Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Quatro on July 15, 2012, 08:27:11 pm
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Is there an elegant way to put a treble cut circuit on switch?
I was thinking of switching a cap in and out across the first plate resistor on a mini switch but was worried about the voltage on the switch (260v on one side of the resistor, 120v on the other). Good idea, bad idea? Better way to cut treble with a switch?
thx
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Is there a reason you want the cap across the plate resistor? If all you're looking to do is create a switchable treble cut, you can add the cap in an area of the circuit without high voltage. Try comparing the sound of adding a cap across the first plate resistor vs. adding a cap to ground between the first and second stages (for instance, across a voltage divider resistor to ground). You'd probably have to use a different value cap there to get a similar frequency response, so bear that in mind when comparing.
Another option is to use a cap connected from cathode to grid. That will give you local negative feedback on high frequencies. There is info on calculating that cap's value at http://www.freewebs.com/valvewizard/gridstopper.html (http://www.freewebs.com/valvewizard/gridstopper.html) under "Increasing Miller Capacitance".
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Is there a reason you want the cap across the plate resistor?
Only because I know it works.
Thanks for the advice. I think i will use the cap between the grid and the cathode.
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I've never tried a switched version. Would definitely be interested to know how it turns out!
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why don't you take a look at the Vox cut control. it works really well and you could turn it in to a switch with a fixed value or use a switched pot.
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why don't you take a look at the Vox cut control.
Will it work on a cathodyne?
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Yes, as long as there is a phase inverter it will work. Putting a cap across the output of the inverter cancels the frequencies the cap passes since they are out of phase. The bigger the cap, the lower the cut off is. Add series resistance to lessen the effect. Care with this control however, as it can load the PI too much. Words better on amps with sensitve power valves like EL84s.
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Care with this control however, as it can load the PI too much.
How is one careful in this situation? By not going too high or low with which components?
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I'm going to try implementing the switchable cathode-to-grid cap in one of my amps. Attached it one way it could work to switch between two values. This is untested; so far I've just experimented with a decade box.
Vox cut control is a great idea and might work quite well for your purposes, though.
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I'm going to try implementing the switchable cathode-to-grid cap in one of my amps.
OK, but you have 27K standing between the cathode and grid.
Brad :think1:
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Yes - 27k grid stopper to help prevent oscillation and blocking distortion when gain is up all the way, etc. It could be omitted. I tested the decade box both ways (with hi-cut cap connected pre- and post-grid stopper) and decided to keep the treble-cut cap connected before the grid stopper for now. I know that the 27k grid stopper and the 500k gain pot will act as a voltage divider, but it wouldn't attenuate the treble-cut cap NFB signal that much unless the gain pot was set very low... My idea was that when the gain pot was set very low, the NFB would be attenuated some, too, in order to allow more high frequencies and act kind of like a bright cap. Maybe. But also could be completely wrong about that... heh heh :think1:
Thoughts?
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One problem would be if the cathode resistor is going to appear in parallel with the gain pot as far as AC (audio) is concerned? In which case the grid stopper might be a problem. I still get confused about how that works when there are multiple caps and resistors involved. Can anyone clarify if that would present an issue?
In any case, Quatro, or anyone who wants to try this, maybe you should leave the 27k grid stopper out for now, until I can do more testing...