Tube DIY Asylum

RE: Improved Marantz 9??

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This Post Has Been Edited by the Author

In Reply to: RE: Improved Marantz 9?? posted by Tube747 on March 21, 2017 at 10:56:17

Well, you certainly have an easy one-- almost
ANYTHING will easily outperform a Marantz 9-- my
all-time candidate for THE WORST tube amp of all time.
I had several. And all the "Mac" stuff as well-- even
the biggies-- the MI-200 and the 1000 mono.
Back to the Marantz 9 and the "8".
The 8 used two EL34, had a two-diode solid
state rectifier, used ordinary parts and capacitors.
Like the 9, it was finished nicely in Brown-painted
steel. The "8"-- and its later improved model
actually had a tiny window on music. You could almost
stand to listen to it, and it didn't blow-up tubes
like the 9 which used 4 EL34.
The Model 9 was EXTREMELY unstable. Almost anything
would send one or more EL34 into running a "red" plate.
Thermal runaway, of course, and parasitic oscillations
Galore!
The 9 also had ZERO bass by today's standards, and it
homogenized music into a mish-mash of signal collisions,
all vieing for your speaker's attention. Unstable?
Ridiculously so!
To make matters even worse, capacitor-input imparted a
sort of "in a Tin Can" sound to everything it tried to
play, which it could not.
The stock amp was "voiced" so that what it did reproduce
favored the midrange and upper midrange. No, it didn't have
any "highs" by today's standards.
Dr. Charles A. Halijak-- Dean of E.E. at Huntsville, Alabama
(Alabama State U.), used the Model 9 to teach students
WHAT NOT to do in designing an amplifier for music.
Various students devised "fixes" for the 9's-- shall we say--
less than exemplary performance. Basically, low-DCR chokes were
applied, utilizing choke-input, all negative feedback was
eliminated, plate currents were reduced, a Common-Mode cancelling
network was applied across the power tube's cathodes.
When all was said and done, you had a decent amp with 15 watts.
Of course, it had at least 10 times more power than the old
much higher watt rating because it was now a stable amplifier.
Those crappy old electrolytic capacitors also got replaced
by film or foil types, or by military-grade Oils such as G.E.
Pyranol, etc.
Better coupling caps also got installed and Gee Whiz! The
thing was starting to almost perform with a Stone-Stock
McIntosh MC-60-- which could be greatly improved also, by
applying the same methods.
-Dennis-

Edits: 03/21/17

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