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Citation II hack

Scott -- the value you are now using for your feedback resistor (3K) to produce the feedback reduction you indicate (18 db) is completely in line with what would be expected for the changes made. By changing the ECC83 stage to a 12BH7 (a very good move), the open loop gain (OLG) has been reduced significantly, meaning that the feedback resistor needs to be reduced as well to maintain the same relative amount of feedback.

Also, because of reduced Miller Effect in the 12BH7, the HF response of the OLG model has now been increased. This -- along with the reduced value of the FB resistor -- required the feedback cap to be increased significantly to achieve the same relative response characteristic as before.

So now, on to the most important question: were these last photos taken with or without a step network in place?

Dave
 
Sorry for the delay,I had a personal emergency,It is without the step network.So should I try for the best square wave with less feedback,or 20db feedback that gets atrange when I raise the output to 5v?
 
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I found some calculators that say the lesser feedback is 10.9 db,with feedback the amp would have ~35 db of gain after feedback, .5v will yeild 58 watts,so the numbers seem good.is there a problem wih using less than 20 db of feedback?
 
I'll be curious Daves input, but on amps running UL output stages I don't
have a problem with 6-12db of overall feedback and how that works out.
 
It's an upgraded/modded ST-70 with the colors inverted in MSpaint.
(I did the pic on accident while resizing and left it ever since here) ;)
 
Hi Scott -- With the unequal oscillation, I figured the shot was without a step network, so one will be needed, and almost always is.

I'm not sure how you arrived at your 10.9 db of feedback figure, but based on information you provided earlier, with the OPT side of the feedback loop grounded, it took .023 vac to produce 5 vac output. With the feedback loop closed, it now took .18 vac to produce the same 5 vac output. That is an input sensitivity reduction of 7.83 times, or just shy of 18 db. If the reduction had been 10.0 times, then that would equate to exactly 20 db of NFB.

To Keg's point, UL amplifiers can certainly operate with less than 20 db NFB since the UL connection itself provides about 4 -5 db of feedback as well. Since I like a well damped bass, I would never go below say 10 db of global NFB in a UL amplifier, and generally prefer about 14 to 18 db depending on what the target goals are.

Since we're just trying to get in the ballpark at this point, first try a step of 100 pf and 10K and let's see what that looks like. It should clean things up notably.

Dave
 
Ok, I did make a mistake,I forgot to ground the fb loop for that test.
I have to remember to reread the instructions untill it sinks in.
 
What I was saying was that .023v yeilded 5v 45db gain(ungrounded)(according to the calculator)When I connected up the loop I could only go down to 1.5v before strange things started happening 5v to 1.5v was 10.9 db according to the calculator.
I have to go back and remeasure things the right way and get back to you.likely tomorrow.
 
Hi Scott -- First, I hope everything is ok regarding your personal emergency.

OK. Two things:

1. A drop in output voltage from 5 vac to 1.5 vac is in fact does represent a 10.9 db change -- all of which leaves me somewhat puzzled about your reference to .18 vac input earlier to produce the same 5 volt output, which is not mentioned in this post. Most likely however, it is all caught up in.......

2. You are now experiencing one of the basic problems in developing a stable NFB amplifier: the amplifier goes unstable before you reach your target level of NFB that you are trying to stabilize in the first place. When it becomes unstable, all bets on any accurate gain measurements are off the table. That is why often the process is one of adjusting the FB network, then the step network, then the FB network, etc. until you reach your goal, or determine it cannot be reached. In any case however, gain measurements are only valid with the amplifier stable.

Dave
 
The .18 was the distorted 5v square wave pic with 20db feedback,the 1.5v(with.18v in) was the best looking square wave with the 10.9 db feedback.

I have been going to the hospital to visit my mother,she has end stage cancer,she is having some moderate problems,but seems to be recovering for the moment,and may come home soon.
 
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