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Old 7th March 2006, 09:58 AM   #11
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I seem to remember a PP design from somebody (?) in a recent thread where the output tubes had a CCS under them. When a capacitor was put between the cathodes and the centre tap of the OPT primary the sound was about equally good but louder. Can't remember the reference here.
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Old 7th March 2006, 11:23 AM   #12
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Quote:
Originally posted by Giaime
Imagine of the tube being a resistor, the value controlled by the grid. The CCS sets a current throught this chain (OPT+tube+CCS), but single values of "resistors" (aka. the voltages at the cathode or the anode) can change, keeping the sum constant. If you inject a positive signal at the grid, the current try to increase, the CCS keeps it constant but you have shifted down the voltage at the anode (because "CCS" seen as a resistor has it's value increased).

Or am I wrong?
If the cathode current is constant then the anode current is constant so the voltage drop across the anode load resistor is also constant. = no output signal. The ccs will appear as a resistor of almost infinite resistance from the point of view of *change* of voltage versus *change* of current. Hope this helps?
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Old 7th March 2006, 12:12 PM   #13
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That Ixys thing has a slope resistance of about 10k according to the datasheet. A LM317 wired as a constant current sink has a slope resistance of 250k. That's 25 times more constant. Here's someting I made a while back that has a slope resistance of at least 400 megohms.

http://www.diyaudio.com/forums/showt...7225#post67225
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Old 7th March 2006, 04:03 PM   #14
timpert is offline timpert  Netherlands
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@Dan and Giaime,

Circlotron is right about the constant current source, its infinite dynamic resistance effectively prevents any AC (and thus signal) current to flow in the current loop formed by supply, transformer, triode and cathode circuit. The SE power stage is typically plate loaded, this configuration has all components (including the load) mentioned above wired in series.

Quote:
I'm using a CCS-loaded cathode follower in my preamp, unbypassed. It works, so? You don't need to bypass the cathodes of SE output tubes if you use a CCS.
The cathode follower has the load in parallel with the CCS, so the infinite dynamic resistance of the CCS is shorted by the load, allowing for AC to flow through the loop. The cathode follower is a different topology from a "normal" SE output stage, so what works in one case doesn't necessarily work in the other.

Jurgen
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Old 7th March 2006, 04:27 PM   #15
sgerus is offline sgerus  United States
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Jurgan,

Thank You,

Now I understand why ccs WILL NOT work with SE output.
(At least when the output trans is in series with the triode)

Just for kicks, I have attached a pdf document on ccs. On page 10 they show a DHT with the output trans in parrallel with the triode, via a coupling cap.....

I don't know if that's still considered a SE amp, but it looks like that's a way to use ccs with a DHT.
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File Type: pdf active loads and signal current control.pdf (58.9 KB, 113 views)
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Old 7th March 2006, 04:42 PM   #16
timpert is offline timpert  Netherlands
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Hi,

The circuit on page 10 is called parallel feed, or parafeed (do a search on that last term, you'll find a lot of info). It is 100% SE, and because it doesn't have any DC flowing through the primary, considered by many to be superior to the traditional approach.

Jurgen
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Old 7th March 2006, 05:14 PM   #17
Giaime is offline Giaime  Italy
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Ah, so I was wrong. Well, I learned something.

Could you use cathode output then? SE output tube as cathode follower.

But I think that's better to develop a servo biasing system, that senses the current throught the tube and sets a negative grid voltage.
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Old 7th March 2006, 05:42 PM   #18
timpert is offline timpert  Netherlands
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Hi,

In essence, the classic cathode resistor circuit does quite a good job as a servo circuit! It tries to preserve the bias point as much as possible, providing an excellent compromise between constant-current operation and fixed bias voltage operation, which both can be a tad more problematic when the tube ages. It does so by means of a very easy to understand and calculate first-order control loop, which by its very nature is unconditionally stable. Its simplicity makes it virtually bulletproof to boot!

So I very much doubt that any active control circuit will be able to beat the cathode resistor. Okay, you could save some dissipation, but if you're worried about that, maybe SE class A isn't for you...

But yes, using a DHT as a cathode follower is a very real option. You'll have to cope with the very high grid voltage swing this topology requires to achieve full power, but once you've nailed that issue (it has been done), a very low output impedance (and thus high damping factor, many speakers like that) will be yours without having to resort to global NFB.

Jurgen
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Old 7th March 2006, 06:18 PM   #19
sgerus is offline sgerus  United States
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Jurgan,

If the ccs could replace the cathode resistor, and the cathode capicator
could be left in (in parallel with the ccs), that would allow the ac to flow, right.

The 10m45 ccs chip gets wired with just an input and an output.

Then maybe this could work after all?
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Old 7th March 2006, 08:57 PM   #20
timpert is offline timpert  Netherlands
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Hi,

That should work just fine. Indeed, the decoupling cap should be left in parallel with the CCS. For a 2A3, the CCS should be adjusted to about 60 mA. Good luck!

Jurgen
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