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

[Mark A. Gulbrandsen]

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

[Mark A. Gulbrandsen]

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

[Mark A. Gulbrandsen]

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

[Zen Mod]

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....

[grimberg]

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!

[Steve Eddy]

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

[Mark A. Gulbrandsen]

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

[Mark A. Gulbrandsen]

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.....

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

[tubelab.com]

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.

[Steve Eddy]

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?

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.

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