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In Reply to: RE: CCS in a circlotron??? posted by Lew on January 03, 2015 at 09:47:58
LewYour conclusion is correct , there is no advantage by putting two CCS in circlotron circuit , it is same effect as we put there two fixed high values resistors inside of circlotron circuit .
I not claim that bootstrapping of DC coupled driver stage is superior solution but before anything else I`m must say next :
By repairing and by serious upgrading for one of my customer his one very old but very good sounding SS amp finally I try to replace simple passive bootstrap circuit around voltage gain stage with active one(CCS) which was consist from one BJT , diode and few resistor , result was significant improvement by measuring THD level , but that original smooth and well defined final sound presentation have gone , special on bass spectrum ,after that I was put back original passive bootstrap circuit and everything was fine again.
Guided with this experience I want try to bootstrap DC coupled drivers stage in the same manner , only with passive components .
Main idea is to keep voltage across R1 & R4 Constant ,since if voltage across those two cathode res.is constant than the current is constant to, drivers tubes will work with constant current to across the whole audio spectrum and drivers stages linearity will be improved .
Original Rk with value of 47K want to divide in two power resistor connected in series , R1&R4 will be now 39K and R2&3 around 4K7 ,optimal value of bootstrap capacitors(Cb & Cb`)I need to found by experiment and with listening test to, I will try with many different values for Cb ,anything between 2u2/450VDC and 100uF/450VDC .
Finally I think that by this type of bootstrapping of driver stages some influence exist on the output impedance of circlotron power stage to , I expect some lower output impedance to .
And yes I have no doubt that both CCS and SLCF work just fine by your OTL amps .
__
"Art which does not have the appearance of art is true art."
- Old Roman saying -
Edits: 01/03/15Follow Ups:
Your depth of understanding is far superior to mine, of course. Can you explain what's happening in the bootstrap schematic? Funnily enough, I recently modified an old Silvaweld phono stage which incorporated a bootstrap by taking a lead from between two resistors that in series provided Rk for the CF output stage back to the cathode of the previous gain stage. I removed the "top" one of the two CF cathode resistors and the resistor that went back to the previous gain stage cathode and replaced the latter two with a CCS, such that between B+ and ground I have CF-CCS-22K resistor. It sounds good, but is it better than the original design? I do not really know. In fact, I do not really understand what's going on with a bootstrap topology.
LewFirst of all I don`t think that I have any superior understanding over yours how this tube electronic circuit works :) , since many time by exploring some electronics circuits I feel like absolute beginner .
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Second I`m not familiar with that specific Silvaweld phono circuit,but I almost can imagine that circuit and modification that you have done .
Any way here is the close look on simplified schematic with only one half of circlotron power bridge and with only one associated driver tube .
I will try to explain in simple as possible way how that bootstrap technique Ideally have to work .
-If the both values of delta VAC1 and delta VAC2 applied across R1 cathode resistor have same values of AC voltage and the same phase status than VAC voltage across R1 resistor is ideally 0 VAC ,
If the AC voltage across R1 is now 0 VAC that further mean that no AC current can flow(leak) via R1 res. (from upper cathode point viaR1& R2 to ground ), so AC current passing across R1 is now (ideally) O , (no looses.)
This further mean that R1 is now virtually transformed on much higher value than 39K for ACV (ideally on infinite high value), so gain of cathode follower is now improved ,linearity to .
That also mean that current via driver tube is now ideally constant too .--------------------------------------------------------------------
Of course that this solution need to be proved practically by me , think that first I have to found ideal ratio of values between R1,R2 and Cb by experiment,measurements,and listening test to achieve constant positive effect across full frequency band of interest , and yes all this explanation from above is pretty simplified (just principle of bootstrapping), since in this circuit exist another variables that have to be take on account for the final design .
__
"Art which does not have the appearance of art is true art."
- Old Roman saying -
Edits: 01/04/15
Thanks for reminding me of the concept of "bootstrapping". I now recall that I've read about it in principle in one of the basic books. So, what are the determinants of the value of R1? And what is happening with CBx? You show a sine wave of equal amplitude on either side of CBx, so I guess you size it to pass the audio signal(?) I guess I need to think more before asking more naive questions, because I have a bunch of them right now.
I like your Old Roman saying. It's quite true.
LewI don`t find that any of your questions is naive , but actually is very good questions .
I think that value of R1 + R2(cathode load res.) need to have usual value as for standard non-bootstrapped circuit ,R1+R2=43-47K , that value is mainly determined from actual DC voltage values on bipolar DC supply sources , and in the same time establish driver tube optimum static and dynamic operating point.
To found optimum value for Cb(x) is little more complicated , and need to be carefully chosen to allow equal AC voltage on both side of capacitor over full frequency spectrum of interest, but on the same time to allow near as possible same AC voltage across R1 too .
Some math can be done but there is some variables in this circuit too ,as output power tube(s) transfer characteristic , and load(speaker)resonance points which can influence VAC signal level and phase on the right side of Cb(x) .
So final math will be pretty complicated for this circuit , and I think that easy way for me is to made experiment , measurements and listening test and to empirically found that optimum ratio of values for Cb(x) on the one side and R1/R2 ratio on another side.
__
"Art which does not have the appearance of art is true art."
- Old Roman saying -
Edits: 01/04/15
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