Marshall 8040 (V40) problem

[Mesjoggah]

Hi, i'm new to this forum and got a question.

Last week i had my weekly jamsession with a friend of mine, we always play a couple of songs and in one of them i play the bassguitar. since i dont have a bass-amp i plug it into the marshall and play it at low volume to make sure it doesn't blow up. Accidentaly i stepped on my equalizer and it got a boost and i immediately kicked it out and finished the song. later i plugged in my normal guitar and i noticed it sounded different then it did before, i don't know what it is but somethings missing.

I checked the speaker with a multimeter, it's a 4ohm speaker and the meter said 3.8 so i think that's okay. Then i did a test with a battery to the + and - and the cone got away from the magnet so that's okay too i guess. I did a visual control and saw no breaks or other damages, after these control i think the speaker is okay.

Later i realized that when i turn the volume down to 0 i can still hear myself play at a low volume.

Anyone has a clue?

[JohnR242]

Tube maybe.

Well, I would have to wonder if the boosted bass input rattled the preamp tube (Valve). That could be the reason why your regular guitar sound is not as it used to be and why you can still hear some sound with volume all the way down.
Hopefully it has not broken down any caps.
Try another tube 12ax7 is what is used in this amp I believe. You could also test it with a 12AT7 or a 12AU7 if you have them and not the 12AX7. The only difference is that the AT and AU are a lower gain. They all have the same pin outs.
Playing Bass through an amp designed for guitar is rather risky as they are not happy getting very low frequencies thrown at them. Low volumes may let you do it but your still safer using an amp designed for a bass IMHO.

Hopefully the tube (Valve) acted like a buffer and took the shock.
Good luck.

[Mesjoggah]

Thanx, I will give that a try, i have another 12ax7 lying around here somewhere so that's easy to do.

[Rob Mercure]

Mesjoggah,

First off don't worry about playing bass through a guitar amplifier the lowewr frequencies won't hurt the guitar am but the guitar amp may not have low enough frequency response to pass them - when I design a guitar amp I aim for a cutoff just above the power line frequency (60 Hz for me, 50 Hz for you I think) to limit hum. But....DO NOT under any circumstance play the bass guitar through a guitar speaker! The guitar speaker isn't designed for the long cone excursions of bass frequencies and I've seen several over the years that were damaged in this manner - and, unfortunately, this may why your speaker sounds "different." Do you have another speaker to test the amp through to determine whether the problem is the amp or speaker?

Now as far as hearing the guitar with the volume control turned down - and I assume you meant the amplifier's volume control - this means that when the control is turned all the way down the rotating contact isn't touching the grounded terminal and fully shorting out the guitar signal. You probably need either the control replaced or cleaned. Your control probably would have manifested the same symptoms before using the bass.

But reality is complicated and it probably is a good idea to make sure all the tubes are seated and plugs and jacks inserted just in case I'm wrong and the bass vibrations have rattled something. But please save yourself some worry - and probably future expense - and don't play bass through the guitar speaker. If you need to play bass on a song switch the head to a bass speaker.

Rob

[Mesjoggah]

Mesjoggah,

First off don't worry about playing bass through a guitar amplifier the lowewr frequencies won't hurt the guitar am but the guitar amp may not have low enough frequency response to pass them - when I design a guitar amp I aim for a cutoff just above the power line frequency (60 Hz for me, 50 Hz for you I think) to limit hum. But....DO NOT under any circumstance play the bass guitar through a guitar speaker! The guitar speaker isn't designed for the long cone excursions of bass frequencies and I've seen several over the years that were damaged in this manner - and, unfortunately, this may why your speaker sounds "different." Do you have another speaker to test the amp through to determine whether the problem is the amp or speaker?

Now as far as hearing the guitar with the volume control turned down - and I assume you meant the amplifier's volume control - this means that when the control is turned all the way down the rotating contact isn't touching the grounded terminal and fully shorting out the guitar signal. You probably need either the control replaced or cleaned. Your control probably would have manifested the same symptoms before using the bass.

But reality is complicated and it probably is a good idea to make sure all the tubes are seated and plugs and jacks inserted just in case I'm wrong and the bass vibrations have rattled something. But please save yourself some worry - and probably future expense - and don't play bass through the guitar speaker. If you need to play bass on a song switch the head to a bass speaker.

Rob

Thanks for your reply, i know it's dumb playing bass through a guitar speaker -won't do that again.
To get back to your answer, i do have another speaker but it's 8ohm while the other speaker is 4ohm, i don't know if that's a problem? and i am not gonna try before i know for sure, i don't want the risk of having two fried speakers