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Old 12th September 2012, 05:35 PM   #1
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Default Tell me why this wont work.

Hi all,
I am working up an idea for a direct coupled Plate to grid feedback amp using Toroidals as outputs. It will be a PP, amp which requires exact current matching between halves. i have always achieved this before by using cathode CCS and bypass caps - but I would like to attempt to do it without the caps this time. I have designed a conceptual building block using a CCS above the transformer winding and wonder why this has never been tried and what I am missing if it hasn't. I suspect the inductive kick at the top of the transformer is probably enough to ensure the CCS has a short and brutal life - but I could be wrong.

Any thoughts ??

Shoog
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Old 12th September 2012, 05:46 PM   #2
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A CCS works by adjusting the voltage across itself until the rest of the circuit provides exactly the right current. A cathode CCS only has to change by a few volts to achieve this. A CCS in the anode circuit of a pentode might want to change its voltage by several kV, but it can't so it will either drop almost no voltage (but current will still be too low) or almost all the supply voltage (so the valve is pushed into bottoming).

To get the best results from a CCS it needs to work against a low impedance, not a pentode anode!
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Old 12th September 2012, 05:54 PM   #3
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But the transformer is acting as a shock absorber taking up all the voltage variation developed across the pentode.

Alternatively assuming that the Pentode was replaced with a triode - then theoretically it would work ?? Gary Pimm suggests such an arrangement for a PP output stage, but he only uses one CCS for both sides. My idea would be to split the primary and use two CCS's to enforce current balance within the transformer.

Shoog
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Old 12th September 2012, 06:10 PM   #4
DF96 is offline DF96  England
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The transformer doesn't absorb DC.

A triode would work better (i.e. less badly) for DC, but there is still the problem that for signals the CCS is in series with the transformer so the CCS needs to be bypassed by a capacitor. I can't think of any reason to put the CCS in the anode circuit. I don't even like CCS in the cathode circuit of an output valve, as a CCS sets the wrong parameter: average current instead of quiescent current (but that is a separate issue).
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Old 12th September 2012, 06:14 PM   #5
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The better is CCS, the less current variations will be caused by tube, the less voltage you will see on your transformer. With ideal CCS you will see no output. With real CCS output will represent distortions of CCS.
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Old 12th September 2012, 06:15 PM   #6
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To get power out from those trannies the upper end should be connected to groud via large capacitor, because there must flow AC-current. Now there is a CCS instead which represents almost infinite impedance and no signal current can flow and no power can come out.
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Old 12th September 2012, 06:36 PM   #7
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I can see that placing a Cap to earth would make this work.
However I do not see that the transformer is incapable of developing an AC signal if it is shunt fed a constant current - again I return to the analogy of a spring with one fixed end and one free end.
Fundamentally I am more than willing to admit that this is no a runner - but the idea that the +B of a normal amplifier with a large cap is behaving any differently to a CCS, with respect to AC, just doesn't seem logical. There is no AC signal at the last cap of a valve power supply. The fundamental difference between the two is the relative impedance.

Shoog

Last edited by Shoog; 12th September 2012 at 06:39 PM.
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Old 12th September 2012, 06:42 PM   #8
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It should be fairly clear. A change in voltage at node 7 can't force any current to flow, therefore no AC magnetization. As drawn, AC voltage at node 1 tracks AC voltage at node 7 = no joy.
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Old 12th September 2012, 06:43 PM   #9
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Constant current source by definition provides constant current. That means, it's dynamic resistance according to Ohm's law is dV/dI, so if dI =0 (constant current, zero current change with changing voltage) that means infinitely high dynamic resistance. According to Ohm's law, voltage variations on transformer will be equal to dV = dI * Z, i.e. dV = 0 * Z. Voltage drop no transformer in series with infinite resistance will be zero.
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Old 12th September 2012, 06:49 PM   #10
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Quote:
Originally Posted by Shoog View Post
... again I return to the analogy of a spring with one fixed end and one free end.
Shoog
Using your analogy: with a CCS at the anode you have a spring with both ends free.
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