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In Reply to: RE: NOVICE SRPP problem please help posted by 6dj8triode@gmail.com on May 20, 2024 at 05:37:53
A real SRPP takes the output from the upper cathode, and has a low load impedance (a few Kohms for a 6922). It's a power amplifier.
What you want is a "mu follower," which is closely related. It takes the output from the lower tube's plate and works best into a high load impedance; it's a voltage amplifier. The upper tube acts as a current source, reducing distortion and allowing the gain to be nearly equal to the mu of the tube - around 33 for a 6922.
Follow Ups:
A previous discussion that went into opinions on SRPP and Mu follower starting with drlowmu's post about half way down another thread (linked below), got me to revive an old breadboard of the Baby O circuit I had put together years before on a scrap of ply.
After listening to it as drawn for a few days I changed the output to the plate of the lower triode.
It made enough of a sound difference to be surprising. Only a 333 Ohm resistor between one connection and the other, hardly anything , yet to my ear enough better to listen to with the lower connection that I'd leave it that way.
I believe I remember reading a comment of Gordon Rankin's in response to one poster that suggested he had reservations about the SRPP but I was unable to find it in a search today, not recalling which of the forums I'd seen it on.
Thanks Paul and Triode Kingdom and Tre
Mu followe need to add one more coupling cap which is a draw back. For the current source function, do you recommend the transistor current source, if yes, what type?
But If you have objection using transistor current source what is the reason?
I am new to current source technology domain
I'm agnostic about current sources - they all have their own strengths and weaknesses. The cascoded depletion-mode MOSFET designs are widely discussed on the various tube forums, so that might be a good place to start.
TK's suggestion to use a plate choke is a reasonable option. It kind of depends on the context, i.e. the rest of the circuit and power supply.
(For my Bottlehead designs, I use the Camille Cascode Constant Current Source, a.k.a. C4S, licensed from the late John Camille. But it's not well documented in the public domain.)
It seems Mr. Blencowe allows the mu follower to have an output at both the voltage amplifier plate, and at the load tube's cathode. After seeing Gary Pimm refer to the source/cathode output of his MOSFET designs as the 'mu' output, using the lower output Z seems a reasonable way to go... :)The cascode MOSFET current regulator can be used as a load, with output at the plate, and with output at the lower device source. The current regulator Paul mentioned has no low Z output.
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Edits: 05/22/24 05/22/24
As I noted above, every option has its pluses and minuses. I use the C4S in my kit designs to avoid adding a trimpot to set the current, giving up the "mu output" because many (including me) think the lower-plate connection sounds slightly better. YMMV :^)
A complete discussion of all the pluses and minuses of all the options seems beyond the scope of a single post.
no trim pots here... :) Those do sound a wee bit iffy to me.
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
I never insert or attach semiconductors to the signal path of tube audio circuitry. There's no valid reason to do this. What you're contemplating can be accomplished with a plate choke.
Regarding your questions about gain, voltage and resistor values, can I suggest that you download and learn LTSPICE? This is a free program that will allow you to model the circuit and experiment with various configurations. Duncan Amps has a library file with numerous triodes for use with LTSPICE. I believe it includes the 6DJ8.
Thanks Triode Kingdom
I went through some topics in here regarding CCS, very interesting 🤔. It seems that there are two different groups of people saying opposite opinions
I will try to look at LTSPICE
"Definition of SRPP" should be a sticky note at the top of every audio Website. :)
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