Recycling tubes

[Kiira Triea]

Hi Tony,
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| A little while ago I asked the list about information about valve
| amplifiers. I am thinking of doing a valve project, perhaps a
| pre-amp. If I have some old valve amps, can I use the tubes inside? I
| know they don't last forever, how can I test if they are alright? I
| read somewhere that hum will increase as a valve amp gets older, is
| this true?
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I am just finishing up a preamp project... it uses a basic cascode
circuit and has balanced inputs (w/ Newcomb transformers) and
has 48v. phantom power. There are circuits on the web for power
supplies of various types and if you want a balanced input for mics
you should contact Jensen... though I guess there are other resources
there. Jense has a collection of app notes and schematics they will
send you for free. 

As far as your tubes go.. if the heaters glow ok and they are not
microphonic then they are probably ok for testing. Tap them gently and
see if you get any sound out of your speakers to test for
microphonics. Otherwise you will probably want a set of matched tubes
for your final product. 

Oh.. you probably know this but be darn sure to discharge those filter
caps after turning the amp off to resume working on it. :-) 

Good luck and write if you have any questions... 

Kiira