explain fender's hi and low inputs to me

[iknowjohnny]

i'm sorry if this is an extremely stupid question, but can someone explain how the hi-lo inputs work on a fender? I stared at a schematic for a while trying to understand but what i kept seeing didn't make a lot of sense. It looks like there are two 68k resistors that are grid stoppers, tho not one the tube pin itself. And when plugged into one input it puts the 68k's in parallel so it's halving that value to about 33k. Then when in the other input it doesn't so your grid stopper is now 68k. what am i missing here? A larger or smaller grid stopper, especially by only 33k doesn't cut the input level down ! i mean, yeah a tiny bit of high attenuation, but nothing to consider a lower gain input as i know it does by playing them. So i do realize w/o a doubt that i am wrong and it's doing something else. But why i can't see this in the simplicity of the input on the schematic is beyond me. What is it i'm missing?! (i've never built a fender or anything with multi inputs)

[Jana]

You almost had it figured out.

In the high input it puts the 2 - 68k resistors in parallel to give 33K resistance.

In the low input one of the 68K resistors is grounded. So, this gives you a voltage divider with two 68 k resistors--cuts the signal in half.

If you plug into both inputs (two guitars for example) then the 68K resistors serve as mixer resistors.

[Badside]

It also means that on the Low input, the input impedance is lowered to 136k instead of 1M, loading the guitar's pickup more (although this doesn't matter anymore if you have a buffered pedal between the guitar and amp)

[Structo]

That was my understanding is it was to make the input a bit more friendly for guitars of different impedances.

Or basically, if it doesn't sound good in #1 try #2.

[iknowjohnny]

Wow, so it goes from grid stopper only to a VD and with a 68k ground reference instead of 1M ?! Seems awful low. thats bizarre. I tried smaller resistors once in place of the 1m and once i got down to about 180k it just killed everything.

[Bear]

Anyone want to offer a further explanation of the effects of bridging four input amps?

[Jana]

"That was my understanding is it was to make the input a bit more friendly for guitars of different impedances."

I don't think that was a part of the equation. They're all high impedance inputs.

You have to remember the era when all this came about:

Distortion was not a desirable result. The low input was there to attenuate hotter signals such as steel guitars, etc.

It was also there as a feature to give two inputs to a channel. Remember, there is no attenuation when using both of the inputs. It was not uncommon for two guitar players, or a guitar and a bass to plug into the same amp. It wasn't fun, but we did it. Not everybody could afford an amp.

Then, take a fender with a normal channel and the reverb channel and you have four inputs! Bass, guitar and two hi impedance mics into one amp! What a sonic mess, but it worked--sorta.

Four holers-- Marshalls--same concept. High and low inputs for each channel. One channel is bright and the other is normal. Bridge the channels and you have a bright volume and a bassy volume. Works awesome!

Another thing to do with the high and low inputs is to daisy chain amps together. Guitar goes into high input of amp 1. Low input of amp 1 gets connected to high input of amp 2. Etc. Ad infinitum.

[Andy Le Blanc]

It was a cheap short cut.

Older model fenders used to give you two in's, then they gave two in's
where one was actually a parallel gain stage.

the modern 1 and 2 is a cheap resistor simulation of a single and parallel in.

CHEAP.... no other reason. Keep the feature, use the gain stage elsewhere.

[Bear]

So bridging a four holer (that doesn't have a triode per input): the channel with the jumper going out acts like the high input, now matter which input is in and which is bridged out, right? But for the second channel, assuming that isn't daisy chained out through it's second input, it matters whether it's into the high or low input, right?

(Only four holer I've owned was a Fender reverb amp, so bridging wasn't useful enough to have tried this out.)