Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: llama on February 01, 2013, 12:04:38 pm
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I'm getting the mad scientist bug again, and found a variety of Variable Capacitors . Some smaller ones have a range from 7 to 350 pF.
Where in the circuit would a small shift in the capacitance be most useful?
Some of them have several plates and can measure over 5 inches in length.
My purpose to find a location where the swing will make a tonal difference and then use a similar value of a fixed cap to finish.
-LLama
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No personal experience, just what I've read: across the plates of the PI, across the plate resistor of the preamp tube (especially a small pentode like the EF86 or 5879), and on your tone and/or volume pots. Tubenit will have more ideas, I'm betting.
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I'm getting the mad scientist bug again, and found a variety of Variable Capacitors . Some smaller ones have a range from 7 to 350 pF.
Where in the circuit would a small shift in the capacitance be most useful?
Probably nowhere in an audio/guitar amp.
These things are for adjusting a circuit operating at frequencies above the audio range, typically into RF. To make an R-C circuit that allows the cap variation to impact audio, you'd need prohibitively-large circuit resistances.
You might find some value in using the largest of those (the 350pF one) as a variable plate resistor bypass when trying to figure a desired value of plate load bypass to reduce a stage's treble. You'd do that, then replace the big variable cap with a small ceramic or mica cap and button the amp up.
Problem is, you might lack a way to accurately measure the resulting capacitance that you liked. The measurement might vary with the placement of measuring leads and your hands/arms. And the variable cap is usually bigger than a simple cap-substitution box where you had a switch selecting different caps to apply different capacitances to a circuit for taste-testing.
Those things might be gold for a radio guy, or someone who has a similar device performing high-frequency compensation in an old oscilloscope, or (if you have some giant ones) as a frequency dial in some signal generators. I've got some audio signal generators that use a 3- or 4-section variable cap for the frequency dial, but that thing is about 4" square by 8-10" deep, and the circuit design allows for 10MΩ+ resistors for low-frequency ranges (which is generally not true in a guitar amp).
While you might find some use in a guitar amp, there are easier, cheaper, smaller ways to do anything these variable caps could do in the audio range.
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This is the only kind of audio gear that uses variable capacitors that I know
http://theremin.us/126/126.htm (http://theremin.us/126/126.htm)
(http://theremin.us/126/126_schematic_p1_rev_1.GIF)
(http://theremin.us/126/126_schematic_p2.gif)
K