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Old 9th October 2003, 04:54 PM   #1
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Default Would this work?

Option 1:

Code:
                           o B+
                           |
                           |
                           |
                           3||E----o
                           3||E
                           3||E
                           3||E
                           3||E
                           3||E----o
                           |
                           |
                         _____
                        /     \
              o---------| 2A3 |
                |       \_____/
                |          |
                |          |____
                \          \   |
                /          /   |
                \ R     R  \   = C
                /          /   |
                |          |   |
                |__________|___|
                           |
                           |
                           O CCS
                           O
                           |
                           |
                           V
Please forgive the ASCII art, I don't have a good drawing program here. Anyway, that's an idea for using a CCS in a 2A3 (or any other) output stage. I left the cathode bias resistor in there because I don't have any other way of making a 45V bias supply. The grid leak should probably be referenced to the bottom of the cathode bias resistor, otherwise the bias will include the drop across the CCS, which we probably don't want, right (unless we calculate for it). I'm not sure if the bypass cap should bypass only the cathode resistor, or should go straight to ground - do we want the AC going through the CCS? Probably not.

Anyway... those IXYS chips should be able to handle 60mA easily, so is there any reason why this is a bad idea? Has anyone tried anything like this?
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Old 9th October 2003, 04:57 PM   #2
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Option 2 would be to move the CCS up above the OPT, i.e. between the OPT and the B+ point. The PS cap is still connected to the B+ point (i.e. before the CCS). Putting the cap after the CCS makes the CCS part of the power supply, and the cap will feed the 2A3 so it won't really be a CCS any more (but it'll be a nicely regulated PS).

Code:
                           o B+
                           |______________
                           |              |
                           O CCS          |
                           O              = PSU C
                           |              |
                           3||E----o      V
                           3||E
                           3||E
                           3||E
                           3||E
                           3||E----o
                           |
                           |
                         _____
                        /     \
              ----------| 2A3 |
                |       \_____/
                |          |
                |          |____
                \          \   |
                /          /   |
                \ R     R  \   = C
                /          /   |
                |          |   |
                |__________|___|
                           |
                           |
                           |
                           V
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Old 9th October 2003, 06:41 PM   #3
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Default Re: Would this work?

Quote:
Originally posted by Saurav
Option 1:
so is there any reason why this is a bad idea? Has anyone tried anything like this?
For one, the CCS will try to maintain constant cathode current. This means that cathode voltage will exactly follow grid voltage changes. (It won't allow any change in the G-K bias voltage, because that would cause a change in current based on the Gm of the tube.) And also, since Ip = Ik, and delta I = 0, you will get exactly zero power into your load resistance.
And finally, the tube's plate resistance is multiplied by the CCS, making it a very good CCS for the OPT. But that of course is useless...

In the second case you have the same effect, though not as significant because the tube isn't acting as an error amplifier. I'm sure the CCS is still quite good though, so you'll have the same net result.

From your circuits, I don't see what your goal was. If you did bypass the CCS with a cap, you would attain constant bias no matter what tube is in place. (You can omit the bias resistor because the CCS pulls the current to - 60mA you said? - it adjusts its terminal voltage until whatever around it puts 60mA through.)
If stable bias is the case, then it will be mu+1 (I think) times more effective in the cathode than the plate circuit, it will provide the necessary bias voltage, and will not waste plate voltage.

If your goal was to reduce distortion, well distortion is a fact of life and a side effect of producing power. Only way around it is to cancel it, as with NFB or a PP stage.

Tim
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Old 9th October 2003, 06:51 PM   #4
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The second option would work if there was a way for the varying anode current back to the psu. If you remove the transformer (replace by short) and take the signal off the anode (via a cap if necessary) with a load resistor to gnd or B+, that would work quite well.

Jan Didden
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Old 9th October 2003, 06:55 PM   #5
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Quote:
you will get exactly zero power into your load resistance
OK, that would be slightly counter-productive

So does no one use CCS's under cathodes? And how does a CCS work as a plate load for an input/driver tube? Even there it would try to hold a constant current through the tube.... OK, so the current changes due to the signal show up across the load (i.e. grid leak of the following stage), so it works because the CCS is in parallel with the load.

If that is correct, then I need to find a way to put the CCS in parallel with the OPT. Which would probably end up looking like a parafeed circuit, with the CCS replacing the choke, and the OPT connected through a cap?
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Old 9th October 2003, 06:58 PM   #6
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Quote:
From your circuits, I don't see what your goal was.
I'm not sure, actually. It was just an idea that popped into my head, so I thought I'd throw it out for comments. That, and a vague idea of trying to take the advantages that a CCS brings to an input/driver stage and apply them to an output stage. And my amp has a single ended 2A3, so that's what I was thinking about.

Quote:
If you remove the transformer (replace by short) and take the signal off the anode (via a cap if necessary) with a load resistor to gnd or B+, that would work quite well.
But then I won't be able to drive an 8 ohm speaker, will I? So it comes back to putting the transformer in parallel with the CCS, and connected through a cap like you said.

Or am I still missing something?
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Old 9th October 2003, 07:20 PM   #7
dhaen is offline dhaen  Europe
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Quote:
Originally posted by Saurav


OK, that would be slightly counter-productive

So does no one use CCS's under cathodes? ..snip...
7N7 used one in his "Instock" design on this thread":

More 6158/13D3

It's on a Long Tailed Pair, so maybe it doesn't count.

Cheers,
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Old 9th October 2003, 11:05 PM   #8
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Yeah, LTP is a balanced system. You can draw actual power from that because one side's current can go up, the other's just has to go down to compensate. The purpose of a CCS here is just that, to balance it.

You can use a CCS in a PP class A amp.

Saurav: yes you'd have to go parafeed. The disadvantage is, you have to run approx. 1/2 B+ across the CCS, so that the ideal maximum voltage range will be 0V to B+, peak to peak. With an inductor, its flyback effect allows peaks from 0V to 2B+, or even more if the duty cycle is odd (say, 0V for 90% of the time would balance with like 20B+ on the flyback peak! ). So you only lose something by not going through the transformer.

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Old 9th October 2003, 11:14 PM   #9
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Quote:
The disadvantage is, you have to run approx. 1/2 B+ across the CCS
Yup, I realized that after I typed out that post. So that's roughly equal dissipation in the tube and in the CCS, i.e. ~ 15W for a 2A3. Which is quite a bit of heat to dissipate in an IC.

OK, here's another idea. Assuming you had the B+ to spare, you could replace the parafeed choke with a CCS + R. The R would be in series with the CCS so wouldn't hurt the high impedance, and would reduce the amount of power dissipated in the CCS.

This is starting to look like it's more trouble than it's worth though, like a brute force way of adding a CCS to the circuit just for the heck of it. Oh well, it was fun to think about
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Old 9th October 2003, 11:39 PM   #10
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Why don't you just use the active load on the plate and use a coupling cap to connect it to an output transformer? In other words just Parafeed it. You could get away with using a PP output transformer since there would be no DC in the primary. Just forget about the center tap a wire it up.

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