Yet another 12B4 line stage, or is the 12B4 better than the Grounded Grid..... - diyAudio
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Old 20th November 2005, 11:02 PM   #1
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Default Yet another 12B4 line stage, or is the 12B4 better than the Grounded Grid.....

At the urging of a couple of other DIY members that claimed the 12B4 would stomp the Rosenblit Grounded Grid I decided to persur building one.... I left a trail of metal shavings fomr Cody to Rock Springs Wyoming as I partially assembled this on my last service trip. This IS alot more fon that watching cable TV on the motel rooms!

My grounded grid follows this basic diagram. I can't remember where I found this one but it was within a thread on this site that I found the link to it. Mine is pretty much a stock 12B4 as was my Grounded Grid. The only change I made to the grid when I built it was to use a different feedback resistor(less feedback) and a variable regulator IC in place of the fixed 12 volt IC. This allowed me to use the designed 12.6 volts on the filaments of the 12AU7's instead of Bruces 12 volts.

How do they compare.....? Listening tests were done with my Dynaudios driven by my DIY overbiased/overbuilt Aleph 2s. Source was from my Pioneer DV-45 CD player, a long time favorite of mine and one which has the proper DACs for the proper type of disks! All three types of CD's were used in listening tests.

The Grounded Grid is very detailed and open and extremely neutral and very dynamic. Overall its a very pleasing line stage to listen to once a good tube type is found. Yes, its somewhat tube fussy as I finally ended up settling on NOS Tung- Sols in mine. This seemed to be the best choice forthis line stage. JAN tubes while less microphonic were much more strident sounding on the high end. Did I say microphonic....... Yes. The grid suffers from this plague to some extent although I never found it to be a real problem. My stereo rack pretty is well isolated and on spikes.

Overall the grid has been very good and I've lived with it now for well over a year.

Then commeth the 12B4!

What can I say when something is so dramatically better. To try to sum it up I'll say that this simple circuit is the real meaning of more for less. By that I mean more music for less parts used. Somewhat the same basis for the Aleph series of power amps too.

This line stage has detail and top end that have to be heard to be believed and seemingly unlimited dynamic range. Its also drop dead neutral. As a comparison to the GG the 12B4 has alot more leading edge comming through than the Grid does and thats the single most obvious difference between the two. The bottom end and midrange performance are very simmilar to the grid but again with the 12B4 revealing even more detail and nuances deep in the music. This is very evedent on plucked bass and vibes and is pretty amazing. Background noises in recordings start to get annoying with the 12B4! Overall this is a much more musical line stage than the grid. Its hard to believe that high end tube equipment manufactirers have not hit on this tube, their engineers must be .

Are there any drawbacks? YES! This line stage has just about zero Power Supply Rejection and requires the very cleanest power you can give to it. Does it need to have a CSS or LED's to sound good. NO! This stock circuit is simply amazing running off my regulated bench supplys. The CCS mod might raise the PSR a good bit but good regulated supplies as Dr. Gizmo used to do and as I am doing also works very well. Its either going to take alot of iron or some well designed regulated power supply to get this unit hum free.

So have I retired my GG? No, it'll serve in another system in another room. Its also a very good piece but the 12B4 does indeed blow right past it.

Simply put the Grounded Grid runs at Warp 5 and the 12B4 runs at warp 9.

Build your self one!

Mark
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Old 20th November 2005, 11:04 PM   #2
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A small tribute to Dr Gizmo... Nothing beats good regulated supplies!

A rebuilt Daven attenuator to 50K controls input level. The girlfriend hasn't said anything bout the supplies yet......

Mark
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Old 20th November 2005, 11:18 PM   #3
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This is the basic 12B4 that I found. Hopefully I'm not stepping on anyones toes posting this here..... note that I used 220 UF for the cathode bypass caps. That is the only deviation from this drawing that I did.

Mark
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Old 21st November 2005, 12:36 AM   #4
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lookin' at short data for 12B4 (seems that Ri is in 1K range) you can try smaller Ra -maybe in range of 3K9 to 4K7.......your ears will decide

another one- I always place 1M resistor from grid to gnd- I don't trust any pot after several burned 12AX7s in guitar amps I repaired.....

and- bigger output cap can be good,depending of course of next stage input impedance

just a few thoughts....
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Old 21st November 2005, 01:11 AM   #5
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Default Questions and request

Mark,

The 12B4 is not currently in production, or is it?
Which brands should I look for?
Who carries 12B4s?
Could you please post the schematic for the power supply you used?

Thank you!
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Old 21st November 2005, 01:18 AM   #6
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Quote:
Originally posted by choky
another one- I always place 1M resistor from grid to gnd- I don't trust any pot after several burned 12AX7s in guitar amps I repaired.....
Or better still, MagneQuest makes some very nice grid chokes.

se
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Old 21st November 2005, 01:30 AM   #7
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Hi Grimberg,

12B4's are readily available, no ones buying them. I got mine locally from Salt Lake Instrument.... G.E. made NOS JAN 12B4's for 10 bucks each, they have a 4/71 code date on them. No low frequency noise, microphionics or other garbage that others have mentioned having problems with in other 12B4 threads. I suspect that the quality of the tube used has alot to do with those problems and its hard to beat a JAN tube.

I find it odd that a tube designed for verticle amp service would work this well in an audio circuit but what the heck. Its got about the lowest plate impedance of any tube out there sans the 2A3 and 6AS7.

The b+(260 volt) supply is an Eico bench regulated H-V supply. I have a pcb for the Chineese Lite Audio tube regulated supply and I now have a reason to build that one up. The filament supply is a 3 amp regulator IC fed from the already regulated Tektronix supply down below the line stage, I used an NTE1929 I.C. in mine cause thats what I could get locally. I plan on adding a simple but well bypassed zener shunt regulator into the line stage B+ later this week if time permits to see how that might work out. You definately will need ultra clean DC on this design!!! While the actual line stage is inexpensive to build expect to plow alot of dough into the supply end of things. Thats where the 12B4 gets to become alot more expensive than the Grounded Grid does!

Mark
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Old 21st November 2005, 01:33 AM   #8
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Quote:
Or better still, MagneQuest makes some very nice grid chokes.
Steve,

No offense, but I'm definately not a big fan of iron in the signal path. Its nice for once to listen to a really great piece of gear should I say unobstructed...... I can't see why you guys would want to add unnecessary stuff to it. Or please elaborate as to why.....

Quote:
another one- I always place 1M resistor from grid to gnd- I don't trust any pot after several burned 12AX7s in guitar amps I repaired.....
I agree with this but I don't expect to wear out my Daven attenuator..... at least not in my lifetime. If I had placed an Alps or the like pot in there I would definately add something to be there "in its place in case".

Mark
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Old 21st November 2005, 01:53 AM   #9
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I have used the 12B4 before, usually as a driver, before I discovered PowerDrive. If you like 12B4's you should check out the 7233. It has a plate resistance of 230 ohms, about one fourth that of the 12B4.

These were used as series regulators. They were also used in HP sweep generators as the sweep source. May be overkill for a line stage.
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Old 21st November 2005, 02:26 AM   #10
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Quote:
Originally posted by Mark A. Gulbrandsen
No offense, but I'm definately not a big fan of iron in the signal path.
Yeah? Then why on earth are you using tubes?

Quote:
Its nice for once to listen to a really great piece of gear should I say unobstructed......
Well, a grid choke's not what I'd call obstructed. We're talking something on the order of 1,000 Henry so at AC it's rather an open circuit.

Quote:
I can't see why you guys would want to add unnecessary stuff to it. Or please elaborate as to why.....
Well, I'm no tube expert but I believe that having a reliable DC path to ground at the grid helps keep the tubes from blowing. A choke is preferable as it offers a relatively low resistance at DC but has such a high AC impedance it rather takes itself out of the picture.

Perhaps someone else can explain this better than I.

se
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Old 21st November 2005, 05:12 AM   #11
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Mark,

Great to see you took up the challenge !
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Old 21st November 2005, 07:36 AM   #12
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Mark,

Are those standoffs from a tek scope?

dave
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Old 21st November 2005, 12:55 PM   #13
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Quote:
Are those standoffs from a tek scope?
Yes they are. Let us give a moment of silence in honor of that unknown TEK scope that allowed me to use its organs for a transplant.......BTW: Not scrapped out by me... jist so everyone here is clear on that issue . Thats something I just couldn't bear to do.....

Mark
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Old 21st November 2005, 01:34 PM   #14
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And how about a 21 gun salute for the fine plug ins that lay down their life for my HP amp. http://cgi.ebay.com/ws/eBayISAPI.dll...MEBI%3AIT&rd=1


How can people commit such atrocities
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Old 21st November 2005, 03:39 PM   #15
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Quote:
Originally posted by Mark A. Gulbrandsen
honor of that unknown TEK scope
Mine was declared dead by Allen Wright before i made it parts... lovely beasts those.

dave
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Old 21st November 2005, 04:18 PM   #16
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I felt the same way about my tek scope until I tried doing any serious measurements and couldn't stop the drift. Then I thought about the calibration sticker on the front thats 40 years past due. Thats right it was due back in 1965. If you parted out a tek scope on ebay the price of the ceramic terminal strips alone could buy you a much newer model. Plus a 10Mhz scope is no good when an opamp oscillates at 40Mhz or so...
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Old 21st November 2005, 07:47 PM   #17
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Hey,

I need a pre to my Zappulse class D poweramp. Could this one drive the 17kohm input, or maybee lower?

I too, woyld be glad to get a schematic of the Powersupply.

I`m absolutely a newbee, but would anyway try building a tube pre, and this one looked absolutely interesting.

Thanks to Mark for the great work :-)

Venlig hilsen Jan
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Old 21st November 2005, 09:28 PM   #18
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Quote:
Originally posted by Mark A. Gulbrandsen


Steve,

No offense, but I'm definately not a big fan of iron in the signal path. Its nice for once to listen to a really great piece of gear should I say unobstructed...... I can't see why you guys would want to add unnecessary stuff to it. Or please elaborate as to why.....


Mark
"signal energy preservation"

no RC stage can compete with inductive loaded stage in that field.

one need to hear good choke or OPT stage to catch what I mean ,but-like always -it's matter of taste ,not universal truth.

same as - one way speaker against multi way speaker -to each his own
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Old 21st November 2005, 10:58 PM   #19
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Bordin Sap on 12b4 - AV Review forum

"Now, I'm waiting for my tube pre-amp to finish from a hi-fi shop. Last month, by chance, I did A/B testing between my solid-state preamp with a DIY one, using 12B4 tubes. I was so stunning with the results. The 12B4 preamp produces the best sonics I've ever heard from a hi-fi system! Tons for details, musical separation, tonal balances, extreme high, authoritative bass, airy, and much more. Highs are a lot better than my solid-state preamp. I would say even better than my friend's Mark Levinson No. 383."
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Old 21st November 2005, 11:07 PM   #20
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All three types of CD's were used in listening tests.
Country, Western, and what's the third?

-j
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Old 21st November 2005, 11:38 PM   #21
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Quote:
no RC stage can compete with inductive loaded stage in that field.
What I'm talking about is not a simple RC stage..... but an active circuit. I have tried chokes but not on this sort of project. Since a simple pass transistor can make a cap look at least 10 times bigger(the multiplication of the caps value is based on the hfe of the device being used) you'd need a whole lot of choke to do what a 100,000 mfd cap could do. Change that simple pass transistor to a mosfet and it looks 10 times larger than the simple pass transistor version. Run that through another pass transistors and all thats left is a few microvolts of noise! Less space, weight, and a whole lot less cost! Take something like the power supply from the ARC PH-3 phono preamp. That power supply elevated to 300 volts would be totally suited to this sort of line stage. Cost to build the active circuit would be less than 25.00. This is why all the expensive iron just doesn't make sense to me....... I can achieve cleaner DC with cheap active circuitry and it would cost hundreds in iron and the results still wouldn't be as good.

Mark
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Old 21st November 2005, 11:51 PM   #22
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Quote:
All three types of CD's were used in listening tests.
(Mark G.)
Quote:
Country, Western, and what's the third?
(J.)

I like that J! I think Mark was referring to SACD, Redbook, and DvD-A, but I may be wrong as well.

Or could it be Country, Western, and Classical Country? I mean what else is there?
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Old 22nd November 2005, 01:16 AM   #23
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Actually all comparisons were done using my old Gene Autry and Cab Calloway 78's that have been transfered to CD..... . The Cabster sounds simply uncanny through the 12B4! The background noise from the 78's sounds extremely detailed and realistic..... Might as well can all this 12B4 stuff and just play the dang 78's!

Mark
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Old 22nd November 2005, 01:48 AM   #24
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Default Re: Yet another 12B4 line stage, or is the 12B4 better than the Grounded Grid.....

Quote:
Originally posted by Mark A. Gulbrandsen
Then commeth the 12B4!

What can I say when something is so dramatically better.

I can say, I told you so.

Quote:
Are there any drawbacks? YES! This line stage has just about zero Power Supply Rejection and requires the very cleanest power you can give to it. Does it need to have a CSS or LED's to sound good. NO! This stock circuit is simply amazing running off my regulated bench supplys. The CCS mod might raise the PSR a good bit but good regulated supplies as Dr. Gizmo used to do and as I am doing also works very well. Its either going to take alot of iron or some well designed regulated power supply to get this unit hum free.
Agreed, but it's not vastly different from most CC tube stages. With an excellent PS, they really begin to shine.
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Old 22nd November 2005, 02:35 AM   #25
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Default Same circuit in CD player output circuit

I'm late to this thread, but wanted to add that I have been using an almost identical 12B4 circuit as the output stage in a Pioneer DVD/CD player (voltage output DAC). I use 8.2K plate loads, but a CCS in the cathode (well bypassed) to hold current constant. Elaborate power supply filtering (LCRCRCLC). Sounds simply fabulous. No noise, even with headphones. I left the opamps in so I can switch back and forth to **** off solid-state guys. I'm about to do a similar treatment to my Sony SACD player. Screw the warranty!
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