Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: eleventeen on August 27, 2010, 07:33:15 pm
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To achieve a nominal B+ voltage from a too-low-volts power transformer with no CT, a voltage doubler is indicated.
On a salvaged power transformer from a full-wave set (like an old TV power transformer) it's pretty much a given that you'll not find a bias tap, hence you'd normally take a leg of the HV secondary, throw in a reverse diode, work out a sufficent dropping resistor, and make your negative voltage.
But you cannot do both!
You can't grab one of the HV output legs of a non-center-tap power transformer and make a bias supply using a reverse-biased (eg; "backwards") diode IF your B+ circuit is a voltage doubler.
In other words (or symbols, actually) THIS WILL NOT WORK!
(http://i172.photobucket.com/albums/w32/ttm4/v_dub_no_go.jpg)
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Can't you just pull it of the leg before the doubler? Or am I mistaken! Someone else weigh in please!
Jim
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Any of the Bogen CHB amps did it.
Dave
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Dave, you're absolutely right. I knew it had been done somewhere. Must be that .22 cap separating the voltage double from the bias section.
http://schematicheaven.com/hifiamps/bogen_chb100.pdf
^^^^^^^^^^^
Update: It worked!
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This looks like a job for cathode biasing!
Dave