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79 Champ Q's...

4/28/2006 8:55 PM
Justin Thomas
79 Champ Q's...
Hello everyone,  
I'm trying to get my 79 Champ up to spec and had a few questions about some terms and techniques used.  
I know how to take voltage readings safely and discharge caps.  
The problem is that my voltages seem VERY high-all of them. The wall V is ~124 every time I check it, so all of my readings are WAY over what the schem says by about 70V and up. Plate voltage on the 6V6 is 431. Is that normal for a 79? I am using the AA764 Schemo...spec is 350V.  
The questions is: What exactly is meant by measuring "the voltage across a resistor?" Or, "the voltage drop across a resistor?" Using the 6V6 cathode bias resistor as an example, would this mean: the voltage at pin 3 is 30V with the neg. probe to ground. On the opposite side of the 1K resistor (what's in the amp) I read 0V. So would the voltage drop be 30V? Or am I doing this wrong?  
And, on Aiken's site, I read that you take the voltage drop across the resistor, and divide by the resistance value to get the cathode current? Which would be .03A? And If I multiply .03A by plate voltage (from Lord Valve), I would get the plate dissipation, which is ~13W? :D Ouch?  
Hopefully I'm not misapplying anyone's info...  
Is there a more direct way to take current measurements with my meter (Fluke 112)? As in, stick the neg. probe HERE and the pos. probe THERE and that's it? Thanks everyone, for your time and understanding...  
4/28/2006 10:58 PM

AA764 is not the right drawing for a Champ from 1979. The PT in the AA764 has a 640VAC CT HV winding. The one in yours should have a 730VAC CT one. I looked at the 1976 and 1981 versions, which are identical for our purposes. Drawing number 045393 for reference. It fits your amp.  
B+ is 420VDC. Remember that your mains are high by 4 volts nominally. If the B+ is about 3.5 times the mains, then every extra volt on the mains will be three and a half times larger on the B+. SO 124VAC on the mains adds about 14 volts, so 434VDC. Darn close to your 431 isn't it.  
Get the right schematic. The Champ-CBS on Schematic Heaven is right for yours. Not a great copy, but readable with effort.  
VOltage across a resistor means the voltage from one end to the other. Like distance across a bridge, or miles across America. Since we usually look at things with respect to the power sources, if the voltage at one end of a resistor - at the voltage source - is higher than the voltage at the other end, we say the voltage has dropped acrosss the resistor. Like if B+ is 420v at one end of a 1k resistor, and 410v at the other end, we say that 10 volts were "dropped" across the resistor.  
Ohms Law describes the relationship between voltage, resistance and current. Current flowing through a resistance causes a "voltage drop" across the resistor. Or put a voltage across a resistor, and we then know how much current is flowing through it. So if 10 volts are dropped across a 1k resistor (1000 ohms) Ohms LAw tells us I=V/R 10v/1000 ohms = .01 amp or 10ma.  
Yes, if you have a 1000 ohm cathode resistor with 30v on one end and 0v on the other, then yuo have indeed 30v drop across 1k. That is .03A or 30ma.  
DIssipation is voltage across the tube, NOT simply plate voltage. Since the cathode is 30v, and the plate is 431, we have 401v across the tube. Times .03A yields 12 watts.  
Remember this single ended amp runs in class A which means it is running at max power all the time. 12 watts is fine.  
Note: the schematic calls for 470 ohms in the cathode circuit and about 24VDC on the cathode. And the cathode cap is rated at 25VDC. Your 30v is over that caps rating. You should put at least a 50v cap there. I'd stick a 100v cap in if I had the amp here.
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5/1/2006 2:02 PM
Justin Thomas

Hey Enzo,  
Thanks for the clarification. I managed to get all my books out of storage over the weekend, and I've got a great copy of the CBS schemo all to myself.  
I put a 470/1W on the 6v6 and remeasured. By the same math with the new voltages I ended up with 19W dissipation. I think I'll stick with the 1K.  
All of the cathode bypass caps (on 12AX7 and 6V6) are rated for 50V- were replaced by previous tech.  
I also removed the Line Out that the previous tech had installed. Oh boy, a hole for something else fun...  
Thanks again Enzo! Now I just gotta fix my friend's 65, that he got FOR FREE off the back of a pickup truck...  
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