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Best/favorite CF NOS noval twin triode?
Best/favorite CF NOS noval twin triode?
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Old 17th April 2008, 03:39 PM   #21
SY is offline SY  United States
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Best/favorite CF NOS noval twin triode?
No. Cathode followers can be designed well (with all due modesty, the CF line preamp design I published is a good example). It's just that your proposed CF isn't. To get that 0.08dB extra gain (inaudible) compared to a properly designed CF with an appropriate tube, you're picking a very inappropriate tube, then using it in a non-optimal way. It compromises both the load-driving ability and distortion for a demonstrably inaudible benefit.
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Old 17th April 2008, 04:10 PM   #22
oldeurope is offline oldeurope  Germany
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Default OT This is an outrageous!

Quote:
Originally posted by SY
No. Cathode followers can be designed well (with all due modesty, the CF line preamp design I published is a good example). It's just that your proposed CF isn't. To get that 0.08dB extra gain (inaudible) compared to a properly designed CF with an appropriate tube, you're picking a very inappropriate tube, then using it in a non-optimal way. It compromises both the load-driving ability and distortion for a demonstrably inaudible benefit.
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Old 17th April 2008, 04:11 PM   #23
kevinkr is offline kevinkr  United States
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Best/favorite CF NOS noval twin triode?
Default Re: D3a, E280F

Quote:
Originally posted by panos29
Not exactly cheap and not even double triode, but D3a and E280F are definitely excellent (maybe the best?) for cathode followers when triode strapped and even better in pentode mode CF.
Ssssh!

Another contender as a dual triode is the 5687, not cheap but not terribly costly either, high GM, good plate dissipation rating, and likes to be run at high-ish currents. Certainly seems like a good choice for a CF to me.
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Old 17th April 2008, 05:56 PM   #24
oldeurope is offline oldeurope  Germany
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Default #23

Hello Kevin,
this one has a low of 17. This gives a loss in gain. Maximum gain is 0,94.
In some applications it is very importand to get a gain close to 1, e.g. bootstrap.
The ECC83 provides you up to 0.99.

Kind regards,
Darius
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Old 17th April 2008, 06:26 PM   #25
kevinkr is offline kevinkr  United States
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Best/favorite CF NOS noval twin triode?
Default Re: #23

Quote:
Originally posted by oldeurope
Hello Kevin,
this one has a low of 17. This gives a loss in gain. Maximum gain is 0,94.
In some applications it is very importand to get a gain close to 1, e.g. bootstrap.
The ECC83 provides you up to 0.99.

Kind regards,
Darius
I understand that, OTOH if you need to drive 10K with good linearity or typical headphone impedances the 12AX7A won't do that at all.

My experience with 12AX7A and CF applications exactly mirrors SY's and I have all the same objections. I stopped using them nearly 10 years ago for everything but error amplifiers in power supplies because frankly there are many tubes I feel are better for specific applications.

The 12AX7A was a low cost tube intended for cost sensitive applications, it is a jack of all trades and master of none. There are better choices in almost all cases where cost and ready availability are not the determining factors.

In a situation where gains approaching 1 are required you really will get better performance with a high gm pentode (pentode connected) - most other applications don't require it.

Are you using mu followers by any chance??
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Old 17th April 2008, 10:23 PM   #26
Vargas is offline Vargas  United States
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For what it's worth, the 5687 is a modern computer duty version of a 6N6p.
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Old 18th April 2008, 06:56 AM   #27
oldeurope is offline oldeurope  Germany
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Default ECC83 and equivalents...

Hello Kevin,

Quote:
Originally #23 posted by kevinkr

My experience with 12AX7A and CF applications exactly mirrors SY's and I have all the same objections. I stopped using them nearly 10 years ago for everything but error amplifiers in power supplies because frankly there are many tubes I feel are better for specific applications.
Oh, this makes me sad. SY wasn't able to use the ECC83 because he had some grid
current.
Quote:
Originally # 13 posted by SY


At 2mA, you'll have some grid current issues with actual 12AX7s.

The gain for an ECC88 in this position is 0.97. The difference between that and a tube with a mu of 100 is less than 0.1dB.
I explained him how to use the ECC83 without grid current. See #14 #16
I am sure this will help you making low loss triode cathode followers, too.

Kind regards,
Darius
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Old 18th April 2008, 01:38 PM   #28
kevinkr is offline kevinkr  United States
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Best/favorite CF NOS noval twin triode?
I guess you totally missed the point SY and I were trying to make. I rest my case.

You may now continue to use the inferior performing 12AX7A to your heart's content, and I will stick to the results 28 yrs of engineering experience and what my FFT analyzer, not to mention my ears tell me.

FWIW if you ever change your mind take a look at some of the tubes the German Post Office came up with late in the tube era, and then still tell me the 12AX7A is better.

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Old 18th April 2008, 02:11 PM   #29
GordonW is offline GordonW  United States
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Quote:
Originally posted by dsavitsk
How about a 6DT8 or 12DT8. mu = 60, Gm=5500, and they are like $3 each.
I'm with this suggestion. My first instinct was to suggest the 12AT7/6201... sort of the "industry standard" for this, since they were MADE for "cathode-drive" (ie, cathode follower) operation... it's specifically stated in the datasheets. However, a 6DT8/12DT8 is functionally identical to the 12AT7, save for having a different pin out and greater heater/cathode voltage-difference handling... DEFINITELY useful in a cathode follower under high-voltage-swing conditions. And, with the mu and gm of the 6DT8, you can get right at .95 gain... at MUCH higher current demands and much LOWER output impedance than a 12AX7 or its ilk. Not to mention the plate curves are better for the 6DT8/12AT7/6201.

OTOH- if you need mega-swing or mega-current capacity, the 6BL7 is an absolute monster as a cathode follower. +-200v? It'll do it. It'll take in excess of 15ma per section to do it right, but output impedance is like, just over 100 ohms!! Gain is only about .9... but as long as that's usable, you can run ANYTHING off of them...

One other good choice, though more expensive, is the 6n30P.... it'll definitely work well. Very high transconductance. It will lose a bit more gain than a 12AT7 or 6DT8 (about .9 vs. .95), but if you want linearity over a WIDE range of voltage swing... THIS tube is just about as good as it gets, IMHO, short of the rare 5842...

Regards,
Gordon.
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Old 18th April 2008, 02:33 PM   #30
oldeurope is offline oldeurope  Germany
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Quote:
Originally #25 posted by kevinkr

...The 12AX7A was a low cost tube intended for cost sensitive applications, it is a jack of all trades and master of none. There are better choices in almost all cases where cost and ready availability are not the determining factors.

In a situation where gains approaching 1 are required you really will get better performance with a high gm pentode (pentode connected) - most other applications don't require it.

Are you using mu followers by any chance??
Quote:
Originally #28 posted by kevinkr
I guess you totally missed the point SY and I were trying to make. I rest my case.

You may now continue to use the inferior performing 12AX7A to your heart's content, and I will stick to the results 28 yrs of engineering experience and what my FFT analyzer, not to mention my ears tell me.

FWIW if you ever change your mind take a look at some of the tubes the German Post Office came up with late in the tube era, and then still tell me the 12AX7A is better.

Hi Kevin,
I like more technical discussions. I am not a tube dealer.

BTW: Did you read post #1 ?

D.
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