Marshall Model

Cannoy identify what kind of valve goes into my old valve guitar amplifier?

I acquired an old valve guitar amp not long ago and i think it needs new valves trouble is the valves that are in it have no writing left on them and there is one valve completely missing how can i find out what type of valves to put in it. Could i use a multi meter to see what voltage etc is going into the valve socket and then find a valve with matching voltage.
Thanks for your help Marc
the amp is home made and needs new vakves but the valves in it are old and say nothing on them and one valve is missing so how do i know what valves to put in it

Hi Marc,
Replacing the tubes in the amp is actually far less complicated than it seems.. but a quick word of warning first.

Whatever you do, dont go probing around the back of the amp with a multimeter. The voltages inside amps are absolutley LETHAL (even when completely unplugged), and if you touch the wrong spot it will all turn sour very quickly.. for the amp and most importantly yourself..Get an amp tech to show you how at the very least..

OK so as far as the valves go.....
There are two main types.. the pre-amp valves (the small ones - usually 3 or 4 next to each other). And there are the power-amp valves (The larger ones, either in a pair or as quartet).

The pre-amp tubes shape the tone of the amp, while the power tubes amplify the signal to a nice loud volume, and also shape the tone to some extent when the amp is nicely cranked to a high level.

As far as the pre-amp tubes go... you just need to use the same 'type', which 9 times out of 10 will be a "12AX7" valve also known as an "ECC83" valve in England... Same thing, just a different name.

You can swap and mix the pre-amp valves as you wish, and as long as its generally a 12AX7 tube, you can use any brand.. (although some brands produce better sounding tubes than others)... (you can also safely change them yourself without any hassle.. just like changing a light globe.. almost).

If you want the amp to really sound nice, you can replace all the pre-amp tubes with some decent new ones.. (Depending on whats already there)..

******************************************
Then there are the Power Valves.. These are the larger sized ones and will usually be one of 3 or 4 types of valve.
a) EL34 (common in Marshalls)
b) 6L6 (common in Fenders)
c) EL84 (seen in Vox, more treble, less volume)
d) KT66 or KT88 (seen in some marshall models)

Each one has a fairly different shape and size and are quite easy to tell apart..Just look up some pics on the web.

You need to replace the tubes with the same type u already have in the amp, (usually) .... so if it takes 6L6's then u need to get a set of 6L6's. If they are EL34's then u need to get a pair of those...(any amp tech or guy in a guitar shop will be able to identify the valve type if you are having probs).

There are a few very important steps when putting in new power tubes though..
1) Always replace BOTH valves (or all 4) at the one time.. dont mix and match.

2) You will need to get an amp technician to adjust the "Bias" on the amp depending on the type of tube and power rating it has. This allows the amp's idle power to be adjusted to the right level.. If its too low the amp will sound awful, but if its too high, the valves will burn out quickly.. so its about finding the acceptable range to put it into.. This is essential when replacing the power valves and will help get the best life and tone out of your amp.

Also dont use the amp with one of the tubes missing.

Putting new valves in can be a very rewarding exercise and will give a new lease of life to a dull old amp. There is a huge selection of different brands and optins u can chose depending on the type of sound you want and the size of your wallet. but you can get good quality pre-amp tubes for around $20 each. Power valves for around $50-$100 a pair.

Some good 'brands' to look out for are: Groove tubes, JJ Tesla, Sovtek, Electro-harminox, Svetlana..
Like strings, its all about personal preference..

Good luck!
Tsafan

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