12B4 Preamp

[Zen Mod]

sorry , Ray - I didn't realize that is you asking , even if I salute your efforts and enthusiasm previously ;

unfortunately , I'm not so familiar with many of 12B4 premp variants , even if I participate earlier in thread , drawing one of first tube shunt regs ( pretty crude and simple , by demand) .
that thread - linked to our Serbian forum , is with CCS load for 12B4 ; I resume origin is from here , big DiyA , and variety of various CCS-es is pretty well xplained in article made by local (P10 and gang) , and you can find it here : http://choky.diyaudio.rs/downloads/diyAudio-CCS-beta3.pdf .
unfortunately , schematics are crucial , so I can't even imagine ways how you are ( already been ) doing things like these .
if you are in mood for that - write specifically how we can help you , and I'll try ..... even if I'm pretty ignorant in many bits 'n' pieces locally available to you , just because I'm living on other side of big splash .

 

[crazyfrog]

IRFP9240 upgrade?

I checked this thread recently looking for an improvement of my 12B4 preamp. Will probably try that CCs cascode but would like to know if someone put another transistor than IRFP9240? Or any suggestions? Thanks.

 

[Poindexter]

That's a good looking circuit, Eli. Have you tried it without the gridstoppers? If the circuit is naturally stable, the sound will be substantially improved without them.

Respeckfully,

Poinz

 

[Salas]

Or any suggestions? Thanks.

IRF9610 is faster, same Vmax.

 

[bequerel]

maybe this will help you :

12B4A preamp - DIYAudio.rs

pics and schematics aren't in Serbian , and you don't need to register to see graphics

edit: shunt reg is further down the thread , one per channel

An externally hosted image should be here but it was not working when we last tested it.

This is one of the best DIY executions I have ever seen!!
Have a look at the last page of the thread Zen Mod links to, guys!

 

[crazyfrog]

upgrade cap on cathode?

I have on hand two Blackgate caps NX 1000 µF 25V. Would it be too much capacitance putting those on 12B4 cathodes?
Thanks
Pierre

 

[crazyfrog]

Seems 1000 µF is not too much for bypassing but not necessary.

 

[lt_texan]

I aways wondered, why not just cut off the #7 pin instead of putting a gridstopper on it?

 

[crazyfrog]

I aways wondered, why not just cut off the #7 pin instead of putting a gridstopper on it?

That question calls for another: why two grid pins on 12B4?

 

[DavinCarter]

The two grid pins are probably for cooling. Typical television use may push the control grid slightly positive. The spec sheet shows grid current for +5 and +10 vollts.

 

[lt_texan]

Interesting, I would have never thought of that.

 

[RobertHaze]

I'm about halfway into building a 12B4A preamp. I confess that I am distracted by all the circuits out there.
My friend who used to work on tube TVs told me there is no reason to terminate both grid inputs. I'm using a tube prototyping board and went ahead and cut the lands off from the other grid. The active grid has a 3.3K ohm metal film feeding a 100ohm grid stopper. Should I use another 100 ohm Grid stopper to connect the unconnected Grid to the 3.3K resistor?
It looks like there are two conflicting design philosophies on the 12B4A preamps- a high voltage 300V B+ with a 1.5K cathode resistor, and a lower 180V B+ with a 500 ohm cathode resistor. Is there any sonic benefit to using either one?
I plan on using cascoded Mosfets for the current source. Supplies will be regulated.

 

[rayma]

Should I use another 100 ohm Grid stopper to connect the unconnected Grid to the 3.3K resistor?
there are two conflicting design philosophies on the 12B4A preamps- a high voltage 300V B+
with a 1.5K cathode resistor, and a lower 180V B+ with a 500 ohm cathode resistor.

There's only one grid, with two different pins both connected to it.
Most tubes can be used with a range of operating conditions,
usually the higher current/power dissipation point is better.

 

[RobertHaze]

1. If not using 2x 3.3k carbon on pins 2,7 most 12B4A will simply oscillate.
2. Mikvous has done all those B+ and bias points with resistor anode loads and the LM317 cathode CCS one. We have listened to them all. 330V B+, cascode mosfet anode load, and 15mA with 1k2 cathode resistor bypassed, is surely the best we experienced. Opinions may vary but this is the best advice I can give you in my experience.

I'm having issues with my 12B4 project preamp. I can't see any oscillation on the output, but I'm using a very old Fluke 97 Scopemeter.
It does sound distorted though. OK on simple signals but things get dark and muddy with complex musical arrangements.

I have been unable to post any pictures to DIYAudio. It will not work if I try to posts pictures from my Facebook page.
I'm using a stacked MOSFET Constant current supply feeding the plate of the 12B4 through a 147.5 ohm resistor stack. Each MOSFET has a 1kOhm gate stopper resistor. I have a 910 ohm resistor stack on the cathode to ground. It is bypassed with 100uF Nichicons. I have a 3.3Kohm MF grid stopper resistor on both grid terminals. The cathode resistor current is 15mA.

I'm using regulated 270V supplies for B+.
As I mentioned, the preamp sounds dark and muddy and easily gets congested on complex music signals. My instinct is to increase the B+ but I can't get much more than 300V without exceeding my raw supply.
Any ideas?

 

[disco]

Perhaps you can use the "snipping tool" for a screenshot? Operating point and loading is of importance here, see post 39 and on.