Since we've been on the topic of cathode feedback and the like in the past couple of days, I was wondering if someone would take the time to explain something to me that I am having my problems in understanding fully.

I've been working my way through trying to understand the various nuances of Claus Byrith's EL34 SE amp, and I'm confused with the global NFB loop.

The schematic can be found here.

Please forgive what's likely to be a very basic electronics question - but I've been working hard on trying to understand this and I just don't get it.

The design uses cathode feedback over the output transformer secondary as well as global parallel injected voltage NFB in which voltage feedback is mixed back into the grid of the driver stage. The amount of global NFB is variable from 0 - 10 dB over a 100kOhm potentiometer. I'm having problems coming up with the calculation as to how this circuit delivers 10dB of NFB.

The way I understand it....

10dB of NFB means that the feedback factor in this case shoud be around 0.041.

The stage has an open loop gain of 53. So, 53 / (1 + 0.041 x 53) = a closed loop gain of 16.3.

20log (16.3 / 53) = -10dB.

So far, so good.

The feedback factor is calculated using the voltage divider rule R1 / (R1 + R2). Calculating the case for the full 10dB of NFB, R1 is the 100k pot in parallel with the 22k resistor which gives about 18k.

18k / (150k + 18k) = 0.107, not 0.041 - we're out by a factor of 2.5. So in this case this the feedback factor of 0.107 would be approx 16.5dB NFB and it's supposed to be 10dB so obviously something else plays a role here.

But what?

I'm "guessing" that this has something to do with the fact that cathode feedback is taken off the secondary as well as is global NFB. So the signal is being divided before reaching the global NFB circuit, and only 1 / 2.5 of the feedback signal is further divided by the global NFB circuit and mixed back at the grid of the driver?

But what resistances exactly play a role in this division and how?

The resistance on the lower global NFB limb seen by this mysterious divider could be 22k || 100k || 150k ~ 16k. Which would mean that the other limb of this mysterious divider shoud have an R of around 24k. Right?

How do I calculate this? The cathode resistor has a value of 330R. The EL34 has an internal resistance. I can see the primary of the OPT as well.

Help?

Hopelessly yours,

Griz