Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 6v6Pin1 on August 23, 2015, 12:40:27 pm
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I'm trying to understand the stout tone control. I see the 0.0047 cap is going to the volume pot and the 0.01 cap is going to the tone pot.
What happens as I turn up the tone control? Is it as simple as sending more of the 0.01 signal to ground? Is it somehow filtering out low frequencies?
What is the function of the 100k resistor going from the tone pot to the volume pot?
Thank you!
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I see it as a simple treble bleed tone control. Looking at this schematic...
http://el34world.com/projects/images/HoffmanStout18watt_No%20Switch_Schematic.GIF (http://el34world.com/projects/images/HoffmanStout18watt_No%20Switch_Schematic.GIF)
...when you turn the pot to zero, the wiper moves all the way down, putting a ground on the right side of the .01µF cap. This gives maximum treble bleed which causes the tone to be muddy/bassy. Turning the pot all the way up puts a 500K resistance in series with the .01µF cap. This gives the minimum treble bleed which causes the tone to be more trebly (brighter). This is the original tone control from the Marshall 1974 (18W) normal channel, which is the basis for the Stout. I don't know that the 100K does anything. I've never disconnected the 100K to see if it really has any effect.
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I'm not talking about replacing the 100K with a piece of wire. I'm talking about removing it and creating an open circuit.
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I hadn't considered that some of the signal from the 0.0047 cap was going to ground through the 100k resistor. Interesting.
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> Turning the pot all the way up puts a 500K resistance in series with the .01µF cap. This gives the minimum treble bleed
AND it wants to short-to-ground the signal on the top of the volume control (as 6v6Pin1 notes).
A dead-short would be no fun.
The 100K limits the shortiness.
But the real point is that 100K is a heavy bass-cut against 0.0047uFd. About 350Hz, over 2 Octaves of bass shave.
Hmmm. The other way, 0.01uFd against the ~~60K of the plate network gives a 280Hz Hi-Cut. "Only" your bottom couple octaves.
Small "error" in that full Low-cut also takes a slice out of mid/highs. 3 or 4 dB. That's mighty good for such a low parts-count.
(You find similar grounded-wiper 2-way tone controls in stuff like 1941 Plymouth radios, why I know it.)
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In my experience the control works like this: at about 8 - 10 on the dial is when the bass cuts pretty good and the control is at it's brightest but not as bright as a treble bleed and/or the Fender tone control circuit. When dialing back to seven and lower you begin to get more bass to your signal and gradually drop treble also as you go down.