How to test Guitar Amp Without a Guitar?

[Mikay]

Hi, Have been given a Guitar amp cab (Marshall JCM1C) and wont have a guitar for a little while.

I just wanted to test it to make sure its functioning but without a guitar struggling a bit.

I have plugged my phone into it using a 3.5mm jack to the 6.35mm input on the front but the sound it produces is horrible.

Just trying to determine why it sounds awful and to determine if the valves need replacing. Uses 2 x EEC83 preamp and 1 x EEC82 power valve.

Or perhaps it would sound awful since im plugging a phone into it rather than a guitar!

[Chuck H]

While it's impossible to tell if it's a good sounding guitar amp or a bad sounding guitar amp without plugging a guitar into it, it's entirely possible to test it's function and affect repairs without a guitar. You need a signal generator, a dummy load, a DMM and an oscilloscope. DO NOT start swapping tubes, changing all the film caps to NOS Mustards, performing popular "mods" or any other such shenanigans. Just treat it like any other electronic appliance and test/repair it for function.

[Enzo]

Your phone is probably just creaming the input. I had a guitar next to me the whole time I worked on amps, but a guitar gets in the way and takes up my hands. SO mostly when I just want to check out an amp, I used music. A CD player (a real one, not one with headphones), a cassette deck, or even just a stereo receiver tuned to a music station. I use the line out from the receiver or from those players.

The music is a full range sound, so I can test the tone controls as well as the volumes. I can hit the reverb with music, and it reverbs or it doesn't. Same with trem, I can hear it working.

What it won;t do is tell me some subtle nuance of guitar tone is to my liking. But it is fine for functional checks.

[oc disorder]

Mikay, there may be a few reasons why your phone sounds so um.. bad plugging it in like that.

Welcome to the forum and BTW the sun has risen here in Aust. in case anyone was wondering !

Your phone or mp3 player for that matter puts out a stereo signal .
Ignoring the fancy 3'rd segment for switching or whatever on some phones the mini jack carries 2 signals Left and right with a common ground which is usually the barrel part of the jack usually known as sleeve.
The left appears on the tip and right on the ring of the jack.
A guitar input is just a tip and sleeve so a stereo lead mini jack to 6.5 stereo jack plug could have its ring (right signal) shorted which the phone may not like. Often just one channel of a stereo can sound quiet um.. weird on its own.
As well as the above possibility, the phone is churning out quite a strong signal intended to move headphones or ear buds.
This is way more than a string vibrating over a magnet can generate so the phone can be overloading the first stage of the guitar amp.

As well as that there are impedance issues which can be complex which can alter the sound.

I couldn't find much on the web . It either seems too simple without examples or over complicated.
eg
How to plug your iPOD (or MP3) into a guitar amp | Bloke's Post

microphone - matching input and output impedance in an audio circuit - Electrical Engineering Stack Exchange

Line signal to microphone input

I knocked up this basic circuit which could help.

Basically it mixes the right and left together with 2 x 10k resistors then feeds a mono volume control which you can adjust
down to a typical guitar level. I just guessed the values but it should work. Someone may suggest alternate values but the basic
principal is the same.

This should make it sound a bit better but a guitar amp is not exactly a hi-fi amp!

[Tom Phillips]

Hi, Have been given a Guitar amp cab (Marshall JCM1C) and wont have a guitar for a little while...

Consider taking the amp to a guitar store and ask to try a few guitars in your price range with your amp. You are being honest that you are planning to get a guitar. Right? Someone in the store may even play guitar through your amp and give you an opinion.