Guitar amp input impedance question

[soundguy]

I should be able to figure this out by now but its little things like this that make me remember that actually, Im still an idiot-

Lets say Ive got a fender bassman. That bassman is providing a load on a guitar pickup plugged into that amp. Part of that load is influenced by the resistor hanging off the input jack in the amp, right?

Fender Bassmans have two input jacks per channel. This allows you to plug into the amp and with another cable, split your signal to another amp. Lets say that other amp is another identical bassman. So now, in this scenario, the guitar has too much of a load on it and sorta sounds muddy and totally lackluster.

If I designated those two amps to forever work in parallel, could I half the value of the pull down resistor on each of the input jacks on the channels running in parallel? These seems too simple to work...

Id like to figure out how to permanently wire those two amps to play nice together with a guitar without having to build an active box. Ive tried fet splitters before and while they sound ok, they certainly sound and Id like to go for the least color and change in interaction between gtr/amp as possible. I can always feel when Im playing into one of those fet splitters...

thanks!

dave

 

[CJ]

It's not the 1 M resistors to ground, it's the 68k series resistors. Since you are going in thru 1 68k, back out the other and then into another one on the second amp, you are now going thru 210 k series resistance. Just bypass the 68k's with a jumper. They are there just for a passive mixer so two guitarists don't interfere as much.

Wait, maybe there is only a 68 k on one of the jacks.

That's why number two always sounds duller than number one. so i think you only need to bypass the 68 k on the first amp, then plug into channel 1 on the second.

A lot of amps don't even have the 68k, Mesa for one. The days of two guys sharing an amp are over. Unless it's the kiddies playing.

OT:Boy I wish there were more kiddies starting up garage bands. I just read a newspaper article on radio stations. It seems that rock stations are dissappearing at an alarming rate. Some big towns don't even have one rock station left. Dang Clearwater. Theyt turned one of the coolest stations we had out here into Ranchera music. Not that I don't like Ranchera, but now were down to one or two good stations.

 

[electronaut]

[quote author="CJ"]but now were down to one or two good stations.[/quote]

****... if you want terrible radio options, move to Chicago. The only decent stations are the microwatt college stations that you can hardly hear from a couple too many miles away.

Sorry for the OT.

 

[CJ]

Satelite Radio. I knew there was an answer, but who wants to pay for radio?
:evil:

 

[Mbira]

They just turned our radio station into ranchera music too!!!

I was just looking through my Aspen Pittman book trying to find out more about that input resistor. I never knew it was there specifically for two guitars. I looked through the whole book trying to find an amp with only one input to check on it. What do you know, the Mesa Boogies were the only ones!

Joel

 

[soundguy]

ok, so the loading is definitely coming from the 68k? Is there any reason to have that resistor in there at all then, if you only ever use the amp with one input?

this is an incredibly useful bit of info to know...

dave

 

[pstamler]

[quote author="soundguy"]ok, so the loading is definitely coming from the 68k? Is there any reason to have that resistor in there at all then, if you only ever use the amp with one input?[/quote]

Well, it does provide a certain amount of RFI (radio frequency interference) filtering.

Which Bassman model are you using?

Peace,
Paul

 

[VacuumVoodoo]

One of the reasons to keep the series 68k resitor in the input is to provide HF roll-off to prevent RFI.

A ferrite ring on the grid wire directly at the tube socket is a better solution.

 

[clintrubber]

You want to connect those two amps permanently ? A solution could be to make an Y-cord (female TS to two male TS - or a variation on this, since most people will not like those male-to-fem TS connections, mechanical PITA). This way you get say 500kOhm input impedance, which might be a bit less than desired - you might already start noticing it.

If I designated those two amps to forever work in parallel, could I half the value of the pull down resistor on each of the input jacks on the channels running in parallel? These seems too simple to work...

If indeed a permanent setup, increase the 1M in both amps to 2M2.
So double them. Then you should be close again to the original situation.

There's no AC-coupling, right ?
Another thing: you're not getting hum, ground loops etc ? Otherwise another issue needs to be solved.

Bye,

Peter

 

[Scodiddly]

Fender didn't put in that second jack for running another amp. It's for plugging in more guitars!

Anyway, the inputs are usually the same exact circuit on just about every amp Fender made. 1M resistor to bleed DC to ground, 68k to match up to the tube grid. The impedance is pretty high, and the 1M resistor is a big part of setting the impedance.

 

[clintrubber]

As I always thought to understand it this arrangement can do all sorts of things: have different input-sensitivities (at however a lower input-Z for the #2 input, ~2*68k), the mentioned more gtrs at once, and for linking to other amps.
Don't know how it's meant by the designer, but it's sure used as such. It's sure a compromise/less optimal situation for some of the uses mentioned above.
Must say I've never had the need for a lower sensitivity, but who knows, it might come in handy when sending stronger signals than e-gtrs to an amp. Those signals are then likely not bothered by the lower input-Z.

 

[Gus]

The guitar (plugged into 1)will see the first 1meg to ground then the 68K and then the 2nd channel 68K

then out of 2 to the next amp it will see 1 meg to ground for about 500K loading (1meg + 68K) || (1meg +68K)

Both amps will need to have the 2.2 megs installed.

When I build effects. I note a tone change when there is < 500K input loading on a strat ot tele type wired guitar. This can be a good thing for effects but is often bad for guitar to amp sounds.


Looking at the fender bassman schematics I have there is another overlooked tone change, if the guitar alone is plugged into 1 the grid stopper R is 68K in parallel with 68K for 34K. This can be a part of what you are hearing

When driven by effects like the output stage of a TS9 or a buffer the grid R value seems to effect the tone to my ears.

 

[Guest]

You are looking for a perminent change so both amps can be played together, rigged as you want with instrument to input 1, cord out of input 2 to input one of the next amp.

The loading in this arrangement, as mentioned above, comes from the 1M grid leak and 68k grid stopper resistors on each input position. To get the two to play nice together without too much tonal change, bypass the 68k grid stoppers on both number 2 inputs and raise the grid leak resistors to 2.2M. I think this will do the trick with the least trouble.

 

[scott_humphrey]

[quote author="pstamler"][quote author="soundguy"]ok, so the loading is definitely coming from the 68k? Is there any reason to have that resistor in there at all then, if you only ever use the amp with one input?[/quote]

Well, it does provide a certain amount of RFI (radio frequency interference) filtering.

Peace,
Paul[/quote]

This is true. If you bypass those 68k resistors completely you will be picking up radio stations ala Spinal Tap. Also, depending upon how the amp is voiced, it might make the highs a bit shrill.

Those resistors are best soldered directly to the first preamp tube. They will work best for RFI filtering if they are located there physically.

If you have an amp and you want to plug just one guitar into it (without any line outs), try disconnecting the second input jack, then connecting input jack number one to the first input tube through a 33k resistor with a 1 meg resistor to ground. Using a value smaller than 33 k will give a brighter sound, while a higher value will roll off the top end.

 

[Analogique]

[quote author="soundguy"]Id like to figure out how to permanently wire those two amps to play nice together with a guitar without having to build an active box. [/quote]

Half-circuit'ing the front-ends is certainly going to risk altering the drive/load ratios of the pickup and grid circuitry ... problem is the Zin of the circuit is dominated by the Zin of the tube which doesn't vary all that much with bias - you'd have to drop the capacitor bypass in the cathode circuit to increase the total Zin that much ... and that would change the sound and feel of the front-end too much probably ...

... you could go through one preamp only and use the cathode-follower stage at the back of the preamp to send signal to the output stage section of your other amp(s) - tone stack ordering being your choice ... this scheme gives pretty good preservation of feel and tone, and good noise immunity - though you might have to lift the power ground on your slave to avoid ground loop hum ...

... loading of the phase-driver circuit on the preamp output is quite negligeable when adding a second feed to the lowZ follower node ... you could probably add up to four "slaves" this way without too much linear and non-linear tone loss ... my sims showed a 400 ohm Zout for that 12ax7 follower stage - I'm pretty sure the Zin of the 5F8-A PI stage is well over 1Meg ...

... jcm

 

[Bryson]

[quote author="scott_humphrey"][quote author="pstamler"]


If you have an amp and you want to plug just one guitar into it (without any line outs), try disconnecting the second input jack, then connecting input jack number one to the first input tube through a 33k resistor with a 1 meg resistor to ground. Using a value smaller than 33 k will give a brighter sound, while a higher value will roll off the top end.[/quote]

in most Fender, Marshall, etc., plugging only into jack #1 puts the signal through the 68k resistors in parallel (34k).

I wonder how much different it'd be using a Y-cord to amp #2 (instead of jack #2), leaving the 68k resistors in parallel in amp #1.