a sweepable range of 500R to 1.5K, should be hotter than Ill ever need and colder than Ill ever need. My Plate voltage is 382v. My Cathode Current is 30.4ma, which to me still seems a touch high.
*edit* Being that the hum is 120Hz, its got to be something in the B+ causing the hum.
Am I on the right track to solving my oscillation problem? Im at a loss as to the low frequceny hum. Ill put it up on my spectrum analyer later today to find the exact frequency of the hum.