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Old 19th September 2012, 07:44 AM   #11
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thanks SY, yep just enough capacitance to lower the cathode negative feedback from 100% (no bypass cap ) to about 80-90%
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Old 19th September 2012, 11:52 AM   #12
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Thanks SY, of course it says lower damping factor in the corner, not higher. R73 is confusing me though, it connects to the speaker side of Ra and it adds something to the NFB. So I thought it had to do with cancelling the effect of Ra.
I had never seen a bridging option on a tube amp before. Two model 9s, one switched to inverting, connect your 16 Ohm speaker to the 4 Ohm posts and off you got with 140 Watts.
Marantz added the CFs to drive the grids of the 2+2 output EL34s. As DF96 says, a CF can add low impedance and distortion to a signal. But the grids a driven through a 68k+1k. Ok, the 68k is shunted with 150p. A bit of treble boost? So why the CFs and the series resistors?
A shame no one showed me this schematic before!!
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Old 19th September 2012, 12:01 PM   #13
DF96 is offline DF96  England
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Quote:
Originally Posted by grometeer
As DF96 says, a CF can add low impedance and distortion to a signal.
Only when a CF is unnecessary. Where a CF is appropriate it can reduce distortion by acting as a genuine buffer.

The 150pF will give a little phase advance, to aid loop stability. Because of the overall NFB they won't boost treble in the output. The CF is probably needed to drive the output stage Miller capacitance when in triode mode. Probably not necessary for UL mode, but simpler to leave in place for that too.
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Old 19th September 2012, 12:49 PM   #14
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Hello DF96, I hope you donīt mind me quoting you less than precisely.
I was just wondering why they decided to use the relatively high 68k resistor to drive the grids. I cannot just be a grid stopper, the 150p makes that impossible. Then I thought it might be used to create a dominant pole, but with the different (EL34īs) input capacitance of UL and triode mode this is not likely, either.
Does the phase splitter work better in any way if it is loaded with pure resistance, compared to a complex RC load?
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Old 19th September 2012, 02:05 PM   #15
SY is offline SY  United States
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Those 68k resistors provide better overload recovery after clipping. They're bypassed to compensate for the output tubes' Miller capacitance. Very clever trick.
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Old 19th September 2012, 02:12 PM   #16
DF96 is offline DF96  England
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Yes, without the 68k the CF would be capable of storing quite a lot of charge in the coupling caps if the output hits grid current on peaks. A conventional (grounded cathode) driver can't do this so much because of the anode load resistors which are the only pull-up available. An interesting example of how a design decision (to use CF) has consequences which then have to be foreseen (or found in testing) and dealt with.
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Old 19th September 2012, 03:19 PM   #17
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Thanks all of you !
I have download the "Service Manual" but don't know how to upload !
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Old 19th September 2012, 04:04 PM   #18
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That makes it clear! Thanks.
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Old 5th September 2014, 02:30 PM   #19
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Smile Marantz M9

Two High voltage rectify diode is in wrong direction.
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Old 5th September 2014, 02:51 PM   #20
SY is offline SY  United States
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They look correct to me.

edit: Nope, you're right.
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