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Old 16th January 2006, 03:31 PM   #1
mbonus is offline mbonus  Croatia
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Default Alembic F-2B

hi everyone!
I'm new to tube projects,but i recently
found schematic of this bass preamp
and i want to build it for myself.
Like i said,i don't have experience with
tubes so i will be grateful if someone could help
me with this project. I found only this scheme
so i don't know some details...
in power supply i don't see gratz,just one diode.
is it ok to have only half-wave rectifier?
which power of transformer and voltage on secondary do i need?
thx!
here is the scheme:
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Old 16th January 2006, 04:00 PM   #2
ilimzn is online now ilimzn  Croatia
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It is a voltage doubler, not a half-wave rectifier, note capacitor in series with transformer winding. My guess at the winding voltage would be about 100VAC
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Old 3rd March 2006, 04:03 PM   #3
Moondog is offline Moondog  Netherlands
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I am also starting to build a pre-amp, based on this F-2B schematic. Alembic is using the Stancor PS-8415 transformer which has 125V secundaries.

Please note the schematic you posted has faults! Pins 6 and 8 of the tube are mixed up!!
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Old 3rd March 2006, 04:55 PM   #4
kevinkr is offline kevinkr  United States
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Definitely a half wave voltage doubler, and I concur that the transformer secondary ought to be in the 100V - 120V range.

The pins on the first section of the 12AX7 are reversed, plate =6, grid=7, cathode=8 on a 12AX7A.

It is important to keep that power transformer away from the 12AX7.

Build the thing in a nice metal box, preferably aluminum as it is much easier to work than steel, and using a socket with shield for the 12AX7.

For this application I would recommend the Sovtek 12AX7LPS, it's quiet and the spiral filament construction cancels the hum inducing ac magnetic field around the filament.
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