Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on February 12, 2011, 12:22:46 pm
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Please I would like to understand how acts in detail the Gulbrasen Expression Control
Jojokeo told me
An expression control is foot pedal operated - a variable voltage divider which can control volume, tone, and effects in general terms but in this organ's use it was for volume control.
but despite my efforts, I'm not able to understand it :rolleyes: :sad: :rolleyes:
also, in the schematic, there are two resistors labeled as "VSR" and without any indication about values, is that a special resistor ?
Many Thanks
Kagliostro
http://www.jimmyandsharonwilliams.com/gulbransen/schematics/APS5.gif
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> two resistors labeled as "VSR" and without any indication about values, is that a special resistor ?
They must be. Probably Varistors, voltage-variable resistances.
The 100K 2W and the Expression Pedal (fancy foot-operated switched variable resistance) form a voltage-divider from the +325V. Result is 175V to zero V. This is filtered with 4mFd (to reduce clicks), then applied through 100K to two VSRs. As VSR resistance drops, signal drops.
No, I don't think it is useful because varistors have changed a lot since those days. The old ones were "soft", resistance dropping slowly at higher voltage. Modern ones change resistance much more quickly. The audio signal would be distorted.
If you must fool with the plan, use photo-resistors and LEDs.
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Whow! 7000 posts. :shocked:
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Thanks PRR
Probably Varistors, voltage-variable resistances.
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As VSR resistance drops, signal drops.
now I can understand how that acts :smiley:
Kagliostro