Yet another 12B4 line stage, or is the 12B4 better than the Grounded Grid.....

[Zen Mod]

.....and send the old critters to His Zenness for proper disposal 😉

exactly!
🙂

 

[LowRedMoon]

I'd like to build this preamp but there are so many variations mentioned that it's not so easy to decide on a circuit and build it.

Is anyone willing to share a final, good, working schematic for this circuit and an accompanying power supply?

Thanks!

Adam

 

[JoshK]

Adam,

I think you are suffering from analysis paralysis. You are most likely fair new to building, am I right? I think it is much more important to build one of later circuits in these pages, doesn't matter which really and when it is working then you can worry about tweaking, and trying other ideas out. There is no real "best", some may be slightly better than others, but all should be exceptionally good.

 

[motherone]

I agree with JoshK. I'm building a "Basic" 12B4 preamp based on an old audio express article in the while I figure out how I'm going to do the casework for one of the designs in this thread.

This one just has a cathode resistor rather than the LM317, uses a voltage doubler, and runs off a wallwart with back-to-back transformers 😀. You can always do tweaks to the design once you have something built.

 

[arnoldc]

Here's mine which I shared to another member. My operating conditions are different from most you have seen here- I'm on the low voltage, low current side. You can start with this and tweak as you like. However, this *is* my sonic preference. Since I have a really small chassis (see my post several pages back, it's a stainless chassis), I
m using MOSFET regulation. Again, this *is* my choice.

 

[pengboon]

Here's a summary (kind of...). 🙂

For the tube circuit itself:

1) Anode/plate: you want to decide if you want to use plate chokes or resistors. Once that is decided, you will know what voltage to have on the PSU, generally about 150V for chokes, or 300V for resistors.

2) Cathode: Various options here, but interchangeable and relatively cheap to experiment.
a) resistor: simplest, and goes with or without bypass capacitor.
b) LM317 CCS: Requires a bypass cap
c) LEDs: yes another not too conplicated option

For the PSU:

1) Simple CLC or LC or CRC have been shown on the thread, simplest to implement.

2) Tube VR: require some extra current from PSU and calculation for the tube VR current. 300/150V is conveniently obtained via 1 or 2 0D3 or VR150 tubes which is relatively cheap too.

3) SS regulation. Kits are available, and some schematics have been shown.

Most importantly, have lots of fun!! 😀

 

[LowRedMoon]

Thank you very much for the replies. Arnold and pengboon - thanks for the schematics and recommendations. I'll try both SS and tube regulation and decide what works best for me.

Arnold - what transformer are you using?

Thanks again!

 

[arnoldc]

Hi Adam, it's a locally made transformer, custom made to my specs.

Here's an external link for the innards -

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

The LM317 CCS is bypassed with 100uF Black Gate capacitor.

 

[Zen Mod]

Here's mine which I shared to another member. My operating conditions are different from most you have seen here- I'm on the low voltage, low current side. You can start with this and tweak as you like. However, this *is* my sonic preference. Since I have a really small chassis (see my post several pages back, it's a stainless chassis), I
m using MOSFET regulation. Again, this *is* my choice.

wimpy

 

[arnoldc]

I was actually waiting for you to say that 😀

 

[mach1]

Arnold,

FWIW I run my 12B4 linestage at a wimpy 21mA - it displays much more speed and articulation than at 30mA, which sounds thick and congested by comparison.

pm

 

[arnoldc]

Hi mach1, the reason I'm running mine at 17mA is because of speed. I find it faster and snappier on my system (driving SET amps) that way. And I also noticed the smoothness on the top end, with nice touch of decay compared to a really sparkling, sometimes strident character of the 30mA operating point.

 

[Zen Mod]

Hi mach1, the reason I'm running mine at 17mA is because of speed. I find it faster and snappier on my system (driving SET amps) that way. And I also noticed the smoothness on the top end, with nice touch of decay compared to a really sparkling, sometimes strident character of the 30mA operating point.

what you really need for best (with that I mean best for you,in particular system) is sometimes not achievable for common amateur ;
you need variable bench HV supply-just because you must vary both- Load resistor and current through stage (as consequence you must change biasing also and Ub - for this you need variable supply)
to define which sweet spot is yours 😉

as Rload can be used everything in range of 1 to 10 of tube's Ri .......
so -for 12B4 something as 1 to 10K
in my experience- lower Rload demands higher Iq

I mention that to Mark somewhere in thread-but -as always-he is willing to think about my thoughts usually 3th time when I wrote them

seems that nobody tried 12B4 with,say,2K2 as Rload..........

I will not,just because I don't have any 12B4,and this toob is (was) not so common in this part of world 😉


hehe -in any case-I'll always say that choke or-even better- OPT is better solution than resistor......😉

 

[LowRedMoon]

Hi Adam, it's a locally made transformer, custom made to my specs.

What I mean is, what are its ratings? I see that you're not using a centre tap and getting your ground reference from the bridge. Would it be applicable to use a CT tranny but not use the CT?

 

[Zen Mod]

................. Would it be applicable to use a CT tranny but not use the CT?

it's always possible,but in that case current capability of xformer is halved;
when used with CT,each half of sec conducts just half of whole periode; usually wire thickness is calculated with that in mind;

if CT is not used ,both halves are connected in series, conducting current all the time

 

[LowRedMoon]

Arnold - what CCS circuit are you using?

 

[pengboon]

Here's mine which I shared to another member. My operating conditions are different from most you have seen here- I'm on the low voltage, low current side. You can start with this and tweak as you like. However, this *is* my sonic preference. Since I have a really small chassis (see my post several pages back, it's a stainless chassis), I
m using MOSFET regulation. Again, this *is* my choice.

Arnold, I thought you were using LED bias from a previous posting?

 

[LowRedMoon]

Please excuse all of the greenhorn-style questions. Some of us are relatively new to this stuff and are prone to asking simple questions.

That said - Arnold, I'm trying to make sense of the VReg you posted (this is the first time I've looked at a SS VReg). There are two + and - points on the circuit. I'm not sure what to do with them.

Is your PS made to compliment 2 channels?

Thanks!

 

[arnoldc]

adam, I'm sorry for the crappy schematic... the left side is the input from the bridge rectifier. The right side is the output to serve 2 channels.

The CCS is LM317 as posted by Brian Beck. Its bypass is a Black Gate 100uF capacitor. Just compute your resistor as 1.25 / desired current, i.e., 1.25 / 17mA = 73.5 use 75ohm.

pengboon, when that picture was taken, I have the LM317 in place. The LED stack goes into the same place. In fact, right now I'm using an unbypassed 470R cathode resistor. I'm trying to pick a flavor to match my amps, and which amp (I've got three) 😀

 

[pengboon]

Arnold, I see... with 17mA, what is your plate voltage? I remember you using 6-LED stack right, that gives 9V on the cathode? I'm trying to figure out how many LEDs I need for my setup which is with VR150 and plate choke (DCR 1.4k)...

thanks!