Hi-Lo Input tricks

[jordan86]

Was at my kid's school yesterday for their band concert. The jazz ensemble had two kids on electric guitar way in the back that you couldn't even hear (probably intentional). They were on crummy little beginner solid state combos, that just made me wish that I'd known and could have offered my hardwired Princeton or old 65' Showman. The Showman would have really been a treat. Those kids would've left the bass players and the entire horns section in their dust. Anyway....it sent me down the rabbit hole of the concept of two players sharing an amp with hi/lo inputs, which leads me to this question and experiment.

How does input impedance change on the high and low inputs of a typical Fender if you are using both inputs?  I just did a little experiment in the LOW input for fun. When I plug a dummy cable into the HI input, the LOW get much hotter. Vice versa, the HI input stays mostly the same but gets a touch softer. There's a VERY subtle tone change in the mids/highs that makes me think the impedance is slightly different. Attached a Fender schem for specifics.

I do know that normally the HI input sees an effective 34K grid stopper with a 1M to ground while the LOW sees 68K and 68K to ground (2:1 voltage divider). But if you opened the other self grounding inputs jack, how would those numbers change.

I think the high would now see a single 68K grid stopper still 1 Meg to ground. The other 68K on the low jack is no longer part of the signal path? So it would be very close to stock scenario, but would explain the subtle shift in highs/mids. The LOW would see 68K with 1M plus another 68K to ground? Meaning it'd be a touch brighter than stock?

Does that sound right? I know that's probably a fruitless and impractical detail to even belabor but curious minds gotta know.

[sluckey]

Well, you just discovered the main reason Leo put two jacks on those amps... so two people can play guitars through a single amp. He was a genius. 

As you discovered, when using both jacks each one delivers the same volume. The 68Ks (often called grid stoppers) now function as a simple passive mixer to prevent one guitar controls from affecting the other guitar. This is what Leo had in mind all the time.

[jordan86]

Dang, didn't even think about that, Sluckey! They definitely double as mixing resistors when using both inputs. Leo really was a genius. I repeatedly find myself thinking how brilliant people where in the 60's. Most often when reading about tube amps or Nasa stories. Just not the same breed today. I blame television.

[tubeswell]

I blame television.

..., video games, social media, over-protective parents (or obversely, total lack of parenting), too soft on school homework discipline, alcohol, sex, drugs and rock'n'roll...


Apologies for the 

[Keppy]

If you ever want to be WAY too thorough analyzing this stuff: https://robrobinette.com/How_Fender_Input_Jacks_Work.htm