Sensing load impedance

sailorjoe

Joined Jun 4, 2013
364
That is a good point. The problem with that is the impedance at each frequency would have to work out to the result of the equation laid out in the cavalli paper. Unfortunately I don't completely understand it and I've been tied up lately and haven't had time to further analyze it.
My question is how does such a thing affect the operating point? I also wonder if a shifting operating point is the basis of the problem at all. Based on LT spice simulation the DC offset servo in my amplifier keeps the operating point stable with a changing load impedance, at least of a single value. Any alteration from 0v at the output will cause the servo to adjust the impedance of the valve accordingly maintaining 0v at the output.
Seeing as the bottom triodes operating remains stable then the top triode will always be at 75ma/75v respectively in order to maintain a 0v output.
There's also the fact that conventional knowledge of deciding plate resistor value is not determined by load, this way of thinking was determined by John Broskie and the author of the cavalli paper. So the true nature of the plate resistor and the load escapes me. These guys are highly respected so they must know what they are talking about.
Have you gotten around to taking a look at the cavalli paper yet?
Oh, yes, I read the paper very carefully, and I understand it completely. It's very clear that they want Rplate to match Zload. But the implicit assumption is that Zload is constant, or at least is treated as a constant for all audio frequencies. What I'm proposing is to change Rplate to Zplate, and make Zplate match Zload. In theory, then the system would be precisely matched at all frequencies simultaneously. At the same time, the DC operating point of the tubes is left unchanged. Can you run these ideas past some tube amp gurus, like tubeguy or someone else? There's some serious work to design Zplate, and before I do it I'd like to get an opinion you trust.
 

Thread Starter

coinmaster

Joined Dec 24, 2015
502
Hmmm, I don't have any direct connections to anyone that knowledgeable about the subject. The diyaudio forum would be the place to ask but I'm blacklisted from that forum because one of the moderators thinks I'm some sort of malicious hacker or something for imaginary reasons I'll never understand. I think that was just an excuse to ban me because I insulted him for being a jerk, in either case I get insta banned every time I make a new account there.
I suppose I can attempt to email John Broksie, he's always talking about how he reads his emails.
 

Thread Starter

coinmaster

Joined Dec 24, 2015
502
Okay I talked to him twice about it. He didn't give me any specific details as to why but he said it wouldn't work.
He gave me an alternative design that will solve the issue but it is a complex design filled with many tubes and coupling capacitors. Not not worth cluttering the signal path with.

So, it seems that's dealt with then :p
I've got a bag full of things I want to implement into my amp though so I'm not finished yet.
For starters that music buffer from that article I showed you with the power control. Also I want the ability to digitally control the operating points of my tube voltage/currents and make presets which still requires me to find a variable resistor. I just found something today called an octocoupler which is supposed to be a linear electrically controlled variable resistor so I'll have to look into that.
 

sailorjoe

Joined Jun 4, 2013
364
Coinmaster, sorry but I've been away from here for a week, busy with other things. If you'd like to try it, I can take a look at designing an impedance circuit that replaces your plate resistor and matches your headphones. It would be a passive circuit, and it would be a drop-in replacement for the resistor. It's a bit of work, so I'll only do it if you want to try it. But since you've been told by an expert that it won't work, maybe you just want to leave it like it is.
Your call. Let me know.
 

Thread Starter

coinmaster

Joined Dec 24, 2015
502
Perhaps at some point down the line as an experiment but for now I got a mess of other priorities I have to deal with. I'm designing the amp to have digitally adjustable operating points which I have big plans for including but not limited to the sliding bias from http://www.tubecad.com/2007/11/blog0126.htm. I plan on using LDRs to adjust a CVS load with a CCS on the cathode so I can manually adjust the load line during operation but for an OTL/OCL WCF it is a bit complex and I'm in the middle of figuring out how to amplify my opamp servos output to swing +/-150v and get a servo to adjust the supply voltage to the opamp based upon trimmer adjustments on the CVS and other things. Figuring out how to get +/-150v out of a +/- 40v opamp is proving quite difficult.
 
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Thread Starter

coinmaster

Joined Dec 24, 2015
502
I guess it's called a "conversation" on this site, its listed under your account. In any case I was wondering if your impedance device could emulate an impedance load at the output of an amplifier?
 
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