Hi-Lo Input Switch

[DMichel123]

I can't seem to figure out what's going on in the Fender jack circuit... I want a single jack with a switch for Hi and Lo input...

Danny

[Tom Phillips]

Danny,
Are you up for a little exercise?
This is what I did once but it's not available in a form I can post.
Starting with the standard Fender input schematic, I drew out the equivalent circuit as if there was a plug only in the #1 (Hi) jack. Then I did it again with a plug only in the #2 jack. When you compare the two circuits, you can clearly see whats going on. It's then clear how to set up a Hi/Lo switch.
I encourage you to give this a try. It's a good learning experience.
Tom

[DMichel123]

I tried that, and I guess I thought it to be more simple than it is... I will need 2 68K resistors and a 1M, correct?

[Tom Phillips]

Been thinking about it and it's working out to be less complicated to use the Fender arrangement with two switching jacks. What's your goal in doing this?
Most people never use the low input. Do you have a special need? If not, you could just wire one jack in the Hi configuration.

[DMichel123]

My buddy who I built it for uses the low input quite often... and I already have a DPDT switch mounted next to the jack... apparently the Pignose G-60VR and THD Flexi 50 both have this feature... though I haven't had much luck in locating either schematic....

[Enzo]

Why would it need to be more complex than switching one end of the lower jack's resistor between two places?

On the other hand, either way is two holes in the panel and they do the same thing either way. The two jack method still allows the old footswitch in the other jack trick. If you have to reach over there and flip the switch, or reach over there and move the plug, what's the difference? Especially if it only happens when changing guuitarists.

[DMichel123]

Why not?

Why would it need to be more complex than switching one end of the lower jack's resistor between two places?

Uhhh, because Leo said so??? I don't know, but if you look what's going on with both jacks, you got:



IN_______
|
68K
|______OUT
|
68K
|
GROUND

AND:


IN___________34K_______OUT
|
1M
|
GROUND


Simply swtiching the Lo input's resistor from being in parallel with the other to going to ground will not take the 1M out of the circuit.... Then you'd have whatever 68K and 1M in parallel is... I see whats going on, but I can't seem to implement it with a DPDT switch...

[Enzo]

Fair enough, but Leo's main thrust was getting something to work good enough, not the work best it could be. His opening circuit offerings were straight lifts from tube manuals of the day. Hell, having the hi/lo option at all was a luxury. And doing it with jacks was cheap and reliable.

But here is the deal. The amp only matters when you play it, so that assumes the guitar is plugged in. What does that do to the 1M resistor? It makes it disappear. It becomes 1M in parallel with the pickup.

You drew the circuit properly, but try moving the 1M over to the tube grid - your OUT. Now you get either the two 68k in divider or you get the series 34k. My unstated point was that the 1M doesn't matter when the guitar is plugged in. The pickup is SOO much lower in impedance that the parallel 1M is irrelevant.

68k with a 1M across it makes something like 64k. Not much difference. ANd if the pickup is 6.8k (selected for convenience) a 1M parallel drops the whole deal about 50 ohms.

You could leave the 1M out entirely if you only play guitar. But if some floor pedal distortion box with a cap output gets plugged in there, you'll want a DC path to ground for your grid, so the 1M stands there waiting and ready for that. That is all it is there for.

But going further, what if you wired the 1M right to the jack and left it there?

But if you absolutely have to switch it in and out, it sounds like you can figure out the pair of 68k and one half of the DPDT switch, and that leaves the other half of the switch to switch one end of the 1M off and on to ground.

Personally I like to keep as little as possible wired to the input jack, but that is how it would be done.

[Steve A.]

Why would it need to be more complex than switching one end of the lower jack's resistor between two places?

On the other hand, either way is two holes in the panel and they do the same thing either way. The two jack method still allows the "old footswitch in the other jack" trick. If you have to reach over there and flip the switch, or reach over there and move the plug, what's the difference? Especially if it only happens when changing guitarists.

Enzo:

So what is the "old footswitch in the other jack" trick? That reference has me baffled... LOL

Another advantage to the way Fender wired up the two jacks is that the resistors kept them from rotating and unscrewing themselves. With a single jack I usually have to retighten the nut after a month or two, and if it has a plastic bushing all of that retightening can strip out the threads.

One disadvantage with the Fender arrangement is that you can get less noise with a 33k grid stopper right on the preamp tube and connecting it with shielded cable going to the jacks. Here is what Bruce Collins drew up, which is a definite improvement over Leo's design:

http://www.blueguitar.org/new/schem/misc_mod/input.gif

Steve Ahola

P.S. For more years than I'd like to remember, I'd always plug into the high gain input, with all of my guitar controls set to "10"... in certain situations the low gain input works better so I'm glad that it is there, at least I do now. (In the past I just thought it was a waste of front panel space- a perfect location for a small switch or pot. LOL)

[Tom Phillips]

Here is what Bruce Collins drew up...
http://www.blueguitar.org/new/schem/misc_mod/input.gif

I see more difference in that circuit vs. the stock Fender than just the placement of the grid stopper resistor. When the Fender #2 input is used the pickup is loaded with ~132k and the voltage to the first stage tube grid is cut about in half. With Bruce's arrangement the #2 input loads the pickup with ~68k and, considering the high input impedance of the tube, there is effectively no voltage divider attenuation of the input signal. Just the loading effect of the 68k resistor against the internal impedance of the pickup.
Have you used Burce's circuit Steve? If so, how does it sound?
Regards,
Tom

[Steve A.]

I see more difference in that circuit vs. the stock Fender than just the placement of the grid stopper resistor. When the Fender #2 input is used the pickup is loaded with ~132k and the voltage to the first stage tube grid is cut about in half. With Bruce's arrangement the #2 input loads the pickup with ~68k and, considering the high input impedance of the tube, there is effectively no voltage divider attenuation of the input signal. Just the loading effect of the 68k resistor against the internal impedance of the pickup.
Have you used Burce's circuit Steve? If so, how does it sound?
Regards,
Tom

Bruce's circuit works great- IMO it is the resistance to ground which makes the main difference between the high and low inputs in the Fender design. With a single input jack you can try replacing the 33k grid stopper with 68k or 132k and the difference is very subtle (I think that the lower resistance gives a slightly brighter sound). Now try replacing the 1M resistor with 132K or 68k and you will get a big difference in gain.

Steve Ahola

[DMichel123]

You're Right...

I ended up just having the switch go between 68K to ground and 1M to ground... keeping 68K to grid.... works pretty good....

[Richard]

Footswitch in the other jack trick

The footswitch in the other jack trick is nice but to me it seem like I was switching from the normal level (high or low) to an intermediate level. Still a useful feature.

[Enzo]

Steve, plug the guitar into the high gain jack and a plain old on/off FS into the other jack. The FS now switches the voltage divider in and out - a gain switch. Try it, doesn't require altering the amp at all.