Rogers Cadet III integrated
I have recently purchased a Rogers Cadet III integrated amp and I am systematically replacing all the resistors and capacitors in the circuit as quite a few have drifted. I have the original Rogers circuit diagram and the one posted by planet10/EC8010. However I am unsure about a couple of things and would appreciate some help.
1. For the 6.8 pF caps that are soldered across the pair of output valve pins what voltage rating should I choose?
2. I have decided to follow the advice to substitute individual 270 Ohm cathode resistors for the two 130 Ohm resistors presently in the circuit. However what value of capacitance should be used in parallel with these resistors and what would be the best place to solder them into the circuit?
3. In the circuit C1 is shown to be a 40uF capacitor (the same value as C6-the Sprague cap in parallel with the cathode resistor). However in the actual amplifier that I have this is a different looking capacitor. It has a code SP-CR-ATB-50/3-6817 followed by -275+. Presumably the last number is the voltage rating which is quite a bit higher than the one in the diagram. This capacitor also appears in the preamp section of the amp. I have removed these caps from the circuit and tried to measure them with a capacitance meter. The value I get is ~16 uF although they don't all measure the same and I suspect some have drifted.
Hopefully someone can help.
The 6.8pF capacitor goes from an anode that could potentially have 2 x 300V on it to another anode that is probably at 200V. Thus, you need at least 400V rating. I would use 630V.
If you use 22uF capacitors across your replacement cathode resistors, you will have the same time constant, and that will be fine. They can go directly from the valve base's cathode pin to wherever the existing capacitor's negative terminal went.
Don't worry too much about exact values for power supply capacitors - but don't make those two 100uF capacitors in the power supply any bigger. So long as the voltage rating is high enough, you will be fine.
My experience is that ALL of the small electrolytic capacitors are likely to have failed and need replacing. I looked at a Cadet five years ago and by the time I'd measured three capacitors as faulty, I gave up measuring and replaced the lot.
Rogers Cadet III
Thanks EC8010 for your quick response, very useful information.
Looking again at the circuit diagram C1 is connected to the cathode of the triode section of the ECL86 valve. The voltage at this point, I believe, should be quite small. Why this capacitor is marked as 275V, if that is what its number code means, I don't know. Should i just aim to substitute a low voltage 47 uF cap here?
Yes, a 10V capacitor will do fine on the cathode of an ECC83 (which is what the triode in an ECL86 is).
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