Curious if anyone else has done something similar, or if this is my own weird idea. I have tried it in a few single ended amps and it works ok, though I'm not sure if anyone actually understood what I was talking about or uses a footswitch. Basic idea is: HI input wired normally, LO input uses a TRS jack with a ~100k resistor between the ring and sleeve, and the ring is connected to a cathode bypass cap (grounded via ~100k resistor). So, use it as a normal input, the switch closes and you get a boosted LO input. Use the HI input and plug in a two button foot switch to the LO, you can use one button to boost with the cathode bypass and the other to cut with the different input impedance. Obvious limitations with cable length and noise, but for a little practice amp it works just fine. Here's the last amp I did this to. Also mentioned in the garage section, but thought this was a better place to get technical input.
foot switchable LO input
Moderators: pompeiisneaks, Colossal
foot switchable LO input
You do not have the required permissions to view the files attached to this post.
Re: foot switchable LO input
In case my description didn't make sense to anyone...
To add to any confusion; I don't know how to draw input jacks and the HI will be quieter than LO with cathode bypassed.
To add to any confusion; I don't know how to draw input jacks and the HI will be quieter than LO with cathode bypassed.
You do not have the required permissions to view the files attached to this post.
Re: foot switchable LO input
I really like the idea of bigger circuit changes being affected by switches at the jacks. I'm planning a plexi build with a similar scheme to jumper channels and to route an integral JFET preamp.
I'd be curious how different the sound is with the cathode bypass capacitor. If I understood Merlin's book correctly, I thought that resistors 10^10 larger and greater will essentially remove a bypass cap from a circuit (e.g. 25k removes 2.5 uF). I was considering messing around with something similar on that upcoming plexi build but using a pot to control input sensitivity by having the control vary resistance in series with the bypass cap and that was going to be the "rule of thumb" I used to determine the pot's value.
Something else I'm wondering is signal bleed from the cathode onto the LO input. Something like this might help you both completely access the cathode bypass cap with no issues with the resistor's value impacting the degree of cathode bypass and to isolate the input circuit from V1's cathode: https://www.amplifiedparts.com/products ... ak-circuit
I'd be curious how different the sound is with the cathode bypass capacitor. If I understood Merlin's book correctly, I thought that resistors 10^10 larger and greater will essentially remove a bypass cap from a circuit (e.g. 25k removes 2.5 uF). I was considering messing around with something similar on that upcoming plexi build but using a pot to control input sensitivity by having the control vary resistance in series with the bypass cap and that was going to be the "rule of thumb" I used to determine the pot's value.
Something else I'm wondering is signal bleed from the cathode onto the LO input. Something like this might help you both completely access the cathode bypass cap with no issues with the resistor's value impacting the degree of cathode bypass and to isolate the input circuit from V1's cathode: https://www.amplifiedparts.com/products ... ak-circuit
Re: foot switchable LO input
Quite different actually, I think I am still not describing this clearly...
You are correct that 100k will make the cathode bypass cap inaudible, and this is intentional. The resistor needs to be large enough to make the cap inaudible but small enough to charge the cap in a reasonable amount of time so it does not pop when the footswitch is closed. When an instrument is plugged in to the LO input, the 100k resistor is shorted and the cap is in circuit. When and instrument is plugged into the other input, the cathode bypass cap is off, and none of the LO input circuitry does anything until you plug in a foot switch. With a two button, trs, foot switch connected you have access to the cathode bypass switch on one button, and the other puts the two series grid stopper resistors in parallel with the 1M resistor, shunting signal to ground like in a normal LO input.
I guess you could achieve something closer to a regular HI LO setup with a trs jack with additional switching elements, but the jack you suggest doesn't have enough parts to do what I am suggesting.
Re: foot switchable LO input
Less confusing way to draw this. I usually skip the input jack part when i draw a schematic by hand...
You do not have the required permissions to view the files attached to this post.
Re: foot switchable LO input
Ahh I see. I didn't realize you were plugging the footswitch into the "low" input (should have read your first post more closely -- my mistake!). Very cool idea to use the stereo jack. I wonder if you could add a switch onto your guitar to switch the cathode bypass from there if you also installed a stereo jack on that side as well. That'd be pretty cool, though I'm sure pretty noisy.
Re: foot switchable LO input
A 5ish foot cable didn't add much if any noise but I don't think I would want to go much longer than that. I have only tried this on a couple Kalamazoo's, this champ clone, and I think another cheap single ended amp that I wanted to add a feature without drilling more holes. I would rather add foot switching in other ways on most occasions, but this is cheap, easy, and works fine.
Re: foot switchable LO input
I've been thinking about this approach some more in context of a Hiwatt build I did. I used the "SAP" preamp (https://hiwatt.org/Schematics/DR_Pre2InputSAP.gif) with an added SPST in series with the footswitch in case the footswitch breaks, I forget it packing for a gig, etc. The switch was installed in the top "brilliant" input with the bottom input converted to the footswitch's jack. I don't find I actually use the switch a bunch since it's too drastic a change, so I'm considering giving this approach a try for the existing channel's "high" and "low" inputs. That'd also let me wire up the unused triode as the original brilliant channel by freeing up the input jacks. The difference in this setting would be that part of the signal off V1 would be shunted to ground through the sleeve rather than changing the cathode values for a gain stage. I'd have some concerns of feedback/oscillation feeding the amplified signal back to the sleeve, so I'm thinking I'd probably need to increase the 4.7K resistor in the ground shunt to something like 50K (maybe even 100K since my present gripe is that the gain cut is too dramatic). Increasing the value would make it function closer to the original value anyway since I'd also be eliminating the balance control to restore the brilliant channel, i.e. eliminating what was essentially a 25K divider when the balance control was full up.
Re: foot switchable LO input
Interesting, nice to have another example of a cloacal foot switch jack. I would still like to know if anyone sees problems with my design/if it is completely stupid in some way that I don’t see…
Not sure if I am following you with changing cathode values and implementing this in a different amp. Pictures are good… Assuming you kept that circuit the same, making the 4k7 bigger would make it louder by default. Shouldn’t be any problems with grounding noise since everything is pretty close in the circuit. Also, no reason to put another switch in line with the jack, the jack is a switch. You could shove a chopstick in there in a pinch and it would switch just fine… Could also use a jack that is normally open instead of normally closed if that is your preferred setting.
Not sure if I am following you with changing cathode values and implementing this in a different amp. Pictures are good… Assuming you kept that circuit the same, making the 4k7 bigger would make it louder by default. Shouldn’t be any problems with grounding noise since everything is pretty close in the circuit. Also, no reason to put another switch in line with the jack, the jack is a switch. You could shove a chopstick in there in a pinch and it would switch just fine… Could also use a jack that is normally open instead of normally closed if that is your preferred setting.