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Old 15th January 2007, 05:22 AM   #1
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Default Why don't we use output tubes as cathode followers?

That is, instead of plate loading an output tube with a transformer, it seems that you could load the plate with a choke or CCS and AC couple a transformer with a much lower turns ratio to the cathode of the output tube. So, what's the drawback?

Edit, I misspoke. No need, it would seem, for the CCS. Just connect the PS directly to the plate.
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Old 15th January 2007, 06:01 AM   #2
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It can be done but is not easy to achieve. The place where it's most often seen is in OTL amps. The cathode follower having no gain (just less than 1) means that the driver has to provide all of the voltage swing needed across the load, even though the cathode follower OP tube has no Miller capacitance and so is easy to drive in that regard. This is a tall order for most driver tubes, very few of which can achieve adequate linearity over such a large swing. The driver also needs a very high B+ to be able to swing that much, unless you use bootstrapping as a work-around.
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Old 15th January 2007, 06:19 AM   #3
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Default Re: Why don't we use output tubes as cathode followers?

Quote:
Originally posted by dsavitsk
That is, instead of plate loading an output tube with a transformer, it seems that you could load the plate with a choke or CCS and AC couple a transformer with a much lower turns ratio to the cathode of the output tube.
Actually, you can't do that. Let's suppose that, in that example of yours, that you discover that the best loadline for that VT is 2.5K. It makes no difference whether you connect that load between the plate and the DC rail, or whether you connect it between the cathode and DC ground. Either way, the VT will perform identically, so far as Q-Point current and Vpk is concerned.

The load impedance and the internal impedance are two different things. Sure, the cathode follower PA will have a lower Zo, and better speaker damping, but the Rl remains the same for the common plate and common cathode topologies.

Quote:

So, what's the drawback?
You lose all your voltage gain. According to the specs, PP 807s can output 26.5W into 6.6K P-2-P. That means that you have 591.4Vp-p, or 295.7Vp/807. However, you're inputting only 22.5Vp at the grid, for an Av= 13.14. Now, developing 22.5Vp is NBD. However, a cathode follower has less than unity gain, but will still require 295.7Vp across the load, which must come from the driver. While the distortion is greatly reduced by a cathode follower, developing nearly 300Vp in the front end, and doing so without adding more distortion than the cathode follower takes away is not so easily done. In fact, you'd probably be better off using the more common common-cathode connection, and reducing the gain requirement in the front end, and still wind up with less overall distortion, especially if you include local and/or global feedback. Bothy local and global feedbcak help improve the Zo situation and improve speaker damping. The former by reducing the effective r(p) connected across the OPT primary. The latter by reducing the amp's Zo at the speaker connection.
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Old 15th January 2007, 06:46 PM   #4
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Cathode followers can be good on a circumstancial basis.

As stated it takes alot of drive voltage for a cathode follower output. Cathode followers can produce good results however. The ones I've built can control 2 or 3 way speakers much better than there grounded cathode equivalents, also the bass is tighter.

It's a much more natrual form of negative feedback as opposed to loops. By the time you add enough feedback to a grounded cathode topology to get a output impedance equivalent to the cathode follower, you may find your overall gain isn't much more than a cathode follower topology.

As mentioned the thing about the cathode follower topology is distortion caused by the high swinging voltage stages. Using a CCS for the voltage gain stages can produce a clean high voltage signal. Active loaded stages (CCS on plate) do not clip as softly as passive loaded stages (resistor/transformer on plate). so make sure the passive stages start to compress the transients before the CCS stage gets maxed out, for more clean sounding SPL.

Choosing a tube: most say that high mu tubes are best suited for cathode follower outputs. This statement for the most part is true, because a CF gain is roughly mu/(mu + 1). However, some of the lower Rp tubes with low mu can be a good candidate as well. For instance a 6c33c's gain as a cathode follower is roughly .7 which isn't good. But when you consider it can easily drive a 400-600 ohm load, it doesn't take alot of voltage to make good power when compared to tubes that require say a 3k load.
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Old 15th January 2007, 07:40 PM   #5
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Cathode follower tube output stages have been done extensively, but mostly in the P-P configuration.

If you check the Radiotron Designer's Handbook there are multiple references shown, which you can research and look up. Practically every possible variation of cathode loading and combined cathode/plate loading has been conceived of and tried. The most famous of these is the McIntosh "unity coupled" output stage.

SE cathode stages are not very common.
The benefit of better winding ratio in the OT should not be ignored, imho, as it makes it easier to wind a tranny with outstanding specs.

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Old 15th January 2007, 07:52 PM   #6
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I experimented with similar topologies before coming to transistor outputs.
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Old 15th January 2007, 07:57 PM   #7
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-- got OTL sitting and never heard - lack of winter in my area makes too warm to turn on - plus basic idea doesn't appeal vs good transformer and less tube count

have some 1K p-p xformers-maybe they'd work with C-F output stage (?)

"purple cow" and a flurry of C-F output amp schemes appeared in the 1950's or so
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Old 15th January 2007, 08:14 PM   #8
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Hi,

Cy Brenneman makes or used to make a S.E. C.F. amp utilizing EL34's, I think it was named the Cavalier.
I had (have somewhere in the filing cabinet) a schematic, S.E. using the 6V6 as a cathode follower output. I remember wanting to try the circuit but it needed a opt with a 250 ohm primary which I did not have.


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Old 15th January 2007, 08:37 PM   #9
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There is a headphone amp that uses these http://www.hammondmfg.com/collins.htm in essentially this configuration but with 12b4's. These transformers are only about $40 for the pair, and are said to be of quite high quality.

But, since the transformers can carry 12W it seemed like a reasonable and inexpensive way to try to drive speakers too. The output coupling cap is still pretty large, but it is low voltage so a reasonably priced blackgate or other decent electrolytic could probably be used.

Maybe I'll add this to my list of things to try when I get some free time, which should be in about 2009.
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Old 15th January 2007, 11:04 PM   #10
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[IMG]6v6 c.f.[/IMG]

here is the schematic I referrred to earlier. OPT primary should be ~ 250R... similar to a cathode resistor value. D'ont take for granted that the pin numbers I added are correct!

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