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Old 18th September 2012, 02:34 PM   #1
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Default Marantz-9

Would any can tell what is the funtion of "C38" near the V8(EL34) ?
Thanks a lot !
The schematic is attached !
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Old 18th September 2012, 03:07 PM   #2
DF96 is offline DF96  England
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I don't know. There are several other unusual things in that circuit:
1. C8 appears to feed PSU noise into the phase splitter - hum cancellation?
2. C20 etc. network only in one phase - which means the forward path is not minimum phase?
3. Anti-parasitic (?) inductors in only two EL34 anode circuits.

Also no safety resistors in case a bias pot wiper leaves the track.
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Old 18th September 2012, 04:50 PM   #3
kevinkr is offline kevinkr  United States
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In normal operation (not setting the bias) all of the cathodes are tied together and that capacitor appears in parallel with the 3.3 ohm cathode resistor eliminating cathode feedback in the ultrasonic region. Probably something to do the stability margin or ?

IIRC C7/C8 are part of a ripple cancelling network..

The bias pots used in these amps were locking milspec hermetically sealed units, I've never seen a bad one in 30 some odd years. (Mostly Ohmite) I use them or the Canadian made equivalent in my own amps and have never had one fail, I usually recommend safety resistors in my published designs, but almost never use them in my own gear.. Perhaps I'm playing Russian Roulette.. Seriously their inclusion is a very good idea.

I've refurbished a few of these amplifiers in the past, and will say they were amongst the best sounding PP UL amps I have ever heard.
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Old 18th September 2012, 04:54 PM   #4
SY is offline SY  United States
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Part of the confusion is that there's stuff not obvious on the schematic. Chief among these is layout and wiring (these were point to point and some wires were unavoidably long). This was all made more critical by the high feedback and high transconductance tubes.

Caps and inductors were added to stabilize the circuit as actually laid out. PPP with high gm output tubes can be a bear as a practical matter for production amps with expected variations.

edit: cross posted with Kevin and fully agree- these are great amps, I've used them and repaired them in the past and was in "awe and admiration" mode.
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Old 18th September 2012, 05:05 PM   #5
kevinkr is offline kevinkr  United States
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In keeping with SY's comments I would say they are quite special, sound quite special, and the build quality was superlative, certainly the best of the best made here in America. I coveted a pair for quite a long time, but good vintage pairs in running condition command more than $10K here in the US - I believe the Japanese are generally willing to spend considerably more for a mint pair..
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Old 18th September 2012, 05:49 PM   #6
DF96 is offline DF96  England
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Quote:
Originally Posted by kevinkr
In normal operation (not setting the bias) all of the cathodes are tied together and that capacitor appears in parallel with the 3.3 ohm cathode resistor eliminating cathode feedback in the ultrasonic region. Probably something to do the stability margin or ?
OK, I didn't try to trace the switch wiring. Bypassing 3.3 seems a waste of time until it is realised that it is in series with 24/4=6 - still only a small proportion, though.
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Old 18th September 2012, 06:54 PM   #7
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Lots of interesting details in this schematic. Is ST9 an invert switch? If so, does V1A work as cf? There are two more cf s to drive the output tubes, but this one is not inside the feedback loop.
And what is this business whith Ra and Rb? Do they add positive feedback to compensate for wire resistances? Similar to IxR compensation in motor drives.
Slightly off topic, but it seems funny to me that native speakers will call this circuit ppp. If you say " parallel push pull" in Germany people will think you are referring to a circlotron.
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Old 18th September 2012, 10:46 PM   #8
SY is offline SY  United States
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Ra and Rb allow the source impedance to be raised, a fashionable thing to do in that era. The input tube is run as a quasi-cathodyne to select polarity. Notice the early use of 6DJ8/ECC88!
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Old 18th September 2012, 11:16 PM   #9
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Quote:
Originally Posted by thomasfw View Post
Would any can tell what is the funtion of "C38" near the V8(EL34) ?
Thanks a lot !
The schematic is attached !
I can't see the schematic very well, but if the capacitor in question is the cathode bypass cap on the last pair it is to make the lower tube have a higher output signal due to the nonlinearity in the previous stages. (the non inverted signal (cathode) is less than the inverted signal(plate))
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Old 18th September 2012, 11:41 PM   #10
SY is offline SY  United States
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Click the little cross-shaped icon in the lower left of the pic and it will expand enough for the schematic to more easily be read.
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