How to convert PP RCA 30w to triode mode
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20to20
diyAudio Member

Join Date: Jun 2010
Location: W-S, NC
Quote:
 Originally Posted by DF96 You need to think about ratios rather than absolute values. The resistor is low in value because the feedback injection point resistor is also low in value. It is the ratio of these two (27) which sets the feedback; 27 is a perfectly normal feedback ratio; many amps are in the region of 30 for this figure.
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.

Last edited by 20to20; 6th December 2018 at 02:20 PM.

 6th December 2018, 03:02 PM #22 DF96   diyAudio Member   Join Date: May 2007 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.
diyAudio Member

Join Date: Sep 2018
Location: Philippines
Quote:
 Originally Posted by 20to20 If it was a loud buzzing squeal, that sound happens when the polarity of the FB is backward. You can reverse swap the plate leads to the OPT or you can change the grounded speaker terminal to the other (+) side. That will reverse the FB polarity.
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

20to20
diyAudio Member

Join Date: Jun 2010
Location: W-S, NC
Quote:
 Originally Posted by kissabout2002 I just did it and now is working perfectly, ....thanks again
Muy bueno! Thanks for the followup.

 6th December 2018, 08:18 PM #25 Wavebourn   Designer & Technologist diyAudio Member     Join Date: Sep 2006 Location: Pleasant Hill, CA RCA engineers were creative. I like servo to the screen grid. I thought it was my invention, because I did not see that before. __________________ Nothing in the universe is perfect. The ideal things are the ones that are most optimal. Optimization criteria, what matters. When I hear "No Compromise Design", I want to take a sledgehammer and test how impact-proof it is.
 6th December 2018, 08:55 PM #26 20to20   diyAudio Member   Join Date: Jun 2010 Location: W-S, NC Great minds think alike...

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