Modeler "Input Impedance" -vs- Real Tube Amp "Input Impedance" ?

[BenIfin]

As usual, out of my technical depth here.

I understand that with *most* modelers these days, you can adjust the Input Impedance to varying values .... with most people usually setting it to "Auto" or 1M Ohm or letting it "take" the Impedance Value of the first Block in the chain.

In the world or real-tube Amps, do their Hight / Low .... Normal / Bright 1/4" Inputs .... also have Input Impedance Values ?

If so, is there a general value or does it vary from Amp-Input to Amp-Input ?

Also, can these values be looked-up somewhere ?

 

[0XI0]

In the world or real-tube Amps, do their Hight / Low .... Normal / Bright 1/4" Inputs .... also have an Input Impedance Values ?

hell yeah, the gain pot is a resistor, a variable resistor. infinitely variable input impedance. but it depends on what circuitry the guitar signal hits first in the chain, depends on the amp. open up revalver IV and open up schematic tweak mode on a clean fender, you can reduce the input capacitors/resistors value and all of a sudden it's a fire breathing high gain.

also if you run passive pickups through a paper clip into DSP distortion it loads the pickups down and feels like tube amp input stage sag/compression.

ClipToneTM

 

[JiveTurkey]

 

[BenIfin]

hell yeah, the gain pot is a resistor, a variable resistor. infinitely variable input impedance. but it depends on what circuitry the guitar signal hits first in the chain, depends on the amp. open up revalver IV and open up schematic tweak mode on a clean fender, you can reduce the input capacitors/resistors value and all of a sudden it's a fire breathing high gain.

also if you run passive pickups through a paper clip into DSP distortion it loads the pickups down and feels like tube amp input stage sag/compression.

ClipToneTM

Just would like to know ... curious.

 

[MirrorProfiles]

On a Marshall 1959, the high input is 1M Ω and the low one is 68k:

Marshall

How the Plexi and JCM800 Functions

robrobinette.com

a Y cable and a jumper between channels gives you different variations. I’m pretty sure there are resistors at the input stage which set the impedance so things further down the line don’t affect the pickups.

 

[0XI0]

because, for the same reason DS-1 started to sound like shit into literally everything, and everybody blamed the pedal, like it changed what it had been doing the same since 19fucking78, distortion adds so much godamn high end content, that if you don't knock off the excess at the front, de-emphasis, like 3v ac through a bunch of wires, before the clipping, there's too much on the backend. all these modelers don't have any real life input resistance, they try to get the "sound" with dynamics and bias shift but there's no tug. it feels lifeless coming out cause it's a hot dog down a hallway on the way in

 

[BenIfin]

because, for the same reason DS-1 started to sound like shit into literally everything, and everybody blamed the pedal, like it changed what it had been doing the same since 19fucking78, distortion adds so much godamn high end content, that if you don't knock off the excess at the front, de-emphasis, like 3v ac through a bunch of wires, before the clipping, there's too much on the backend. all these modelers don't have any real life input resistance, they try to get the "sound" with dynamics and bias shift but there's no tug. it feels lifeless coming out cause it's a hot dog down a hallway on the way in

So you're saying you like modeled Amps and EFX and think they respond just like the real things

 

[Lysander]

In the world or real-tube Amps, do their Hight / Low .... Normal / Bright 1/4" Inputs .... also have Input Impedance Values ?

Yes.

Also, can these values be looked-up somewhere ?

Not aware of any single list of input impedance for amps, but a Google search for specific models should give you enough information.

In general, input impedance for tube amps is rather high: 1MΩ or more, even for very old designs.

 

[DLC86]

hell yeah, the gain pot is a resistor, a variable resistor. infinitely variable input impedance. but it depends on what circuitry the guitar signal hits first in the chain, depends on the amp. open up revalver IV and open up schematic tweak mode on a clean fender, you can reduce the input capacitors/resistors value and all of a sudden it's a fire breathing high gain.

also if you run passive pickups through a paper clip into DSP distortion it loads the pickups down and feels like tube amp input stage sag/compression.

ClipToneTM

The gain pot has nothing to do with the input impedance, the impedance is set by the grid-to-ground resistor and in all amps schematics I've seen in my life it's always 1MΩ.

If you link two channels with a patch cable the impedance will be 500k cuz there will be two 1M resistors in parallel

 

[JiveTurkey]

Just would like to know ... curious.

I assumed you were getting ready to go split hairs in this vs. that land

 

[0XI0]

So you're saying you like modeled Amps and EFX and think they respond just like the real things

these do!

as long as you run them stereo

 

[JiveTurkey]

these do!

as long as you run them stereo

View attachment 32359

Ben's going to break out in hives from all that UA

 

[Humbug]

Typical input impedance for a tube amp is 1M and is set by the first grid leak resistor.

 

[bikescene]

1meg is a typical value of high inputs, even in really old amps like the Tweed Champ and Deluxe circuits. The 68k grid stoppers that set the low jack input impedance also go back to those Tweed amps too. I never looked at schematics of amps earlier than that.

 

[jay mitchell]

In the world or real-tube Amps, do their Hight / Low .... Normal / Bright 1/4" Inputs .... also have Input Impedance Values ?

Yes. In the case of Fender amps, Input 1 has a 1Mohm resistance, whereas Input 2 has a resistance of 136kohms (in addition to attenuating the signal by 6dB). This applies to both normal and reverb channels.

If so, is there a general value or does it vary from Amp-Input to Amp-Input ?

I Mohm is the most common, but there is no standardized value.

Also, can these values be looked-up somewhere ?

Yes. You can find schematic diagrams of most tube amps online. You'll need to have some very basic electronics knowledge to calculate input impedances from component values.

 

[jay mitchell]

hell yeah, the gain pot is a resistor,

The gain pot has absolutely no effect on the input impedance of an amp.

 

[bikescene]

I had a few minutes to take a look at the RCA tube book that the early guitar tube amps were supposedly based off of. Many of those designs to interface with magnetic phonograph pickups also used 1meg to set input impedance (R1), although some were 1meg pots.

http://www.tubebooks.org/file_downloads/RCA_HiFi.pdf

 

[Boudoir Guitar]

As usual, out of my technical depth here.

I understand that with *most* modelers these days, you can adjust the Input Impedance to varying values .... with most people usually setting it to "Auto" or 1M Ohm or letting it "take" the Impedance Value of the first Block in the chain.

In the world or real-tube Amps, do their Hight / Low .... Normal / Bright 1/4" Inputs .... also have Input Impedance Values ?

If so, is there a general value or does it vary from Amp-Input to Amp-Input ?

Also, can these values be looked-up somewhere ?

"real-tube Amps" vs "modeler" doesn't really have a distinction in this case. The "input impedance" on a modeler is defined by the analog circuitry prior to the DAC or anything else, and in a tube amp (or pedal in front of a tube amp) before any gain stage, tube or otherwise. I.e, to the extent anyone is worried that there is a difference between "real" input impedance, "tube" input impedance, "digital" input impedance etc., that conversation doesn't really make much sense.

 

[jay mitchell]

"real-tube Amps" vs "modeler" doesn't really have a distinction in this case.

Not in a qualitative way, but it really does matter.

The "input impedance" on a modeler is defined by the analog circuitry prior to the DAC or anything else, and in a tube amp (or pedal in front of a tube amp) before any gain stage, tube or otherwise.

Correct.

I.e, to the extent anyone is worried that there is a difference between "real" input impedance, "tube" input impedance, "digital" input impedance etc., that conversation doesn't really make much sense.

The concern is for matching, however. If you're interested in duplicating the sound and feel of a specific tube amp, and if your guitar is passive, then you really need the input impedance the guitar sees to match that of the tube amp. The tube amps I've used all have input impedances of 1Mohm, with some shunt capacitance due to the tube's input capacitance (multiplied by the Miller effect) and the cabling, so I'm able to get a good match with the two modelers I use, both of which have 1Mohm impedances. If you're trying to match a different amp - or a different input on the same amp - 1Mohm may not get you there. FYI, it's trivially easy to build a loading circuit to match an amp that has an input impedance lower than 1Mohm, so there's really no necessity for the modeler to take care of this.

 

[Boudoir Guitar]

Not in a qualitative way, but it really does matter.

I don't quite follow? To clarify, I didn't mean to imply that the value of the input impedance didn't matter. Just that 1Mohm input impedance is 1Mohm input impedance regardless of whether its a tube amp, a solid state pedal, or a digital whatever.