Amplifier input

[Hagstrom]

Hi,
I have found an old projector amplifier that I have start using as a guitar amp. I think it is a Bell&Howell. It contains 3xEF86, 1xECC83, 1xEZ81 and 3xEL84, all mullard valves.
I have only tested it by pluging my guitar into the mic-input, and it makes a nice sound (although clean).

Now the question:
1. I have learned that the guitar signal is much stronger than a microphone. Is it possible I can destroy something by using it this way? Must I (can I?) put in some kind of signal reduction circuit?

2. Are there other drawbacks by using the mic-input?

Gratefull for all advices.

/J

[Gold_xyz]

Hi,

Guitars pickups can generate from 50 to 500mV
http://www.dimarzio.com/media/pdfs/t...uick_guide.pdf

I think that nothing doesn't happen.
(but in every case it would be to see the schematics)
Often guitar amplifiers works in overdrive for to have a big
distorsion.
Also guitar have a volme control for reduce the output.

Before you plug your guitar in this amplifier (and every other)
check in every case a good ground/earth connection !

[Sherman]

You won't hurt anything by plugging in your guitar, at most you could overdrive the input stage (and therefore all the stages downstream) resulting in some distortion. Since you say the sound is clean that doesn't appear to be happening.

However... as Gold_xyz said, be VERY sure that your amp has a good ground or that the input jack is isolated otherwise you could be electrocuted if the jack becomes 'live'.

I strongly recommend replacing the (probable) two wire AC cord with a three wire cord and connecting the ground (a green wire) in the three wire AC cord to the metal chassis in the amp. I've changed a few 2 wire power cords to three wire cords on older amps for this reason. At least one amp was giving the player shocks if the unpolarized 2 prong plug was plugged in the wrong way.

After replacing the power cord, have fun!

[Hagstrom]

Thank you for replies.

Yes, I have thought about grounding the metal chassis but I didn't know if it was common practise to do so on amplifiers; now when I know I will definitely do so since I already have a three-wired AC-cable.

A question about the distorsion:
If I want some distorsion, is it possible to turn up the volume control on the guitar to make the input signal strong, and then bring down the volume with the volume control on the amp?
Or will this just reduce the input signal to the same level it would have if I did the other way around?
I have tried and the sound does change, but I'm not sure if it is for the better or if it is actually distorting...

I guess this depends on where in the amplifer chain the amp volume control is located, right?

/J

[SemperFi]

Dont worry about overdriving the amp, that is how you get distortion. Th output stage at full clip actually stresses the tubes less than at low volumes.
That amp is voiced for HiFi, and may sound good with an acoustic and perhaps jazz with an electric, but too muffled for rock or anything else. Try small valued cathode caps, like 0.33microfarads in paralell with the cathode resistors. This will increase gain in the highs and give better sparkle with an electric guitar.