I see a lot of people talking about modding their tweed amps to lower the coupling cap value for a more modern lead sound. I also see a lot of people talking about putting the preamp tube's cathode bypass cap on a switch to get switchable voicings for their amps. Is there any reason I don't see anyone talking about making the coupling cap switchable? Would it be possible to put the coupling cap on an SPDT to get two separate sounds?
Switchable Coupling Cap?
corliss1
Poster Extraordinaire
Possible? Sure. Functional? You'd like get some pretty major popping noises while making the switch due to switching high voltage. The cathode cap switch has maybe 1.5V on it. Also, those wires are some of the most sensitive for noise in the amp so running things back and forth to a switch may not prove ideal.
Cool, yeah, I thought something like that might be the case. Or worse, I was afraid that if the switch was break-before-make then it could even maybe blow the tube because there would be a split-second in which there's no load connected. I would definitely not want to risk damaging the amp.
However, if a loud pop is the only problem, I'm not too worried, as long as there's no added noise when not switching back and forth. Could such a pop damage the amp in any way?
corliss1
Poster Extraordinaire
Loud pops can stress speakers.
I'd need to think it through more, but I wonder if using a switch to parallel in a second cap would work better. Switch open, small value. Switch closed, larger paralleled value since parallel caps are additive.
I'd need to think it through more, but I wonder if using a switch to parallel in a second cap would work better. Switch open, small value. Switch closed, larger paralleled value since parallel caps are additive.
elpico
Tele-Afflicted
There is a thump when switching, particularly the first time you toggle the switch after turning the amp on, but the amp can't "thump" any louder than it can play so if your speaker doesn't blow from playing your low e string I wouldn't worry about a thump. If it really bothers you it should be possible to greatly reduce the thump by adding high value resistors to the switched ends to keep the caps charged up. I've never bothered. The volume control is after the cap so if you really don't want to hear a thump you can simply turn the volume down before switching.
Fwiw a preamp tube won't be damaged by not having a cap connected. The 100k plate resistor is the tube's main load.
Fwiw a preamp tube won't be damaged by not having a cap connected. The 100k plate resistor is the tube's main load.
Snfoilhat
Tele-Afflicted
Vox's old Brilliance control was a bass cut using and extra cap in series with the coupling cap. It looks to me like they place all the extra components (switch, cap) after the first cap, to keep the preceding stage's VDC off the switch. I haven't played a 1960ish AC15 and don't know if it pops, but it seems like this question was worked out a while ago. There are modern AC15 descendants with multiple cap values on a rotary switch with resistors between all the lugs to try to equalize the voltages across them.
I any event, it seems like putting these components behind a cap may be quieter than in parallel. No need to minimize part counts w/ homebrew amps, if a cap or two extra makes it function better.
I any event, it seems like putting these components behind a cap may be quieter than in parallel. No need to minimize part counts w/ homebrew amps, if a cap or two extra makes it function better.
Attachments
tjmicsak
Tele-Meister
Maybe if the option was to add in an additional cap parallel to another still connected for changing the values there would not be so much pop.
IOW, instead of an A or B switch, the switch option adds in an additional parallel cap leaving the first path still connected. Use the combo for the one value, and leave the single connected for the second value.
IOW, instead of an A or B switch, the switch option adds in an additional parallel cap leaving the first path still connected. Use the combo for the one value, and leave the single connected for the second value.
No pop if the switch just bypass a high value resistor in series with cap.
Design must aim to pop free action, for sure if you do It wrong way you'll have bad result.
I think it's not a common feature because it's not a vintage feature.
Design must aim to pop free action, for sure if you do It wrong way you'll have bad result.
I think it's not a common feature because it's not a vintage feature.
mgreene
Tele-Afflicted
Doesn't Swart make such an arrangement on some of their amps?
elpico
Tele-Afflicted
Like I said you could just add a resistor to the switched end if you're that bothered about it. Like this:
image removed
image removed
Thanks all! I'm not particularly bothered about pops, unless the popping is likely to cause damage, and it sounds like that's not too much of a worry. But as of now, I'm planning on using an ON-OFF-ON switch. The OFF position will be a 560pf, one position will add a .00047uf in parallel with it, and the other will add the stock .002uf in parallel with it. I'm planning on putting this on the volume knob side, not the tube side, since I want the switch to be on top of the chassis.
A 5F1
I'm looking in that schematic, but it's really difficult to see. Where is that switch located?
And when you say "quieter" are you referring to pops, or hum/buzz/rfi? I don't care about the former, but I do care about keeping the later as minimal as possible.
Edit: ah, I found the switch
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LudwigvonBirk
Tele-Afflicted
Agreed. Don't slam your amp with a pop-inducing mod. You can't use that on stage, you might damage the speaker or a component, if you have to turn the amp off to change the sound a little bit...that's lame design.
I have not tried it (in this context) and am not recommending doing it, but! a pot instead of a switch could be interesting to try. I wouldn't drill a new hole before trying it, but with a pot you could get A ("0"), B ("10"), and sounds in between, and no component-slamming pop (^ design criteria met). If you do this experiment please take measurements and share what you find.
