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Old 28th February 2012, 08:24 AM   #11
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Quote:
Originally Posted by artosalo View Post
This is fact.. ?
Or would it be myth instead ?
It's No.3: A question from someone who doesn't know
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Old 28th February 2012, 08:29 AM   #12
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Quote:
Originally Posted by riccoryder View Post
The effect is to cancel some of the internal feedback of the input triode, making it act more like a pentode. If one envisions the triode plate voltage influence on cathode current as plate to cathode transconductance, the a.c. plate voltage times gm(pk) is the feedback current reducing change at the cathode. We're just cancelling some of that current to raise gain.

The same thing is done between the phase splitter cathode and input cathode in the Dynaco ST-35.
Am I right in thinking that the triode effectively has variable resistance (like a MOSFET, but the Pentode is more of a variable current device (both being voltage controlled))?

If so you are saying that the current feedback tends to make a circuit behave like a pentode because its output is more of a current output - like a variable CCS?

Would it make a difference that I am using pentodes to drive the primary that I want to derive the current feedback from - would this just linearise the pentode?

Thanks for all the replies BTW, this must be basic stuff for you all but I want to understand the mechanisms rather than just relying on stabs in the dark
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Old 28th February 2012, 09:58 AM   #13
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Quote:
It's No.3: A question from someone who doesn't know
This is not an answer to the topic.

Can you definitely show the high output impedance causes "rubbish sound" ?
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Old 28th February 2012, 10:15 AM   #14
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High output impedance, combined with a speaker intended for a more conventional low output impedance amp, will give a big peak at the speaker bass resonance. Unless you like reggae, this will be 'rubbish sound'.
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Old 28th February 2012, 01:12 PM   #15
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The reality may not be that black and white only.

Here is something for you to study:
http://www.google.fi/url?q=http://ww...fYjIA6tSvAEXiQ
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Old 28th February 2012, 04:19 PM   #16
DF96 is offline DF96  England
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If you have studied that paper, then you will know that in section 3 he discusses the problems raised by current drive, such as the bass peak I mentioned. Solutions are possible, but they do mean different speakers with either much higher mechanical damping or some form of motional feedback. So the paper simply repeats what I said, but in much greater detail. Thank you for agreeing with me.
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Old 28th February 2012, 04:24 PM   #17
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On this picture feedback by current is positive, while feedback by voltage is negative. The result may be from low output resistance to even negative, depending on pot position. It is opposite to current drive.

Click the image to open in full size.
__________________
The Devil is not so terrible as his math model is!
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Old 28th February 2012, 06:33 PM   #18
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Quote:
Originally Posted by artosalo View Post
This is not an answer to the topic.

Can you definitely show the high output impedance causes "rubbish sound" ?
Err.. I'm the original poster asking the question, sorry.
If I knew the answer I wouldn't need to ask....
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Old 28th February 2012, 06:49 PM   #19
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Originally Posted by Wavebourn View Post
On this picture feedback by current is positive, while feedback by voltage is negative. The result may be from low output resistance to even negative, depending on pot position. It is opposite to current drive.

Click the image to open in full size.
Wavebourn, is the current feedback from the 200R pot at the right/bottom pot (Rk2 perhaps but it's a bit fuzzy)?
I'm guessing the voltage nfb is from Rf1 (60k)..

I.e. as the 6V6 conducts it pulls the centre of the 200R pot upward (current sense) and the anode pulls downward (voltage of anode vs centre point of 200R pot).

It's interesting because at no time does the voltage feedback actually reference the voltage of the primary, just of (B+ - primary voltage - a bit of Rk2) which means PSU ripples etc will not get corrected?

I wonder if there would be a benefit to drive the OPT with a cathode follower and ground the centre tap so the cathode feeds back the actual primary voltage? What do you think - has anyone done this?
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Old 28th February 2012, 07:43 PM   #20
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Found a few tubecad articles on cathode follower outputs, like this one:
Cathode-follower power amplifiers

The main issue seems to be providing heater windings (one per output tube) and a driver circuit that goes negative enough to provide a sane idle current.

Perhaps a smarter move would be to use the aikido trick to feedback the power rail to compensate for the fact it's not a big feature of the usual anode feedback.
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