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Cold Clipper Switch popping

Discussion in 'Amp Tech Center' started by aabbs20, Jan 20, 2021.

  1. aabbs20

    aabbs20 Tele-Meister

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    Hey all,

    I recently completed a hybrid amp of sorts. It's a Princeton Reverb style amp with an added cold clipper gain stage and a 3 way switch a la Robrob. See link below for more info.

    https://robrobinette.com/How_the_Marshall_JCM800_Works.htm#Cold_Clipper

    Everything functions well and it sounds killer but I'm getting a loud pop when I use the switch. I know I'm changing the bias of a triode so maybe a pop is to be expected but I'd like to get rid of it if possible. Is there anything I can do to quiet it down or remove the popping noise?

    Thanks.
     
  2. Jon Snell

    Jon Snell Tele-Holic

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    You could include a mute circuit that mutes the output as the switch is changes by using a DPDT swith. One side runs your bias as described and the other drives the mute timer or use a FET with a charge up capacitor to slowly switch in the resistors in circuit.
     
  3. Pick_n_Strum

    Pick_n_Strum Tele-Meister

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    Would a snubber cap between the middle lug and outer lug help? Someone with more knowledge than me could maybe confirm.
     
  4. Jon Snell

    Jon Snell Tele-Holic

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    A snubber will reduce the effectiveness of the differences required in the settings.
    A snubber is a capacitor with a series resistor of around 100 Ohms. So the gain of the valve will be high at high frequencies and not change.
    Something like this;
    Screenshot 2021-01-21 at 09.29.44.png

    A simple change of switch configuration can give you different results and there is no limit to how many combinations that can be configured. When the FET gets around 0volts on the gate, it goes low impedance, nothing more than that.
     
    Last edited: Jan 21, 2021
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  5. Lynxtrap

    Lynxtrap Tele-Holic

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    ...or you could use a pot instead of resistors, with a "minimum value" resistor to ground to make sure you don't short the cathode completely.
    That way you're not limited to fixed resistor values. There will probably be some kind of faint noise when you turn the pot, but no pop.
     
  6. radiocaster

    radiocaster Poster Extraordinaire

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    Personally I would not use that switch while the amp is on, only when the amp is off or in standby. Do you need to switch in between songs?
     
  7. Paul G.

    Paul G. Friend of Leo's

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    While switching you are momentarily removing bias from the tube, this is the source of the noise. I beat my head against this wall for some time, and pretty much gave up on it. You used to be able to find "make before break" switches, but they've gone the way of the dodo bird.

    You can use the switch while the amp is off (not standby, off) and keep the feature, but I've never cracked this nut.
     
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  8. Jon Snell

    Jon Snell Tele-Holic

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    Using FETs as switches and a capacitor ensures a clean thump free change over due to the time constant of the gate drive.
    The gate voltage moves fairly slowly and allows the impedance to change gradually.
     
  9. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    You could try a couple of 1M resistors across the switch to always have some voltage across all the resistors.

    EDIT: Disregard this post. @Jon Snell points out in the post below why this will not work. Thanks Jon.
     
    Last edited: Jan 21, 2021
  10. Jon Snell

    Jon Snell Tele-Holic

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    That won't work as we are not holding a capacitor at a charge voltage, we are changing the impedance of the cathode.
     
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  11. radiocaster

    radiocaster Poster Extraordinaire

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    Only the heaters are powered in standby.
     
  12. aabbs20

    aabbs20 Tele-Meister

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    Thanks all.

    Jon, I love the FET idea, I just haven't worked with them before. Anything else I should know about them?

    In your diagram it looks like I'm connecting the 10uf cap and R2 to the heater supply. I'm thinking I run a short jumper from a tube pin? Are there any concerns about hum?
     
  13. Paul G.

    Paul G. Friend of Leo's

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    I know, yet when I did this very thing, I still got the big pop when in standby. My guess is that it's because your filter caps are still charged (if you've been powered up before) and there is voltage on the plate.
     
  14. radiocaster

    radiocaster Poster Extraordinaire

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    Ah o.k., you have to wait a little.

    In fact I added some cathode bypass caps in an amp, and I only use the switch in standby or off. I usually don't do it right away when I flip to standby.

    Mainly I'm worried that suddenly adding the caps in the circuit might shorten the life of the tubes and maybe other components. Not sure it really matters, but I don't want to risk it.
     
  15. Jon Snell

    Jon Snell Tele-Holic

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    If you look closely, the capacitors connect to the ground wire.
    I suggested using the heater supply as a way to guarantee the FETs are switched off completely when not in use.
    Assuming your heater winding, from your transformer, has a centre ground, use a 1n400* (range) diode, from each side of the winding and a 100u capacitor to get your 3 or so volts DC for the supply.
    If there is pure AC on the switch supply, it will not produce hum it just won't work correctly.
    Any N channel J FET will work
    See diagramme;
    Screenshot 2021-01-21 at 18.52.54.png
     
  16. Paul G.

    Paul G. Friend of Leo's

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    How about if you have your cold clipper resistor bypass the switch and remain connected to the cathode all the time, then switch in the bypass cap and regular bypass resistor?

    Say 10Kohms for the cold clipper, 1.5Koms for normal cathode resistor. When switched in your resistance will be about 1.3Kohms. That way you maintain bias on the tube at all times. Might work without the complexity of an FET. I might try this later, just for fun.
     
  17. Jon Snell

    Jon Snell Tele-Holic

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    Like R4 in my diagramme does, do you mean?
     
  18. Paul G.

    Paul G. Friend of Leo's

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    ...Now that you mention it... ;)

    I should have enlarged the diagram instead of scrolling past because it was tiny.
     
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  19. aabbs20

    aabbs20 Tele-Meister

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    The FET idea looks pretty complex. I'm sure it will work really well but I don't think I've got room in the chassis for it. I tried a Frondelli style PPIMV and with that turned all the way down the popping is gone. I'm happy with that for now but might look at the FET idea down the road or on the next one.

    Thanks again, you guys rock.
     
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