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Old 14th March 2007, 10:12 PM   #1
JoshK is offline JoshK  Canada
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Default Q on PP & current mirrors

I was reading through some back issues on tubecad and came across JB's articles on using SS in tube circuits to do "house cleaning" duties. The idea that really struck me was using current mirrors in the cathodes of PP pairs.

In fact, this seems like such a great idea (to my newby thinking) that I wonder why it has not been used/talked about/adopted more widely. Can anyone see why this might *not* be a good idea? It seems to avoid the issue with using a CCS in the tails of a PP pair in that this would be limited to class A only, whereas the current mirror is not.

For the driver/input circuit, CCS is fine and dandy and I am not questioning that. Class A for these stages thank you very much, but for the output stage, it seems like the current mirror would be the superior choice. It also seems like it would keep balance in the pair in case of non-matching, drifting, or other variance.

In my case, I am planning a PP 6c33c amp and I've read of the troubles in matching low rp/high gm tubes such as these. CCS in the tails is one way to force dynamic matching of current through the pair, but I am concerned with what happens when the amp is driven beyond strict class A and am wondering if a current mirror is a better choice (and setting bias current seperately of course).

I will still likely plan for class A, but I'd like to have some safety in how the amp performs when driven hard/to the limit/to clipping. Thoughts will be greatly appreciated.

Josh
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Old 14th March 2007, 11:32 PM   #2
Irakli is offline Irakli  United States
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Hi,

On paper (or in simulators) a lot of things look perfect. Real life is little bit different.
I think to make high current high quality current mirror is not an easy task.

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Old 14th March 2007, 11:44 PM   #3
JoshK is offline JoshK  Canada
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Quality I don't know about, no experience there. Current though I would have thought would have been easy, since transistors are such higher current devices than tubes.
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Old 18th March 2007, 04:06 AM   #4
JoshK is offline JoshK  Canada
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Anyone else have experience or supposition on the matter?
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Old 18th March 2007, 04:47 AM   #5
SY is offline SY  United States
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What's the looking-in impedance of a current mirror?
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Old 18th March 2007, 04:56 AM   #6
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Hey JoshK

I too have been looking at this in conjunction with servo 's for setting the bias and allsorts, and I'm still no nearer...

I think that you are right, S-State in support of tubes is a grand idea, you're looking at what 300Ma per side? there are ways of getting a gurrent mirror to deal with that (on paper anyway) cascoding is an obvious one.

BUT here's my question, and perhaps, I'm overthinking this, but I dont think for a P-P stage you need dynamic current matching, rather that the two stages are equal but opposite over a long enough time constant that the music is unaffected? Is that right? (not an expert on P-P output stages, (or any others) complete noob really)

Depending on your bias arrangements then perhaps an easier method of achieving what you need is to build a common reference for the two tubes and use that "somehow" to control the current per tube.

John Broskie's Tubecad site also has a description of a clipping servo for setting bias, which works for either class A or B. THeres also something in Morgan Jones' tome. Too complicated for my little brain to figure though Class A is *way* simpler.

cheers

Andy
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Old 18th March 2007, 02:50 PM   #7
JoshK is offline JoshK  Canada
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Quote:
Originally posted by SY
What's the looking-in impedance of a current mirror?
I take it that was a question to provoke me to answer my own question?

I don't know but I'll have to look into it. But if it were really high and that wasn't good for the cathodes, then you could put the mirror on the plates.

Thanks for the response guys. Food for thought.
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Old 18th March 2007, 04:05 PM   #8
SY is offline SY  United States
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Quote:
Originally posted by JoshK


I take it that was a question to provoke me to answer my own question?
Pass the hemlock, please.
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Old 25th March 2007, 09:31 PM   #9
JoshK is offline JoshK  Canada
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i went back and took another look at the circuit broskie posted. i forgot that the current mirror was bypassed on each leg with caps for the ac signal path.
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Old 26th March 2007, 02:14 AM   #10
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Josh,

What issue of tubecad was JB's current mirror article in?

Dave
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