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Has anyone built a Fender Champ 5C1? I had some transformers laying around that should work great and I ordered the 6sj7 and a 6v6 and built it a day or two ago. When I turn it on, it has almost no volume, unless I use my signal generator and crank it. My 6v6 is running 334v at the plate, 263 at he screen, and around 31 ma just idling. My 6SJ7 is at 263 v plate, 30v screen. I have never built a preamp around a pentode. My voltages are a bit off what Fender recommends, but I can't figure what is really wrong. I have check my wiring and everything. I don't understand the 6SJ7. The screen seems oddly low in voltage, and the bias method is odd. There is no cathode resistor, just grounded. Can anyone shed any light on this?http://music-electronics-forum.com/i...s/confused.gif
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is 30 ma a bit low for class A? it seems that would put me more in the class AB range when looking at the 6v6 charts? thanks again, this is my first post, and this amp is killing me! -
30mA doesn't seem too far out for a 5C1 running "period correct" voltages. The schematic suggests 32mA, less screen current. A 5C1 is single ended and must be class A, it can't be class AB. You could however try biasing hotter by fitting a slightly smaller cathode resistor, or a higher voltage rectifier?
The fact that the 6SJ's plate voltage is the same as the 6V6 screen voltage means that they are connected to the same point on the B+ rail, you should have a 250k resistor between the 6SJ plate and the 6V6 screen supply.
5C1 is "grid leak" biased, in these designs it is normal to ground the cathode.
Methodically check each component off against the layout again, highlighting them off as you go. You may find an error.Comment
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Fixed
I finally figured out what it was. I was wondering why my screen on the 6SJ7 was so high like you had said also. I wasn't measuring a voltage drop across the "250k" resistor so i unplugged the amp and measured the resistor. After using numerous multimeters, I decided just to look at what value the resistor was because I was almost reading open. It turns out I must have put a 25o ohm resistor in instead of 250k. I have never made that bad of a resistor mistake before. I put in 270k due to my high B+ and it works great. Also my voltage all read very close to the ancient fender correct voltages.Comment
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Ha. I just recently did the same thing in a Champ build. I had installed 180 ohm plate resistors instead of 180K's. The preamp was basically shut down. One of those late nights when you don't catch the details I guess.
Back to your original question, I did just build a 5c1. I went by the Fender layout and schemo. It was working but it was not very loud and it's sound was lackluster. After some brief troubleshooting I wrote it off as maybe a primitive circuit and that was the reason hardly anybody was building these things. I disassembled it and built a modified 5f1 type amp instead. Well the other night I picked up one of G Weber's books and randomly opened it up, and right there were a couple of pages discussing the 5c1 circuit, wherein G mentions an ERROR on the layout. Seems the layout has the B+ rail coming off pin 2 of the 5y3 rectifier instead of pin 8. Since I'm far from being a seasoned builder, I'm wondering A) if I blindly followed the schematic here, and B) what exactly would the amp do hooked up that way. I really cannot remember any details about the build.Comment
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