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Old 5th April 2003, 06:30 PM   #1
Jay is offline Jay  Indonesia
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Question Tube Theory

Hi,

I'm building pre-amp consisting of 3 gain stage of paralleled ECC81. As this is quite expensive, I don't want to get an unexpected result by matching it to the wrong amplifier (Aleph).

Too high an output impedance is what I'm worry about. So I need a hint regarding tricks to lower the output impedance, or good site where I can probably learn how to do it (May be I need to replace the last stage with 12AX7?, may be I need to create active load on the last tube's chatode?)

Thanks
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Old 5th April 2003, 07:00 PM   #2
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Replacing the last stage with a 12AX7, even as a CF won't give you a low output Z.

Read this article, as it's an excellent primer on how tubes work, and partly on how to apply them. You need to understand what's happening in a circuit.
http://www.tubecad.com/articles_2003...ier/index.html
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Old 5th April 2003, 07:01 PM   #3
SY is offline SY  United States
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Ways to lower output Z:

1. Use a cathode follower as the last stage. A CF can, religious protestations aside, be designed to give extremely low distortion and high bandwidth.

2. Use a different tube. Something in the 6DJ8 family, for example, run at 10 ma will have a much lower output Z than anything made from an 12AT/AX7. There are a lot of other choices out there with lower rp and higher current than the tubes you're proposing.

3. Use voltage feedback from the last stage. This is the trickiest to implement, but some intelligent choices and possibly mixing this method with the first two methods will get you superb results.
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Old 5th April 2003, 08:44 PM   #4
Jay is offline Jay  Indonesia
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Thanks for the replies…

I have read the cad journal, and I have now simple questions before proceeding with in depth theory and calculation…

Because adding a bypass capacitor in the cathode resistor seems to simplify the choice of the tube and the corresponding bias elements, is it common to use this approach in the pre-amp output? I mean I want to assume that the bypass capacitor is required/mandatory.

Before replacing the tube, do I have to base the calculation on the existing current or voltage gain? (I mean I will first calculate the required gain, which I assume is the gain in the original circuit?) What is the optimum or correct gain, anyway, considering the amplifier input gain stage?

And SY, what is the best candidate for the tube in the last stage? (I want a high-end tube). But I think it will be easy, may be a must, that it is also a dual 9 pin triode.

Thanks again...
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Old 5th April 2003, 09:07 PM   #5
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Default Having A Bypass.

Hi,

Quote:
I mean I want to assume that the bypass capacitor is required/mandatory.
No, it is not mandatory.
A bypass cap shunts the AC signal appearing at the cathode to ground thereby optimising the gain in that stage and lowering Zo in the process ( I simplify).

Ommitting the bypass cap under the same circumstances introduces some degenerative feedback for that stage, hence reducing the gain and upping the output Z of that stage.

Either way, you'd better do the maths with the bypass present or not and than design what you prefer, not the other way around.

One question I have for you: is this a line preamp?
If so must you really use the highish gain 12AT7 series of tubes?

Last remark:

Paralelling sections of dual triodes may be beneficial in some applications but if you can avoid it in a preamp, please do.

Cheers,
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Old 5th April 2003, 09:22 PM   #6
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My $0.02

A 6DJ8 (6922) as line stage connected as a comon cathode(unbaypassed cathode) it will drive an Aleph front end (around 10K) but marginaly with a 10 mA bias, you will notice a loose bass and not to good highs.

I used this setup for about a year, not bad.

As you are constructing this preamp you are better off selecting an even more powerful tube and check if you can do without a gain stage, selecting instead a CF as suggested in previous postings.

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Old 5th April 2003, 10:33 PM   #7
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So...why is everyone averse to a cathode follower? This is only one of the things it was born to do, after all...

Tim
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Old 6th April 2003, 07:57 AM   #8
EC8010 is offline EC8010  United Kingdom
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Default There's nothing wrong with cathode followers.

A 6C45 at 10-15mA makes an even better cathode follower than a 6DJ8, but you will need a large coupling capacitor (4.7uF) to couple to the power amplifier.
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Old 6th April 2003, 09:31 AM   #9
dhaen is offline dhaen  Europe
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Default Causing trouble.....

Quote:
Originally posted by Sch3mat1c
So...why is everyone averse to a cathode follower? This is only one of the things it was born to do, after all...

Tim
Tim,

They're more fashionable now on the forum than they were.
Last year I was actually cautioned by 2 moderators for suggesting that they were usefull! After all, one must not propagate incorrect information

Cheers,
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Old 6th April 2003, 03:54 PM   #10
Joel is offline Joel  United States
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Default Re: Causing trouble.....

Quote:
Originally posted by dhaen
...They're more fashionable now on the forum than they were.
Last year I was actually cautioned by 2 moderators for suggesting that they were usefull!
This is true!
I think there have been some solid arguments for them made since then, and some others have experimentally seen for themselves that the anti-CF league is not based on facts.

Joel
(who always thought a CF doesn't sound like anything)
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