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Old 23rd June 2003, 04:28 AM   #1
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Question output impedence of CF? and a 6L6 question

Hello all...

I just had a thought that made me wonder... Ok, in a cathode follower config, the Zo is reduced by a factor of about 5.1... correct? And in order to drive lower impedences well enough, we need a Zo quite a bit lower than the Z of the loudspeaker. I am needing Z=300 ohm, so Zo of amp should be lower... probably lower than 150, right? And, a lot of tubes that would be nice to use in an amp like this have relatively high output impedences. So, if one paralleled two tubes on output, would this effectively cut the impedence in half much like paralleling loudspeakers?

Ok, the 6L6 part:

I was just looking at the specs for 6L6 to use as an example for the above when I noticed the Rk of the tube. It is low... Rk of a single tube is 300-ohm... if reduced by the factor someone mentioned to me before for cathode followers, then Zo would equal about 58.8-ohms... right? Of course plate resistance is very high... but does that affect anything? And, if Zo is not low like I thought it would be, would paralleling two 6L6s on output lower the Zo to where it might work well for 300-ohm headphones? Thanks to all-- if you can straighten out my jumble.
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Old 23rd June 2003, 05:40 AM   #2
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CF theory applies to any tube. First of all, Rk has nothing to do with Zo, it sets bias. Generally it is bypassed with a cap so it is effectively out of the AC circuit.

I'll refer you to this thread for info.

Humm.. you look familiar...

Tim
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Old 23rd June 2003, 06:10 AM   #3
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hehe yes...

I still have much to learn about tubes and CFs I guess.

Is Ra related to Zo? What is? How can I determine it? Thanks...

edit: You mention Rp. Where do I find this?
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Old 23rd June 2003, 06:22 AM   #4
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Yes, Rp (=Ra) is reduced by the NFB which CF mode causes. It also reduces distortion, which is what makes CFs so nice.

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Old 23rd June 2003, 06:32 AM   #5
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Yes... I just realized that Ra=Rp... forgot plate=anode. hehe

So, generally, they reduce by the factor you mention in that thread (5.1)? Well... that makes 6L6 quite unuseable... doesn't it?
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Old 23rd June 2003, 08:18 AM   #6
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No. That's only for that tube. You have to run the numbers on 6L6 to get the correct figure.

In the end, even if it isn't low enough, you can add as much global NFB as it's stable with. Probably can go down to 10 ohms.. not like it's much more useful than 100 ohms though.

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Old 23rd June 2003, 03:09 PM   #7
Joel is offline Joel  United States
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a) with headphones, which often have sensitivities as high as 100db/1mW, or higher (Sony), you don't need to have the Zo of the amplifier all that close to the headphones' imedance. An example would be a headphone amp I threw together with half a 6F8 per channel as output devices. Drove my 63 ohm Sony's with aplomb.

b) a 6L6 would make a very good cathode follower actually, because it has high current, low Rp, and high transconductance - when wired as a triode. A 6V6 has similar CF performance. The problem with implementing one is that the high current requires a choke load (or CCS), and then your size, and costs go up.
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Old 23rd June 2003, 08:50 PM   #8
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Note, of course, that a 6L6 is just a scaled-up version of any smaller CF, so it doesn't need a CCS or choke any more than the others. It's just cheaper on heat.

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Old 23rd June 2003, 09:01 PM   #9
Joel is offline Joel  United States
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Thank you Tim, master of the obvious.
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Old 23rd June 2003, 11:50 PM   #10
316a is offline 316a  England
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Default 6L6

Quote:
Originally posted by Joel
a) a 6L6 would make a very good cathode follower actually, because it has high current, low Rp, and high transconductance - when wired as a triode. A 6V6 has similar CF performance. The problem with implementing one is that the high current requires a choke load (or CCS), and then your size, and costs go up.
how about a 6L6 strapped as a triode sitting on a 6L6 pentode current sink ? That should make a decent CF but to obtain low Zout , you'll need a higher transconductance valve . E55L is a 10w pentode , strapped as a triode it has gm of about 50mA/V or there's the 6S45 . Both of these should have enough current to drive headphones

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