Testing Amp Valves

mpc

I have quite a few amp valves and I have lost track if they are new or used or even faulty, what's the best way of testing them to see if they are still good to use - other than going to a tech to do?

Fingers657

Might be worth getting a Valve tester.

ecc83

How technical;/practical are you MPC? You could buy s cheap valve amp and mod it so that you could measure things like whether the valve biases to the right anode current at the right sort of grid voltage. You would need at least two digital multimeters.

Pre amp valves are easy. Just build the standard circuit and check anode and cathode volts then listen to it!

Dave.

inewham

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mpc

Fingers657 said:

Might be worth getting a Valve tester.

such as......?

mpc

ecc83 said:

How technical;/practical are you MPC? You could buy s cheap valve amp and mod it so that you could measure things like whether the valve biases to the right anode current at the right sort of grid voltage. You would need at least two digital multimeters.

Pre amp valves are easy. Just build the standard circuit and check anode and cathode volts then listen to it!

Dave.

sadly im not that technical, but I am willing to have a go if I am told what is required etc..

goldtop

Alas, valve testers (affordable ones, which are the only ones that are easy to understand) are almost useless. The Orange one was a decent stab at doing something more clever, but it seems to have been killed off. IIRC there were some reliability issues. 

mpc said:

ecc83 said:

How technical;/practical are you MPC? You could buy s cheap valve amp and mod it so that you could measure things like whether the valve biases to the right anode current at the right sort of grid voltage. You would need at least two digital multimeters.

Pre amp valves are easy. Just build the standard circuit and check anode and cathode volts then listen to it!

Dave.

sadly im not that technical, but I am willing to have a go if I am told what is required etc..

It's not the sort of thing that can be quickly and safely conveyed in text, unfortunately. 

If the valves are Mullards, Brimar, Telefunken, etc (i.e. potentially valuable) then find a tech. Or:

• try them in an amp
• if they work, sell them on Ebay

ICBM

The problem with proper valve testers is that they're designed for a different purpose than what's needed to see if a valve is good for use in a guitar amp - they're for measuring the valve characteristics for matching and grading, and although they do have basic testing functions, they will only show up if a valve is definitely dead, not whether it will survive at the much higher voltages and currents in an amp, or whether it's noisy or microphonic. A valve can easily 'test good' on a tester but fail in an amp, or just not sound any good.

I have two 'valve testers' - for preamp valves, a Mesa V-Twin pedal. This operates the valves at realistic valve-amp voltages, and with it connected to an amp I can easily listen to the gain, tone and noise of the valve and tell if it's good or not very quickly, and tap it to check for microphonics. For power valves, I have an old valve PA amp which has a faulty output transformer (it arcs internally at full power), and wasn't worth much anyway, so I converted it into a tester with a variable bias control with much wider range than normal, hugely oversized screen resistors to make sure that they don't fail before I can turn off at the first sign of trouble, and meter points for valve current. With the variable bias control I can deliberately run the valves at up to 100% of rated dissipation if I want, and with the output connected to a speaker cab I can listen for noise and tap the valves for microphonics. It's rather Heath Robinson though, and I wouldn't recommend doing this unless you're very familiar with working around dangerous voltages.

ecc83

Taking VERY GREAT NOTICE of ICBMs safety warnings, if you can find a beat up but working amp that uses octal OP valves it is pretty easy to make up adapters to plug in B9A valves like the EL84. (I have such an adapter made up somewhere, free if anyone wants it) You COULD adapt from B9A to octal but B9A plugs are hard to find and you can't make them from smashed valves!

The man is right. You need to run audio through the valve and drive the ***t out of it and watch for fireworks.

Dave.