This 30W amp circuit is in the RCA manual. On the next page is a 50W amp. It uses the same cathode resistors on the VA with a 10R to ground. The FB resistor is 1200R. What's the ratio for that? Both are on the 16R tap. There is no common ratio to use generally. There are many design considerations for feedback component values.
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1200/10=120. Generally, higher power amps use higher ratios here because they need to generate more output voltage from the same input. A voltage gain of x30 (30dB) is typical for medium power domestic amps nowadays; more gain was common 60 years ago.
Anyway, my main point is that 270 is not unusually low in value, as 27 is not an unusual feedback ratio. Hence 0.01uF is not unusually high in value. Ratios are what matter.
Anyway, my main point is that 270 is not unusually low in value, as 27 is not an unusual feedback ratio. Hence 0.01uF is not unusually high in value. Ratios are what matter.
I just did it and now is working perfectly, I check also older Rca schematic and on the 30w amp the value of 0.01 uF and 270r are constant, I just have to use a 330r instead because I do not have 270r, and work very well, I swap the plate leads to the OPT rather the speaker terminal because my OPT have 4,8 and 16 ohm, I will do a proper measurement soon, still thank you very much you really solve a big problem, since I built the amp and take off the FB was working well actually very powerful but with some hum and noise problem, not to much but they was bother me, but now is absolutely silent even at the max volume, so very happy with it, I will try later to apply suggestions from other members on triode mode....thanks again
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