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Old 19th June 2017, 10:29 AM   #51
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I would love to see your experimental results showing there is no benefit having the CCS on the output, since Pimm put it there on purpose.
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Old 19th June 2017, 10:58 AM   #52
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That's fair, and your point on closely matched output tubes is well taken.
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Old 19th June 2017, 11:45 AM   #53
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Default Differential input/Differential output stages

My knowledge on this is limited, but I would like to put the following to discussion; A tail CCS is supposed to ensure AC balance even with severely unmatched tubes. This at the cost of DC imbalance even with -real world's- matched tubes. At the input stage what matters mostly is AC balance. DC imbalance can be faced with coupling capacitors. At the output stage first concideration is DC balance otherwise the OPT will be damaged. AC imbalance will be "fixed" with the differential to SE conversion at the OPT. A common CCS is very unlikely to fulfill this requirements. Separate CCS for each tube should allow DC cancelation but most probably we are back to AC imbalance. Typical fixed bias should also do that fine. One thing that I find interesting is that a tail CCS -AC balance- theoriticaly converts the stage to a constant load as seen from the psu meaning less interaction. One CCS common for both cathodes should do that. Can separate CCS do it? And how about simple fixed bias compined with an OPT of high enough(?) inductance?

Last edited by MagicBus; 19th June 2017 at 11:47 AM.
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Old 19th June 2017, 01:19 PM   #54
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Originally Posted by sser2 View Post
They were ignorant about that at WE.
They didn't have high performance CCS's back then - you can bet your bottom dollar that if they had they would have used them.

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Isn't the CCS, a key part of the Tabor's design?
Without it I can guarantee that Gary Pimm would never have designed this circuit. The design philosphy is using the best elements to allow the tube's to do what they do best. I also doubt that the absence of a CCS would be as insignificant as some might suggest since the output stage is the main contributor to the overall distortion profile compared to just about any kind of input stage.

With regard to separate output stage CCS's, if you bypass them with caps between the OT cathodes you get both DC balance and enforced AC balance. It really represents the best of both worlds. A single CCS approach works equally as well but requires constant attention to bias balancing through some other mechanism such as independent screen adjustment.
The penalty, from my experience, with separate CCS's is that once they fall sufficiently out of balance the circuit will tend to enter oscillation and this is likely well before the tubes have reached a full service life.

Shoog

Last edited by Shoog; 19th June 2017 at 01:36 PM.
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Old 19th June 2017, 01:35 PM   #55
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An alternative to using two separate CCS's for mis-matched tubes:

Screenshot - 6_19_2017 , 3_34_49 PM.png
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Old 19th June 2017, 01:38 PM   #56
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Originally Posted by jazbo8 View Post
An alternative to using two separate CCS's for mis-matched tubes:

Attachment 621901
I agree thats a good approach - but is it superior to screen adjustment as in the original design ?

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Old 19th June 2017, 02:11 PM   #57
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Probably not much, just an alternative to try.
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Old 19th June 2017, 02:14 PM   #58
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Originally Posted by jazbo8 View Post
Probably not much, just an alternative to try.
Screen adjustment should be cheaper since you don't need a high power wire wound pot like you would in the cathode's.

Shoog
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Old 19th June 2017, 02:18 PM   #59
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Yes, the darn WW pots are kinda expensive, but it does use less parts than two CCS's with the associated circuitry.
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Old 19th June 2017, 02:22 PM   #60
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Originally Posted by jazbo8 View Post
Yes, the darn WW pots are kinda expensive, but it does use less parts than two CCS's with the associated circuitry.
I would go with two simple TL783's or two of the IXY chips, I bet they would work out cheaper than the WW pot.

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