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In Reply to: RE: if 7B4 is the best driver tube for SE DC 2A3 ... posted by deathtube 667 on March 01, 2017 at 06:59:47
First, the 12BZ7 was recommended-- by me! The pros and cons and the need
to drive into less efficient (and less costly) wiring, etc., for Jeff's new lower-cost build meant that a bit of extra driver power was needed for this less expensive build-- not for what I do in my more expensive build where everything must be as perfect as possible..
The 12BZ7 is controversial in that it's not perfectly linear on paper. It's good for Jeff's application because it has a high amp-factor and will in addition provide some extra current needed to drive less efficient wiring in the build. The linearity problem is NOT really a problem. The 12BZ7 can be run where it will easily be linear enough for its intended application.
With that said, Jeff is the final decider for his amps-- not me!
There you go-- more than one way to skin a different Cat.
I would not have guessed 12BZ7, but it seems logical based on your transfer efficiency concepts to use that tube in a budget build.
Each triode section of 12BZ7 is similar to a parallel 12AX7 from what I have read about it.
Thanks for the information.
You got it!
Incidentally, there are reports of early failures
regarding the use of 12BZ7 tubes.
Plug-and-try, it seems, is not doing the job.
People have simply plugged these into circuits
designed for 12AX7, etc., and see if they
hear something-- different. Trouble is, what are
the new operating points now? DID THEY CHECK THAT
against 12BZ7 Design-Center values? AND, try to
operate UNDER THOSE?
Just plunking a different tube into another tube's
circuit isn't useful tube life data!
The tube is no different in build quality than similar
types made by the same tube factories. Why would one
age faster than another?
It's operating points and nothing else.
I say, no need for you to answer any questions to anyone, on what I am going to build.
The proof of the pudding, will be in the listening, or, the A-Bing, which I would readily invite - when my new monoblocks are completed.
You, Dennis, said it well : " The linearity problem is NOT really a problem. The 12BZ7 can be run where it will easily be linear enough for its intended application."
As you say, there are lots of ways to skin the cat.
Have a good day !!
"The 12BZ7 is controversial in that it's not perfectly linear on paper."
That might be the understatement of the year.
"...will in addition provide some extra current needed to drive less efficient wiring..."
Wire takes current to drive it?
I think you need to explain why.
"The linearity problem is NOT really a problem. The 12BZ7 can be run where it will easily be linear enough for its intended application."
Please show us where you would operate the 12bz7 to make it "linear enough" for a low distortion audio application.
Have Fun and Enjoy the Music
"Still Working the Problem"
The 12BZ7 isn't really any less "linear" than a 12AX7. The curves you posted are almost exactly those of a 12AX7 but with "Ip" multiplied by 2. There's a reason for that. For all practical purposes, each section of a 12BZ7 is almost exactly a 12AX7 with both sections paralleled. 12BZ7 gm is double that of 12AX7, rp=1/2, If=2x, u=100, etc. If the 12BZ7 has a weakness compared to the 12AX7 it's excessive microphonics. Every one I've tried to use was a microphone. YMMV
I disagree with you, I do not think the graphs are similar.
But the graphs, and " maximum swing analysis " are NOT the sole reason to use, or discard, any tube. There are other factors one must weigh. The less-linear graphs I see, won't stop me in my intended use of the tube, one time in my audio chain.
In what gear were you employing the tube, preamp or amp, may I ask you ??
How many stages, one, or more in series??
Aside from microphonics, how did it compare to a 12AX7 sonically-speaking, in your circuit, and from your direct experience ??
Thanks in advance.
There are minor differences in the curves, mostly at extremes. In part no doubt an artifact of manually fitting a smooth curves to discrete data points as was done in those days.
My experience with 12BZ7 was in an attempt at improving drive capabilities of two traditional PP driver topologies that utilized 12AX7: Mullard circuit and gain stage direct coupled to split load inverter. In both situations the sound was pretty much same as 12AX7 but didn't do much serious listening before moving on because of microphonics. Maybe I had a bunch of bad examples of the type. Maybe they'll work out fine in your apps. Fortunately they're still inexpensive so no great loss if they don't work out. As always with tubes like this, YMMV.
Good observations. For preamp use, Low-Noise tubes
are chosen. Anything special about "low noise" tubes?
The plates are shorter! The less area, the lower the microphonics.
(and incidentally, the lesser the musical dynamics-- if all else is equal).
I'll be leaving operating points to the user of any tube-- it's
his amp, not mine....
I love tubes that have a large surface area and low operating currents, and low thermal stress levels.
Lots of dynamics VS distortion levels. Of course, all tubes are
microphones in one way or the other. As always-- there are degrees of trade-off.
Thanks for pointing out your observations to all of us!
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