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Old 7th August 2002, 11:40 PM   #1
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Default 12b4a

Has anyone tried the 12B4A in a preamp circuit? It seems ideal(at least for a non-DHT): cheap, easy to implement, available, gain of five to six, and 1100 to 1200 ohms out. What could be better?...unless I hear from someone who knows better.
Tks, Jack Thomas
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Old 8th August 2002, 12:58 AM   #2
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417A/5842 is better.

Mohan
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Old 8th August 2002, 02:23 AM   #3
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Default why 12b4a?

Any particular reason?
What schematic are you planning on using?

I am interested in it too, but I heard it is microphonic,
but that is just hearsay, I hv not breadboarded it.

Yv
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Old 8th August 2002, 05:25 AM   #4
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I have simulated the 12B4A in SRPP and Constant Current draw circuits and I agree it appears to be ideal. I have plans to build a preamp with this in the future. The 417 is a much different tube related to the 6922 - 6DJ8. gain is too high at 42. Try the 12B4A and report back. Microphonic? I have found microphonic tubes of any kind. !2B$A's are cheap enough to sort them out. Check out TubeCad.com
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Old 10th August 2002, 01:32 AM   #5
dice45 is offline dice45  Germany
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Mohan,
could yo please elaborate why?
12B4A and 417A are completely different tubes.

12B4A is the grandchild of the 2A3 single plate, as the granny it is meant to be used for series regultators (and as vertical TV amplifier); it has similar parameters. µ=6.5, G_m=6.3ma/V, R_p=1030Ohm

417A is a supertransconductant tube with high gain. µ=43, G_m=24ma/V, R_p=1800Ohm
AIWI, the 5842 is a WE 417A without the nasty microphonics of the famous tube. Raytheon is to be preferred in this respect.

I plan to build a preamp having a differential pair of 12B4A as output tubes driving a Lundahl LL1660 output transformer. I intend to have the tube running at considerable current: 34mA per tube. I read raves about the 12B4As sonics on the JoeNet; a friend seconded this and so i decided to go with it. As this is a line stage application and meant to be differntial, i am frightened off from using DHTs; the diff.pair's virtual ground would collect too much noise. So i am happy the 12B4A is indirectly heated.
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Old 10th August 2002, 06:17 AM   #6
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Quote:
Originally posted by dice45
I read raves about the 12B4As sonics on the JoeNet; a friend seconded this and so i decided to go with it.
Dice,

You beat me to that comment. I am going to get some of these and try something out with them too. Here is the recreated data sheet that Bill Perkins did up -- i understand he is sitting on a number of these and selling them cryoed & graded (reason why he went to the effort to create the pdf)

dave
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Old 10th August 2002, 06:57 AM   #7
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Quote:
Originally posted by dice45
12B4A is the grandchild of the 2A3 single plate, as the granny it is meant to be used for series regultators (and as vertical TV amplifier); it has similar parameters. µ=6.5, G_m=6.3ma/V, R_p=1030Ohm

I plan to build a preamp having a differential pair of 12B4A as output tubes driving a Lundahl LL1660 output transformer. I intend to have the tube running at considerable current: 34mA per tube. I read raves about the 12B4As sonics on the JoeNet; a friend seconded this and so i decided to go with it. As this is a line stage application and meant to be differntial, i am frightened off from using DHTs; the diff.pair's virtual ground would collect too much noise. So i am happy the 12B4A is indirectly heated.
Hello Bernhard,

Please keep us informed with your progress and findings about the 12B4As. I picked up 5 recently, NOS/NIB Sylvanias for $1 ea. Do you know of any preference for a particular brand(s) in terms of performance?

I had also heard good things about them, and wondered how they would fare as a diff stage. I hadn't thought about running them quite as hard as 34mA though. At my first look at the curves, I thought about maybe 25mA with a Vp of 90-100V, loaded by one of Pete Milletts PMCCS, parafeedeing into my S&B TX1022 TVCs.

I love the idea and linearity of DHTs, but using speaks with the sort of efficiencies mine have (104-106dB), I'm concerned about noise and hum. The transfer characteristics are great for an IDT.

Cheers
Brett
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Old 11th August 2002, 12:45 AM   #8
dice45 is offline dice45  Germany
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Dave,

i kiss your feet for posting that 12B4 data sheet link. Thanxalot !!

Brett,

unfortunately my preamp is not to happen soon, 1st i have to get 2 linear tracking tonearms designed ready and built, one open baffle speaker and two power amps built and cooperating.

I do not know any particular preference concerning brand, but i also will go with Sylvanias, i have enuff of them to feed pigs. for my application this is not bad at all, tight matching, yanno.

34mA is recommended data sheet operating point and curves show the tube likes that best. The tube is meant to be the reg tube in a series reg, it has to handle lost of current.

low-level differntial pairs and DHTs: does not work, Thomas Mayer tried that out.
Problem is: the differntial pair has a virtual ground at the connected cathodes and with DHTs, this virtual ground is galvanically connected to all hum and noise the heater supply (trannie) picks up by induction or stray-in in reference to chassis/signal ground.

Now it is the designer's choice whether to inject that noise voltage into the virtual cathode or into the grid, same amplitude, just phase inverted. On line input level, 2V RMS@0dB, S/N ratio suffers too much.

In a differntial power amplifier with the 1st stage being IDHT, i will try low-µ DHTs for 2nd stage (2x 71A ) and 3rd stage (2x AD1). But even this signal level demands separate and symmetric heater trannies with additional electrostatic shield windings connected to ground.
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Old 11th August 2002, 01:17 AM   #9
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Quote:
Originally posted by dice45
i kiss your feet for posting that 12B4 data sheet link.
No you don't :^) I think you already owe me one beer if i ever get to Munich, just make it 2 (and get someone to carry me to my room :^)

I did a quick pick of some of the JoeList posts i have archived and attach them here as food for thot:

.................................................. ...........................
Joe Roberts: Not necessarily...how about a SE triode into an output trans with a balanced
output? This is one of the most common topologies in pro gear of yore.

My main line amp is a 12B4 into a 5k:600 ohm MQ output trans. Sounds great. If
I wanted to run balanced, the 125 ohm tap is the effective center tap of the
600 ohm winding.

You can definitely drive the snot out of that long cable with this setup!

Next step is to battery bias that 12B4. Bass is already tight in self-bias,
but I want to hear more of that battery thang.

It was recently mentioned in Valve article by my bud John Day. He took it to a
preamp shootout and it did all the shooting, even though it is one funky
looking piece of gear.

Think I was running the tube at 25 or so mA. low DC B+ (125-135 V or so).
Never wrote it down. Can check if interested. The 12B4 is microphonic so do
something tricky about that.

(Bill Perkins has some of these Cryo treated which is claimed to
reduce the microphonics (as well as other improvements))
-----------------------
Gordon Rankin: This is a great tube for preamps. Make sure you use DC heaters as it is
prone to noise from the filaments.

I used 5K Mills resistor on the plate and 20ma. It worked great.

funny thing I looked up the 12B4, and here on one of those Dog a Day
calandar pages (often littered through my house with schematics on
them...) was my 12B4 schematic. Samurai Spirit (the Dog) Thr, July 23
1998 marked final.

I used the jensen 10K:10K input terminated with a 10K P&G RF11 pot, grid
resistor 100 ohm. Bias on the 12B4 was 90V/-10v/20ma, I used 2 Mills
MRB-12 15K in parallel. Bias resistor 499 mills MRB-5, 33uF/16 BGN cap.
Output 3uf/Hovland, 100K shunt to ground. 6V4 power supply, choke pi,
then split cap.

Proabably would be better with a Jensen P4 input 1:2 using a 20K pot.

I also had some scriblles about battery unit, 8-12V cells, 6V for
heaters, 100 ohm bias (-2V), 100H/20ma choke.

I used regulated dc heaters and placed the 12B4 in Yamamoto sockets and
lifted the socket off the base of the chassis with those rubber standoffs
that Percy sells.

In the end you could bash the damn chassis, nothin...
-----------------------
Kurt Steffensen (the mad Viking & responsible for the Arhus triod Festival); 12B4 is one of my favorite tubes from line signals to driver.
And it makes a magnificent CF.
( Are you guys beginning to explore the merits of this wonderfull little circuit ? ;-)

I use DC heaters , though it is probably not really necassary. Depends upon how much noise you accept.

12B4 is a triode that does what it is told. Nothing less , nothing more.
DEAD neutral !

And it is able to reproduce a clean 24 bit dynamic range.....

Don't know how it will behave as an MC amp.. The electrode structure is to dramatic for such small signals , for my taste.
But I do not doubt , that the white noise could be low. I would worry about microphonics , at such low levels , though.

I once used it for output tube as well. Cool :-)
It makes a good circlotron , Remco ;-)

And by the way , it is an excellent regulator triode :-)

Not a single of my 12B4's are microphonic. But I have heard many say this.
( I may have some 200 hundred pcs., though I havent tried them all ;-)

Mines comes from the same series production ( batch ) made by GE.
But I also have a few Toshiba and hmm is it CBS ? ..
None of these suffers from microphony either....
-----------------------
Bill Perkins: The 12B4 is a low mu, single triode of medium transconductance and rp.
It will *operate* at a plate voltage of 500V and is ideally suited to
applications where large output swings are required, particularly with an
IT.
So if you want to drive an 845, for instance, this device will do that
very well.
-----------------------
Dave Slagle: it is really not that hard to just do it from the ground up....
just kiss (keep it simple.)

build a 300V or so supply (the total voltage can be whatever for the firs
go.. 200-500V)

say we have 300V... OK lets use a 10K plate load resistor.... and want to
run 20ma of current ohms law tells us we will drop 200V leaving us with
100v left for the tube

now go to the curves and see that at 100V and 20ma you will be at -11V bias
or so... so you need to chose a cathode resistor that drops 11V at 20ma
which is 550 ohms. toss a 100mfd cap across it add a output cap of 2 mics
or so... a grid resistor and a output resistor and you are done.

it will make music and sound pretty good... now its up to you to work on
the PS and the operating point to make it sound great.
-----------------------
Steven St.Laurent : What I Did This Week (So Far)..

I've been stuck in the hell that is Sun Microsystems Training (free so i
can't complain too much) for the past week plus, so i've had time to plan
things. During my parole time from work I've managed to "finish" breadboarding
my 12B4 one-tube-wonder. There are some cryo'd units I got from our friend
up north, Bill Perkins.

Basically my current setup is a single tube version of the End of Zen by
Tom Stoppleworth (bottlehead forum has big thread). I junked the input
tube and just run the 12B4. Parafeed (Hammond 157G choke + Angela
3.3uf caps) thru some Hammond 125E's. Input isnt optimum, 1k ohm resistors
on the input, need to play with this some. AC filaments, nothing special.
DEAD SILENT, except for the fact this is within 3 feet of 5-6 computers
and several computer monitors plus a halogen lamp with a noizy dimmer!

Sevendust (noize to you older guys) has incredible bass slam, even for a
poor modern recording. Denial (track 2) is quite impressive. Dave Brubeck
Quartet sounds quite good too, cymbals sound very natural. Squirrel Nut
Zippers really rocked, the bass was quite impressive. Played some ZZtop to
good effect also.

I would say its very neutral, but with more bass. Its not going to make
solid state sound any better, if anything it shows its warts. Its worlds
apart from the sound of my 1626's for certain. Its more like a wire with
some gain and a bass boost.


My only complaint is the gain isnt quite what I would like. I need to
play with it some more. I doubt the tube is seeing a pretty load, coming
off the headphone output from a cd-rom. I've not played with output tranny
loads much either yet.

Any suggestions on improving gain and what to put between the cdplayer and
the tube?
-----------------------
Other comments: Julius Futterman used 12B4s as outputs in some of his early, direct-coupled OTL amplifiers.

A 12b4 OTL can be found at <http://www.infomaniak.ch/~bonavolt/12b4.htm>

The GE's I'm using are during the first ten minutes or so of warming
up. Quiet after that. Running them at 90v/28ma. AC filaments,
biased up to 30v.
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Old 11th August 2002, 02:34 AM   #10
dmcgown is offline dmcgown  United States
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Just as a point of reference, my present preamp is a choke loaded 12B4a (GE, the RCAs are horribly microphonic), using a 60H/30mA Handmade choke (www.hndme.com), with 25mA/155V across the tube. Using Russian 2.2uF PIO + 0.056uF Russian Teflon output caps. It plays good music for me.

I don't think a Lundahl LL1660 with a differential 12B4a is a good idea. The transformer really can't handle the current levels that a 12B4 likes to operate at. You may be better served by other iron, or go with a simple SE common cathode.

David
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