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scholl 6th January 2013 01:37 PM

10M45 as simple preamp...
Hi all!

My first post on this forum..please forgive the spelling. Lol

Anyway, had 10M45s for years and used them as CCSs in tube circuits with good results. Reading up a bit on FETs and MOSFETs I decided to try a simple circuit, running the 10M45 as a simple voltage amp.

K to ground is a 200ohm resistor
A to B+ is a 1000 ohm resistor
G to ground is a 100K ohm resistor

Output taken from K is a 4uf cap to a 30K resistor to ground.

Biasing at 17ma

Gain about 8

Hokked it up and put it in the system to find the humm a little to much so I added a 5H choke and another cap to the powersupply for very quite operation.

Sonicly it has very nice soundstage over 180 degrees on good material but pushed back a bit. Imaging is excellent as is tone. Very easy to listen to. Bass and treble seem lacking but everything is there it's just not in your face.

I've heard many worse preamps.

Anyone else build such a circuit, any better operating points?



scholl 12th January 2013 12:24 PM

Now Balanced
Added a simple direct coupled 10M45 concertina stage with 400ohm A and K resistors for a 40ma ballanced output. Had to change the A resistor in the first stage to 3Kohm. Now have nice unballanced to ballanced preamp good for 16V p-p output. Would be better to increase B+ to 100V.

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