12B4 Preamp - Page 4

freakyone · 11th February 2009, 02:55 AM

and this

sparkle · 11th February 2009, 04:58 AM

Quote:

Originally posted by freakyone
Hello

Has any one tried this one,
Or maybe just your thoughts on it.

Cheers

i have a same schematic and similar values preamplifier in my system - a monster preamplifier...... really, really good.......

psu that you presented could be better .... (other tubes, bigger cap's)....

philipbarrett · 11th February 2009, 11:06 AM

What values would you use where?

inertial · 11th February 2009, 11:59 AM

Excuse me,
Why not an input chocke PSU ?

Cheers,
Paolo

freakyone · 11th February 2009, 06:16 PM

Thanks Sparkle

Just a quick question, is the schematic complete, not missing anything is it?

cheers!

sparkle · 12th February 2009, 05:07 AM

Quote:

Originally posted by freakyone
Thanks Sparkle

Just a quick question, is the schematic complete, not missing anything is it?

cheers!

i have a capacitor in parallel with the cathode resistor....470uF Black Gate.... and a gri stopper 2k2 carbon composite.....0,47uF coupling capacitor is a bit low... better use 1uF (this is if you plan to use amps with lower input impedance).... also use 1M resistor to the ground, after the volume control ..... also the 50k resistor is not the same (don't remeber the value), cathode resistor is 1k5 in parallel with 1k, and i do not know which one i have used for R2 but my current is aaround 22mA (from what i can remember).....

be aware - LED diodes can sometimes produce problems in this ccs.... i have had numerous problems with them - but this ccs sounds great in this circuit... because of it - my next project is to remoe that ccs and use DN2540 cascoded ccs in there......

btw. the capacitors in the psu need to be bigger... i am using 100uF as the second cap - also have a monster inductor for filtering ( i believe it is arround 80H double C core).....

Salas · 12th February 2009, 02:59 PM

Quote:

Originally posted by freakyone
Hello

Has any one tried this one,
Or maybe just your thoughts on it.

Cheers

A friend tried that yesterday and told me that sounds more alive with the MJ BJT CCS of the schematic than with a load resistor, but got some hiss also.
Since he has got only IRFP parts and leds in his bin and no easy access to DN depletion Mosfets, I have drawn this in the attachment for him. If it will work any better, he will let you know I guess, he is a member here.

andyjevans · 12th February 2009, 03:55 PM

In discussing what types to use, has anyone made the point that 12b4s test absolutely all over the place, within a huge range. Your mission, should you choose to accept it, is to find two that measure equally! That's the starting point, the rest is luxury. the point I'm making is thay're a pig to match and you may need to buy several to get some good matches

andy

mikvous · 13th February 2009, 11:56 AM

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 .

Salas · 13th February 2009, 12:40 PM

Nice to know that the 9240 cascode CCS anode load worked best for you. You probably mean that you needed 3.3k (or over) grid stoppers on both 2,7 pins. By adjusting the trimmer you can listen to other bias points as well. I would use a very small sink on the lower Mosfet for long term reliability for this CCS. Maybe I will try that CCS load on my 6V6 preamp too. Interesting to compare the anode resistor signature to a Mosfet cascode CCS active load. It can stand 400V across it, so it can be applied to many stuff.

P.S. If anyone else attempts it, please keep in mind that the 4K5 resistor shown on the circuit is virtual. It just represents the 12B4 that must be connected in its place. Also I think that the 12B4 has a higher rp (around 5.5-5.7K in such a circuit) and will develop maybe 25V more on its anode than shown on the original BJT CCS schematic with the 20mA bias it recommended.

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11th February 2009, 11:06 AM	#33
philipbarrett diyAudio Member Join Date: Feb 2005 Location: Dallas, Texas	What values would you use where?

11th February 2009, 11:59 AM	#34
inertial diyAudio Member Join Date: Aug 2004 Location: Italy	Excuse me, Why not an input chocke PSU ? Cheers, Paolo

11th February 2009, 06:16 PM	#35
freakyone diyAudio Member Join Date: Sep 2005	Thanks Sparkle Just a quick question, is the schematic complete, not missing anything is it? cheers!

12th February 2009, 03:55 PM	#38
andyjevans diyAudio Member Join Date: Sep 2003 Location: London	In discussing what types to use, has anyone made the point that 12b4s test absolutely all over the place, within a huge range. Your mission, should you choose to accept it, is to find two that measure equally! That's the starting point, the rest is luxury. the point I'm making is thay're a pig to match and you may need to buy several to get some good matches andy

13th February 2009, 11:56 AM	#39
mikvous diyAudio Member Join Date: Nov 2006	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 .

13th February 2009, 12:40 PM	#40
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	Nice to know that the 9240 cascode CCS anode load worked best for you. You probably mean that you needed 3.3k (or over) grid stoppers on both 2,7 pins. By adjusting the trimmer you can listen to other bias points as well. I would use a very small sink on the lower Mosfet for long term reliability for this CCS. Maybe I will try that CCS load on my 6V6 preamp too. Interesting to compare the anode resistor signature to a Mosfet cascode CCS active load. It can stand 400V across it, so it can be applied to many stuff. P.S. If anyone else attempts it, please keep in mind that the 4K5 resistor shown on the circuit is virtual. It just represents the 12B4 that must be connected in its place. Also I think that the 12B4 has a higher rp (around 5.5-5.7K in such a circuit) and will develop maybe 25V more on its anode than shown on the original BJT CCS schematic with the 20mA bias it recommended.