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Old 11th December 2002, 02:10 AM   #1
Fossil is offline Fossil  Singapore
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Default Help needed for Marantz 7 clone

I've got a kitset based on the M7 line stage. The power supply for the HV uses 4x RC filters, providing 280V and 265V.
Recently, I bought a 12AU7 (orig 3x 12AX7) to replace the last tube, hoping to tweak the sound.
When I measured the 2 supply points, I have 255V and 235V instead. How is it so? Does it mean that the 12AU7 cannot substitute the 12AX7 without affecting the circuit (bias etc)?
Can someone help to explain how the supply voltage is being pulled down?
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Old 11th December 2002, 09:57 AM   #2
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Hi
The voltage drop is to be expected. There is a voltage drop over each resistor in the RC power supply filters. The voltage drop is dependent of the average current drawn from the supply. If the current is increased, the voltage drop is increased as well and can be computed according to ohms law.
The 12AU7 draws more current than the 12AX7, and therefore the supply voltage drops down.

Torben
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Old 11th December 2002, 10:54 AM   #3
Fossil is offline Fossil  Singapore
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Thx Herrmann!

So what shd I be doing next?
Shd I be limiting this DC current when using the 12AU7 or
shd I adjust the supply voltage to maintain 280/265V for the circuit (probably by bypassing 1 of the filter resistors)?
Or juz leave it alone?
any advice would be much appreciated as I know nuts abt design of tube amps Which is why I bought a kitset!
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Old 11th December 2002, 12:50 PM   #4
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Hi
I would not worry too much about the voltage drop.
In 1981 or so the US magazine "The audio Amateur" published an article describing how to improve the Marantz 7c. Their version of the changed preamp was named Pooge-1. One of the changes was to make a regulated power supply. This supply operated at 240 volts, feeding all tubes. 240 volts is quite close to your voltages.
I would probably just leave it as it is or keep the 12AX7 depending on subjective impressions.
If you want the 12AU7 you might want to make sure that the resistors in the RC power supply network are able to dissipate the extra heat generated due to the increased current.

The Pooge-1 version of the Marantz 7c can be found here:

http://www.geocities.com/mattijs1970...p/maranlin.gif

Torben
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Old 12th December 2002, 01:07 AM   #5
Fossil is offline Fossil  Singapore
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Thx again, Thorben

So grateful that u reminded me abt the resistors' ratings as I forgot all about it.
I've removed the 12AU7 juz in case the ratings r too low.
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Old 15th December 2002, 03:57 PM   #6
Fossil is offline Fossil  Singapore
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Default Update

Juz an update...
Found a Marantz Consolette schematic which uses the 12AU7 at the output. Also downloaded the 12AX7 and 12AU7 data sheets for reference.
After comparing the 2 designs, I kinda understand how to 'configure' the tubes.
My next move will be to build offboard resistor network which can be plugged into the main circuit when I switch from 12AX7 to 12AU7.

If that works OK, I'll listen to the 12AU7 for abt 6 mths before building a constant current source with FETs juz for the fun of it.
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