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Old 23rd July 2012, 01:48 AM   #1
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Default testing tubes

Hello,

I recently acquired a variety of pre and power tubes (everything from 12ax7 to 6v6), they are mostly from old reel to reel machines from the 60's and 70's.

I don't have access to a tube tester and would like to try them on my guitar amps, but don't want to risk damaging the amps.

Here is my question, IS there any risk to the amps if I plug these tubes in?
If so, would it be catastrophic or minor?
Is there any way to test tubes without a tube tester, say some simple DIY test circuit setup? I have some basic electronics and soldering experience.

Cheers
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Old 23rd July 2012, 03:11 AM   #2
TheGimp is offline TheGimp  United States
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The 12AX7s can be tested in a simple triode gain stage.

The 6V6s can be tested in a simple pentode gain stage with the screen tied to a fixed voltage per the data sheet.
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Old 23rd July 2012, 05:08 AM   #3
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by johnny audio View Post
Hello,

I recently acquired a variety of pre and power tubes (everything from 12ax7 to 6v6), they are mostly from old reel to reel machines from the 60's and 70's.

I don't have access to a tube tester and would like to try them on my guitar amps, but don't want to risk damaging the amps.

Here is my question, IS there any risk to the amps if I plug these tubes in?
If so, would it be catastrophic or minor?
Is there any way to test tubes without a tube tester, say some simple DIY test circuit setup? I have some basic electronics and soldering experience.

Cheers
In theory there could be a bad tube that will damage the amp. Some kinds of un-likely internal shorts come to mind. But in real life these kinds of pathological tube failures don't happen.

But simply placing the tube in an guitar amp is a VERY crude test and only gives you a very basic "thumbs up or down" result. Guitar amps have so much gain that even very weak tubes will make noise. This test will "pass" marginal tubes.

Best way to test is not to use a tube tester either. Simply bead board an simple triode gain stage and measure the gain with AC voltmeters. You can d the same with the power tubes. Use a resistor as a dummy load. All you need is two or three resistors and a power supply.
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Old 23rd July 2012, 02:21 PM   #4
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You can't hurt the amp by plugging ANY 12AX7 into it, even if it's shorted. Might damage your EARS... though, by creating serious hum.

6V6 is a POWER tube, so it's a different story. A shorted or gassy tube might blow a fuse or blow cathode or screen resistors (I'd say "big deal" - 50 cents worth of parts to replace...) Power and output transformers don't fail instantly (though the latter CAN burn up, instantly, if you crank the output with no speaker attached).

And if a tube SOUNDS good, it IS good, in that application... regardless of test reading. A 12AX7 works at a fraction of the current it sees in a tube tester... so a low-emission tube may work just fine.
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Old 25th July 2012, 02:54 AM   #5
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Thanks everyone for there responses. I'll plug these tubes in my amps and hear what all the fuzz is about 'vintage' tubes.

Thanks again.
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Old 28th November 2012, 07:01 PM   #6
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Quote:
Originally Posted by johnny audio View Post
Hello,

I recently acquired a variety of pre and power tubes (everything from 12ax7 to 6v6), they are mostly from old reel to reel machines from the 60's and 70's.

I don't have access to a tube tester and would like to try them on my guitar amps,

Is there any way to test tubes without a tube tester, say some simple DIY test circuit setup? I have some basic electronics and soldering experience.

Cheers

There has been many thoughts of a simple diy test setup for the most common 6,3V E-types / 12A.. types . ...looked around and found some ideas, ....which might be of some help?
The "Nos" 12AT7 Telefunken does it "leak"oxygen , or the two triodes...do they match?
Any ideas to the set-up schematic for ECC83 , 6SN7 and EL34 would be highly appreciated.

Two PSU's attached high voltage +300Vdc and neg. -120Vdc....DC heaters.

Schematic set-up link for ECC83, ECC82, ECC81, 6SN7 , EL34 :

https://picasaweb.google.com/1031761...22949922722402

https://picasaweb.google.com/1031761...35953886498290

https://picasaweb.google.com/1031761...36760178462642
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Last edited by Leakstereo20; 28th November 2012 at 07:07 PM.
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