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Old 16th February 2009, 06:29 AM   #41
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whats the gain of this preamp? Zin? Zout? thx alot
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Old 16th February 2009, 11:59 AM   #42
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Quote:
Originally posted by mikvous
Difficulties on matching is not my experience.Main problem is that in my case could not make them stable without over 3.3K grid stopers on both grids.I've made a few different implementations of this tube and to my ears and system sounds as follows:
1. B+ 340v ,7.5 k anode load ,cathode 1k unbypassed .
Sound was dull and borring with no excitement.
2. B+ 220v, CCS loaded like the one on post #30,cathode 500
Ohm bypassed with 470uf Black gate non polar.
More live sound a bit harsh.This impementation also
presented exesive hiss.
3. Same as #2 but ccs loaded with IRFP9240 cascode like the one
my good friend Salas designed.
Hiss was gone, no harsh, more balanced sound.
Have to do more serious listening on this but it seems much
better over all.
Power supply is the same on all the above: rect 5R4 GY >20 uf> 10 H> 440uf> Maida style regulator .

i also had some problems with MJE, 2N ccs.... but in my case the problems were LED diodes.... they are the key problem there (in my opinion).... try to put some small cap's accross them (like 100nF multilayer)... the hiss should be gone....
also because not feeling fine with that type of the ccs i will convert it to cascode DN type... hopefully it will work fine....
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Old 16th February 2009, 12:01 PM   #43
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Quote:
Originally posted by milen007
whats the gain of this preamp? Zin? Zout? thx alot

gain is very near to the mu of the tube.... i can't remember other parameters... i think that Zout is below 1kohm...
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Old 17th February 2009, 02:33 AM   #44
Salas is offline Salas  Greece
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Today I saw Mikvous, I went to his place so to align a pair of Apogee Centaurus with new 1m long ribbon elements. We made a new crossover for those speakers. So I had the chance to listen to the 12B4 with the IRFP9240 cascode CCS anode load as well. Its much more alive and musical than before but still smooth. Clearly preferable than his older resistor loaded version. Especially easy to distinguish the differences with the aid of the cone in closed box & ribbon dipole hybrid speaker. I did not hear any hiss on the ribbons, but such tall elements usually constrict their average last octave energy in the far field in comparison with the rest of their power response. My room measurements confirmed that. Mikvous assured me that with his Seas 8 inch & Peerless HDS speaker (another project we did together in 2008) he could appreciate some further residual hiss reduction when he implemented some bypasses I recommended as in the attached schematic. Off course I would like to compare the minimal part count DN2540 cascode obvious alternative for any subjective differences, but this will take some time until we get some online, could not get them fast in some local store. As for the LEDs being the obvious hiss culprit in the original BJT CCS, I think it is not that likely, since it had only 2, when the 9240 CCS had 6 and still turned hiss way down even with non bypassed LEDS.
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File Type: gif bypassed.gif (5.7 KB, 866 views)
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Old 17th February 2009, 02:39 AM   #45
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hi Salas

sorry guys abit off topic.

the irfp9240, is it the one that your design for high voltage supply (the one i am building)? so that design in also a ccs?

erwin
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Old 17th February 2009, 02:41 AM   #46
Zen Mod is offline Zen Mod  Serbia
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Salas

try to cascode 12B4 with another mosfet ;

there are usually some possible improvements
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Old 17th February 2009, 02:47 AM   #47
Salas is offline Salas  Greece
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Those we just had on hand for the experiment VS BJT. I will get depletion ones and I also think of TO220 faster 9XXX ones. What do you recommend Zen Mod?
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Old 17th February 2009, 02:48 AM   #48
Salas is offline Salas  Greece
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Quote:
Originally posted by milen007
hi Salas

sorry guys abit off topic.

the irfp9240, is it the one that your design for high voltage supply (the one i am building)? so that design in also a ccs?

erwin
The HV shunt has same Mosfet for CCS, but not cascode.
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Old 26th February 2009, 01:55 AM   #49
Salas is offline Salas  Greece
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News on this preamp. A different bias point has been tried. 330V B+ 150V anode, 1k2 capacitor bypassed cathode resistor, 18V on cathode. 15mA Mosfet Cascode anode load CCS. Got surely better for dynamic contrast and stage.
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Old 26th February 2009, 02:07 AM   #50
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hi salas

i would like to build this for me as i have pair of 12b4. would you kindly post the complete schematic with the CCS? as from the schematic of the ccs above i am not sure where the input and output. also cap bypassed cathode resistor of 1K2 you mentioned.

erwin
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