guitar amp

I find that hard to believe. Are you sure? How old are these schematics? It may be tradition. It may be that you are seeing stuff designed by musicians who took to electronics as a hobby. Vacuum tube amplifiers? Again I am flabbergasted. Of course there are audiophile nuts that claim outlandish benefits to the use of tubes. At best they are misguided.
You also have to describe the schematic better. The input impedance you mention (about 100k-500k) is high, especially if driven by some kind of a magnetic pickup. It may be part of RC coupling. Some shunting resistance is need to keep stray capacitance from limiting the audio bandwidth. You always have to worry about becoming unstable and that might depend on where negative feedback is inserted. You have more homework to do.
Bill -- Ferme le Bush

Thanks for your reply. Yeah, the schematics could be old.. . .
About the input impedance, I read that magnetic pickups have an output impedance of average 500k. Here is another form of the question: if you have a mosfet, would you need to have a resistor running from the gate to the ground? Or can you simply connect the input to the the gate?

how many?
you haven't seen enough i guess.
DC grid bias is needed for proper operation as a (sort of) linear amplifier
negative feedback is used to reduce amplifier instability and in some cases improve linearity.
negative feedback in both tube and SS amplifiers is primarily intended to help the power amplifier. section. (as in low voltage gain / large power gain)
for proper operation, it matters how all elements of the system are designed and constructed. yw

Chet Atkins was misguided? Surely not.
jc

Chet Atkins has been gone a while and I do not know about his electronics knowledge. I do know he was an innovative guitarist.
By misguided, I mean not knowing, like the song from Annie, Get Your Gun: anything tubes can do, solid state can do better. In terms of high fidelity, ss can be better. Certain ways of using tubes can give distortions that produce certain special effects easily--but that is not the same as hi-fi.
Bill -- Ferme le Bush

Depends on one's interpretation of the word better. Technical-wise (as measured with instrumentation) you are likely correct. However.. as sounding better to the earpans, this is subjective, and indeed tubes when implimented without too much negative feedback can sound quite lovely and "warm". This has been the the subject of many "debates", and of course the decision on which is "better" lies with the person doing the listening and applies only to that person. All that said, I prefer a tube preamp and solid state power amp with minimum feedback.
jc

In hi-fi, the best you can do is to reproduce a waveform. A distorted waveform may sound better to some. I have no quarrel with that. But that is introducing artifacts. Then the art is to introduce distortion we and others like.
This distortion is like adding spice and other adulterants to perfectly fine natural food to make it taste better. Isn't that why we charbroil steaks instead of eating them raw?
Bill -- Ferme le Bush

I think you could be misguided with the last remark, though the first one could be true.
sQuick..

They all do these day's. I service them regularly.
The grid is biased for a reason. Why not try connecting your guitar direct to the grid of an ecc83 & see what happenes. Valves/tubes dont work like transistors.
if you design & test an amp well, lots of neg fb wont be required. Maybe slide a filtered touch in at the phase splitter stage.
sQuick..

The thing is, tubes are less prone to odd-order harmonic distortion, which is more irritating to the earpans than even-order harmonics. Note neither transistors and tubes are distortion-free, so it could be the lesser of the evils.
Yes, and each person has his choice of "spice", 57, A-1, or whatever. Some choose even-order distortion.
Sorry, I don't see that as a good analogy. Some may prefer them cooked different ways.
BTW, you said: "Chet Atkins has been gone a while and I do not know about his electronics knowledge." Note he was RCA's recording engineer in Nashville for quite a while. Regarding his choice of amps, one of his favorites was a Musicman RD-50 that used 6L6GC output tubes - the workhorse output tubes in their power class.
jc

I am pointing out that distortion of the "natural" product can make the product more attractive. What the bottom line appears to be is that "Vacuum tube amplifiers have more pleasant distortion than do solid state amp0lifiers." That may be and I will not deny any listener of his favorite distortion. Realize that human ears also introduce distortion. With double ended amplifiers and feedback there should be engineering problem getting distortion down to audibly undetectable levels. Moreover, modern electronic technique allows much in the way of custom generated distortion based on frequency, nonlinear, and time dependence.
Bill -- Ferme le Bush

The grid intercepts some electrons emitted by the cathode. They will bias an isolated grid negative in an uncontrolled way. An old technique was to connect the grid to ground with a very high resistance and use the grid current to provide a controlled bias (grid leak bias).
A high grid-to-ground resistance, combined with shunt capacitance, will also result in reduction in gain at high-end frequencies. The same is true for large plate resistors.
Grid bias is usually privided by a cathode-to-ground resistor. If this resistor does not have a capacitor across it there is negative feedback.
From the dim past, I think electrometer tubes put the control grid in the shadow of another element and had extremely low grid currents permitting extremely high impedance grid circuits.
I agree with Salmon Egg that stability is an issue. You could create a nice oscillator. A coupling capacitor and a resistor produce a frequency dependent phase shift. If coupling capacitors are in your feedback loop you may have trouble, lots of feedback - lots of trouble. (Op amps don't have coupling capacitors.) From the dim past, I don't remember that negative feedback was common other than unbypassed cathode resistors.
A major name in amps said he could produce any tube "warm", or other, sound identically with solid state and feedback.
[Sorry if this post is duplicated]
bud--

Yep!!
You are 50% correct on this one. So far as stereo equipment goes, you are correct. At best it is mostly noisy junk. Guitar amplifiers are an entirely different story. They are more responsive, warmer, fatter, and for harmonic clipping (distortion) nothing can touch them....think about it... there is a reason WHY there were and still are soooooo many companies manufacturing stomp boxes attempting to simulate them. Ever listen to a vintage Marshall 1968 Plexi, JTM 45, JCM 800...? Or any Soldano? Just listen to them...they speak beyond volumes. :)
Of course, solid state amps have their place in music as well. Here I would say they are better suited for some types of Jazz, Pop, and to a lesser extent Fusion. So far as digital modeling amps go....they too are junk; they have a long way to go. For now, there isn't anything like the real thing. Again, why are they trying to duplicate it on a chip?