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Old 25th August 2003, 08:44 PM   #1
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Default 12B4A as tube buffer?

Hi all. I was looking around for a low mu triode to use in a buffer/volume control and came across the 12B4A. Frank I believe I've seen you include this tube in a preamp that you designed and posted. I may be mistaken though. It looks like 300V on the plate with a 5K resistor on the cathode and a 500K grid resistor would give you a pretty good buffer with low output impedance. That would be with 34 mA through the tube. The signal would be taken off of the cathode of course. Any opinions?
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Old 25th August 2003, 08:45 PM   #2
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Go for it. Looks nice and linear, like a single mini-6AS7. But with mu=5, it'll drop a lot of signal.

Tim
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Old 25th August 2003, 09:07 PM   #3
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Hi,

Gavin,

I think you mean the 12BH7A, an ECC82 on steroids.
The circuit was a White CF.

If you want a good buffer with high current capability then a penthode is often desirable.

The 12B4A is a very fine tube I like alot and cheap too.

What kind of CF do you have in mind?

Cheers,
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Old 25th August 2003, 09:08 PM   #4
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Oh, my favourite little triode!!!
If I had to live with one tube only for the rest of my life, it would be the 12B4A!!!
I have wet dreams about a 12B4A > IT > 12B4A > OPT amp.
Some day...

Well, enough about that.
I too have plans for a line stage using this tube.
It will probably be a common cathode stage using either plate chokes or 2:1 OPT:s.

As I said its an excellent tube, especially considering the price.
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Old 25th August 2003, 10:14 PM   #5
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Default Santa's Early...

Hi,

Quote:
I have wet dreams about a 12B4A > IT > 12B4A > OPT amp.
Ironman, aren't you?

Have you tried the 6C19P? Not bad either.

Gavin.

Here's something you can even drive your speakers or cans with if you add a 220F cap to the output:

Cheers,

P.S. The cap at the input isn't necessary if the source has an output cap and you use a switched attenuator as indicated.
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Old 25th August 2003, 10:23 PM   #6
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Default Re: Santa's Early...

Quote:
Originally posted by fdegrove
The cap at the input isn't necessary if the source has an output cap and you use a switched attenuator as indicated.
But if you remove that 220n, won't it upset the biasing of the stage?
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Old 25th August 2003, 10:26 PM   #7
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Quote:
Ironman, aren't you?
Yeah, Im on a long and winding (!) way away from plate resistors and coupling caps

Nope, I havent tried 6C19P. Cant find a local source for them.
Sad it is.

That White follower looks like something that almost could drive my 811As in class A2, I think.
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Old 25th August 2003, 10:30 PM   #8
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The 12B4A is a great tube. The cct Frank linked would be fine if you need no gain, but if you do want some, try a Pimm BBMCCS and run it as a mu stage. Gain is around 5 and the output Z should be below 200R.

In a line stage, run DC heaters as they seem to be sensitive to a bit of hum IME.

My fave op-point (grounded cathode) is 120Va-k, 30mA. They seem to last quite a while here too.
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Old 25th August 2003, 10:37 PM   #9
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Default Re:Santa's erly...

Hi,

Quote:
But if you remove it, won't it upset the biasing of the stage?
I know, it should...

BTW, the attenuator is a constant Zo series attenuator so the bias shouldn't change by changing positions of the volumesetting.

I never really measured it with and without it, prefer the way it sounds without the cap so something must be right.

If you feel it should be left in then I've no problem with that.

Cheers,

P.S. Fulling, I tried it to drive a 812 and it works great.
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Old 25th August 2003, 10:37 PM   #10
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Hi Frank. I was looking for a simple cathode follower to use as a volume control. Kind of a passive "preamp" without the impedance problems. Sch3matic I'm not great at caculating gain and I don't have a lot of experience with CFs. What gain figure do you see for the schematic below? A cathode follower should have a "gain" of about .8 or so shouldn't it? Please excuse my ignorance with this circuit.
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