12b4a

[dice45]

Dave,
no prostetic leg, sorry, sold my sister ....
and yes, i am planning to have a bottom extension. One pair of 18" Beymas per side.
The Fertin needs no tweeter. Recently i heard a buddy mutter to himself "... only too big for a tweeter".

[fdegrove]

Hi,

Wow,a simple question generated an awful lot of discussions.
I can not help but noticing a lot of people confusing differential,push-pull single-ended and so on.
It is always hard to give a straightforward answer to a question,the more you know about something the more you realise you don't know anything at all.I assume you singled out the 12B4 since you did not require much ampification,need a low output impedance?
Well I must say that as others I consider this tube to be excellent for the task and surely there are ( or should I say there "were"other types up to the task.
Microphonics wouldn't bother me since you can easily combat that and it will vary from brand to brand and on a gereral note: most valves are if pushed to their limits,designed conservatively they are dead quiet.
Never forget to pay attention to the psu design since this will always be in the signal path.
If I were to design a linestage to drive my amps I'ld certainly go for this one,add a well designed valve regulated psu and you're definitely rocking.
All brands have their sonic footprint since they are all slightly different ( whatever set of parameters was important to one manufacturer, wasn't necesarily for an other etc.)
More to the point I'm convinced that there is a lot less character to a valve than there is to passive components such as caps and resistors.
Conversely, the choice of power supply has as much impact on the sound as all the rest put together.
Anyway,I whish you good luck with your project,

[dice45]

Frank,

you are right, i singled the 12B4A out becuase it has not much gain and together with the LL1660PP and a tiny cathode resistor, it does arrive at the output Z i want to have, not lower not higher, 600 Ohm. Moreover, it is a cheap tube and sounds gorgeous.

The line stage is meant to handle full swing all the time as the GC is located **after** the output trannie. 600 Ohm again. So it needs much headroom. -17V is just what makes my sleep sound.

The linestage is a differential one, so it is not near as sensible to power supply influences as a SE stage would be. In fact, the diff. linestage is a constant load to the PS. I intend to use a SS shunt reg for each stage, p2p-wired right to the terminals of a 10µ MKV cap. Makes 6 reds in total for the whole preamp. Not a very lean design

I have few stage neg-FB and zero loop feedback in the is preamp, so any stage is required to have right the gain and right the output Z i need.

For MM phono, i use 2x EC8010 for 1st stage, 2x 12A4 for 2nd stage, 2x 12B4 for 3rd/line stage. All stages differential/longtailed pairs, 1st 2 stages choke-loaded, 3rd stage transformer loaded. MC input trannie. DC-coupling (or let's call that wire coupling) between all stages, phono EQ split and floating anode2anode. Hot pluggable.
Priority one is phono/vinyl; for line level the wire between 2nd and 3rd stage is replaced by an input transformer.
No coupling cap .... and as the amp is differential, the PS caps do not count as interstage coupling caps, atleast if tube matching is properly done.
The 6922 thread has given me new inspiration how to possibly avoid the MC input trannie and to get along noise-wise. Maybe i try out other tubes and B+ or add a 4th stage.

Mohan,
did not intend to smartass you, i wish i had your experience. Me is more of a thinkthink sort of guy but i think over this amp design since i got infected by that contagious tube disease. I discarded many circuits without ever having had the opportunity (professional work load) to try them out; later i found flaws in the circuitry and contradictions to my spec and went on with another idea.

Now, this preamp structure simply appeals to me for its conceptional beauty.
And listening experiences of different differntial and PP and SE designs showed me a properly differntial amp is what i want. As preamp, as power amp. For the record, i have not settled on Allen Wright's preamp, i simply don't like its sonics and i don't like its conceptional complexity. And i am not afraid to use more signal transformers than needed if they make me like sonics more.

[fdegrove]

Hello Munich.

Actually I'm not sure if you use a MC or MM cartridge ?
However in case you use an MC, just know this is actually one of the few balanced sources available.In this application experimenting with balanced MC tube stages should pay off (on paper at least ) .
If you are considering the 6dj8 stage I suggest you be aware you will need double the amount of components.
Please keep me posted if you decide to go for it.
I can see the thinking behind your phono and line stage and I can confirm you will find sound sleep with the 12B4,depending on your design it will easily swing tens of volts peak to peak.
I just can't help but wonder what a fully balanced system (MC + amps would sound like,yes amps included.A fully balanced OTL comes to mind).
Certainly not a bad idea in these days of RFI and RMI pollution...

Gruesst die,

[dice45]

Frank,
i keep you posted.

I use both MM and MC. Yeah, i know, MC is balanced and floating. I have no problem having the MC floating at -25V. See my other post in the 6922 thread.
Also possible with MM, i would guess.
One of my specialties are glued MMs, see analogue forum board.

Yes, my plan is to go all balanced/differential from MC to speaker.
Allen Wright's power amp is something i could grow old with. But i have some own ideas. Choke-loaded IDHT at the input (6900), 2nd stage 2x 71A driving IS-trannie-> 3rd stage 2x AD1.
Reason for 3 stage design: i have to locate my FR speaker LF-roll-off somewhere. Speaker XO caps are the only coupling caps in the whole system.

I designed phono preamp and power amp together, the think is system-think. My point was not electrosmog pollution (although i admit i cherish differential immunity to that), i simply like the sonics of truly differential amps with zero loop feedback.

[Mohan Varkey]

Bernhard,

Appreciate your thoughts. I value all contributions in this forum. I am still learning what our forefathers knew but did not put pencil to paper.

I did not mean to put Allen Wright and yourself together in one basket. I have a high regard for Allen as a clever designer. I had many interesting discussions on the phone with him when he was in Sydney (mid –late eighties). Like yourself, I did not totally enjoy the sonic character of his designs.

I used to design 30-40W OTLs in the late seventies – mid eighties. I believe that OTL reigns supreme in the musical enjoyment category despite my reservations on “Balanced topologies”, “No two tubes are similar” etc etc.. I used to have Magnepan MG IIB then, which presented a simple resistive load to the OTLs. Later, when I changed over to electrostatics, I tried higher (70W) power OTLs with not so interesting results and a host of other problems including oscillation etc...

When we pick high transconductance tubes, we need to have a good cathode bypass cap. Without this, the transient response will be somewhat diminished. Another option is to use battery bias and then you loose the ‘idiot-proof’ safety factor. Constant current operation does not allow a triode to behave in its natural form and that is to oppose changes in voltage and current. Yes, we can take any tube and operate it in any manner that suits us. I have found that while these types of operation will yield maximum gain, lower output impedance etc., these topologies have one thing in common, that is they are not as faithful to musical reproduction as the simple grounded cathode operation.

There are small windows of operation with any tube where it will excel in tracing the input signal more faithfully. These windows need to be found for a given circuit and this takes hours of listening, component changes and measurements. Once these are found, you will have a system that excels most commercial offerings at any price. This to me is the joy of DIY. This enjoyment is not cost based but performance based.

I shall stop for now.

Mohan

[fdegrove]

Hi,

-Re amps : Am I reading you correctly in that you use 71A and AD1 ? I used to sell these to Asian customers,ten years ago....
Deep sigh.

-Re IS Tranny : If this has to couple a wideband stage I always try to get Zout on the preceding stage as low as possible.It makes life on the tranny easier and gives better coupling at the frequency extremes. (Nothing is perfect).Guess you knew that already.

-In case you use a series filter in your speakers X-over I'd like to know more about it.Perhaps I should post a thread in the speaker section.

Greetz,

[Tom S.]

I'm a little bit of a "johnny-come-lately" to the thread so pardon me if I interject with a 12B4A comment/question in the middle of a design philosophy tangent.

There are cryovac'ed 12B4's floating around? Where? I seem to recall getting a somewhat terse response from tubeworld about the possibility of cryovac'ed 12B4A's. I really would like to get a hold of some of these tubes but I can't seem to find any cryo vendor selling the 12B4A variety. Perkins seems to not respond to e-mails.

Tom §.

[planet10]

Originally posted by Tom S.
There are cryovac'ed 12B4's floating around? Where? I seem to recall getting a somewhat terse response from tubeworld about the possibility of cryovac'ed 12B4A's. I really would like to get a hold of some of these tubes but I can't seem to find any cryo vendor selling the 12B4A variety. Perkins seems to not respond to e-mails.

Tom,

I got Bill on the phone (250 607 6633 IIRC) to ask him if i could get some, but his entire inventory (7-800 tubes) went off to Austrailia (i guess someone saw a bargain). Bill let his domain expire (fixed now) & was without computer for 3 weeks during a recent move so one of those is likely to be the reason for no response.

You could contact Bill to find out who the fellow in Oz is and see if that vendor is willing to sell any, or you could send your 12B4s (or anything else for that matter) to Bill and he will treat them.

dave

[Tom S.]

but his entire inventory (7-800 tubes) went off to Austrailia

!!!

I wonder who would want hundreds of cryovac'ed 12B4A's. Sounds kinda weird. Thanks for the info.

Tom §.