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In Reply to: RE: CCS for 6SN7 driver tube posted by DAK on June 21, 2016 at 21:10:26
A CCS in the cathode of a single ended 6sn7 stage will lower that stages gain.
The CCS presents a large impedance.
The larger the impedance in the cathode, the greater the amount of degenerative feedback.
Note, a CCS as a plate load will present the highest load impedance to the tube and the tube will work into a horizontal load line and the stage's gain will be at it's theoretical max.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Follow Ups:
Tre
I'm confused on what he is trying to do as I haven't seen a schematic...He said he is using 1/2 of each 6SN7..If he uses it as a CF,he won't get any gain..If he uses the first 1/2 of each 6SN7 as a voltage gain amp and CCS loads the plate,a gain of 20 is no problem.
I realize he is only going for 12 watts and it may have enough drive for that but if sets it up for no voltage gain in the amp,he is going to have to have one hell of a strong preamp considering he will losses thru the PS transformer as well.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
My post above has nothing to do with what DAK is trying to do.
I was just adding some info.
DAK wants to build an amplifier that consists of 2 independent SE amps (with a input transformer as a phase inverter at the inputs) bridged at the plates of the output tubes using a push pull OPT.
At least that's what he originally wanted to do, now I'm not so sure. :-)
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
My post above has nothing to do with what DAK is trying to do.I realize that but I wanted a clean place to jump in and address this with the back and forth..After I made this post he responded to my post below. I know the about the separate SE drivers he wants to use for each half of the cycle for the PP output stage...I just have my doubts about not having any voltage gain going into the CF or whatever he plans on using. If he uses one triode of the 6SN7 for voltage gain,he can definitely DC couple the cathode of the second triode to each KT88.I would love to see a schematic of this thing and I can't find it in the link.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Edits: 06/30/16
We don't need a CF to drive a KT88 in pentode mode, do we?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
We don't need a CF to drive a KT88 in pentode mode, do we?
Not in pentode but we are going to need some local FB. He wants a very linear output stage and I just don't get using a pair of KT88s in PP for a measly 12 watts..If you only need 12 watts,use the most linear pentodes out there which are the 6V6 and EL84 tube types.He will get all the power he needs and they are both easier to drive than a KT88..
That output circuit is SE in the schematic and he is talking a PP output circuit driven by independent tubes in SE fashion for each half of the cycle.He is going to have to implement AC balance so one driver tube doesn't become more dominant than the other because the top and bottom halves of the sinewave have to be pretty even I would guess especially in this type circuit with limited power.
I think he would be much better off using one CCS plate loaded 6SN7 to drive each tube but use the first half for a voltage amp and the second half for a CF DC coupled driver..This is if he uses KT88s.If he uses 6V6s or EL84s,a basic voltage amp/cap coupled driver would be fine.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
A lot of arguments could be made against what he is proposing.(2 SE amps driven out of phase by means of a input transformer and coupled at the output tubes with a push pull transformer)
But it's what he wants to try.........
BTW the circuit, as drawn, has feedback. Rfb1 and Rfb2 on the schematic.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 06/24/16
A typical PP amp might have a voltage amp coupled to inverter/splitter coupled to output tubes. One could have the inverter/splitter first if desired (transformer or something else).
I guess I am having a hard time seeing how the finished product can be called "two SE amps" driving a push-pull transformer. Once the SE output transformers are removed (and you have no air gap and core with DC current) you no longer have anything that is "SE" or different from many standard push-pull amps (other than the input transformer doing the splitting).
"I guess I am having a hard time seeing how the finished product can be called "two SE amps" driving a push-pull transformer."
Yes, there's a conceptual error in all this. It makes no sense to me either. Seems somewhat akin to building a bicycle by connecting two unicycles together.
--------------------------
Buy Chinese. Bury freedom.
"Seems somewhat akin to building a bicycle by connecting two unicycles together."
That's a good analogy of what he wants to do.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
That's where so much of confusion comes in..He will need two separate 6sn7s for each channel and he is splitting the phase via transformer in the beginning with NO voltage amplification before or after the PS trafo.
I think the best way for him to do this is to CCS plate load a 6SN7 for each tube. He already split the phase with the trafo and now just use the first half of each 6SN7 as a voltage amp and the second half as a DC coupled follower to drive each of the output tubes..He will have a voltage gain of 20 but if he only wants to drive the KT88s to 12 watts,he will probably have enough gain but can't be 100% sure.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
"But won't he need four SE amps?"
Yes.
"I guess I am having a hard time seeing how the finished product can be called "two SE amps" driving a push-pull transformer."
I understand what you mean.
It's just that what he wants to do is not a "normal" push pull amp.
A normal push pull amp would have a diff amp for the output stage and the driver stage. He doesn't want to do that. He wants two SE amps coupled at the input with a input transformer and at the output with a push pull output transformer.
And yes, in the end it's a push pull amp but a very different way of getting there.
Why he wants to do it that way, I have no idea.
I think the results might be interesting.
That's all, nothing more, nothing less.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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