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Old 30th Jun 2017, 9:46 pm   #220
Diabolical Artificer
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Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,636
Default Re: 807 (maybe) amplifier build. Now EL34

"So an amplifier designed to have a negative feedback loop is designed with the appropriate excess of gain....." I did that David on the front end,the ECC83 I set up as per the Philips datasheet for high gain and low THD, IE 400v HT, 220K anode R, 1k2 cathode R. From memory I get 4v RMS for 100mv IP.

The PS is a LTP cathode running off a CCS off a -80v rail, with + 350v HT though I kept the anode R's small in value, IE matched 36k (2 x 18k). This gives me a good voltage swing, 60v RMS at the cathode follower's into 47k OP grid resistors at 10mA per side. This drives the OP stage with no clipping for an OP of 144w . Not sure what that is in dB, but plenty of excess gain to spare for NFB.

I have tried to teach myself FB and associated maths, and will again at some point try to get my head round it and other similar challenging subjects, but am starting at a very low baseline of knowledge. I'm struggling with the "basics" and can't find an "in". It's knowing where to start, what basics to learn, getting a strong base to build on, before I try to get to grips with the complex maths associated with feedback. To be honest though I've been putting it off. Advice welcome.

Ohm's law, no problem. 1/(2piRC), ok there, but struggle with Thevenin. From what I've gleaned I need to understand Nyquist's thereom. One of the problem's I have is finding the right reading material. Without exception reference reading, The Art of Electronics for example, goes too fast and assumes a certain level of knowledge.

I'll stick at it, I've found a pretty good ish Utube teacher here - https://www.youtube.com/channel/UCdh...bzxebigAlA2rDg and some of these Indian lectures are good too - https://www.youtube.com/user/nptelhrd . What they lack is practical applications of that material. It bugs me that there are no answers for instance to the AOE set questions at the end of the chapters.

Good suggestion Howard, unfortunately the OPT leads are cut and won't reach for a swap over. I've ended up switching over the PS triodes round.

More soon, thanks again for your interest and suggestions, Andy.
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