I'm testing some 6EU7 tubes. I don't have a tester, so I rigged something up. Datasheet says with 150v plate and -1 grid, I should get around 1.2ma plate current. With three tubes I get between .9 and .5ma. What would the typical tester consider weak?
I can't tell you when a tube tester would say weak but I can tell you that no tube tester is 100% and the experts say the only real test is in a working circuit.
I had a Thomas Organ amp that still played. When I checked the tubes both of the 6v6 tubes tested as dead, didn't move my meter at all, but the organ still played fine.
A friend of mine played her marshal till the power tubes refused to make any noise, it faded very quickly like in 4 beats hahaha
I had a Thomas Organ amp that still played. When I checked the tubes both of the 6v6 tubes tested as dead, didn't move my meter at all, but the organ still played fine.
A friend of mine played her marshal till the power tubes refused to make any noise, it faded very quickly like in 4 beats hahaha
The tubes do work in the amplifier. I imagine negative feedback makes the circuit somewhat immune to the actual gain of the tubes.
All the "old farts" from the tube era say to change the tube when gm has dropped to 70% of new (book) value. Certainly this was the consideration when designing a tube circuit. It had to work with tubes down to 70%. That can be a bit of a problem these days when some modern production tubes are not really what they are sold as. Some JJ 7591 I bought recently tested at 70% of book value brand new out of the box. Will try the EH 7591 next time to see if they are any better.
This is the rule I apply when doing restorations on guitar amps (I do have an AVO MK3 Tube Tester), anything which tests below 70% gets replaced. On a recent Twin Reverb check all I did for a friend, all I did was replace the PI tube which tested at 65%. The owner insisted that it had improved the sound dramatically.
Cheers,
Ian
This is the rule I apply when doing restorations on guitar amps (I do have an AVO MK3 Tube Tester), anything which tests below 70% gets replaced. On a recent Twin Reverb check all I did for a friend, all I did was replace the PI tube which tested at 65%. The owner insisted that it had improved the sound dramatically.
Cheers,
Ian
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6EU7.....Guitar amp?
I once retubed a guys amp head and he was back the next day saying that I killed his tone. WTF? The old tubes were still on my workbench, so we started swapping and cranking. Guess what? The old half dead rectifier tube was giving him the sustain that he liked. It went back into the amp and he was happy.
Most circuits were designed to operate in a reasonably linear manner even as the tubes lost Gm. The operation outside the linear range will change considerably as the tubes age.
Consider your 6EU7 in a typical guitar amp preamp stage. The tube might have a 100K plate load resistor operating from a 250 volt supply voltage. With a 1.2 mA plate current the plate voltage is 130 volts. Overdriving this stage slightly will cause the tube to saturate on the bottom half of the wave first since there is plenty of headroom for the plate to swing upwards. This is usually a soft rounding of the wave.
If the tube is weak, drawing say .5 mA, the idle plate voltage will be high, near 200 volts, so clipping will occur on the top of the wave first, and it will me much sharper. This changes the "tone" of the amp.
Granted this is an over simplified explanation since overdriving a tube will cause grid current, thus shifting the bias and therefore the plate voltage, but the effects I describe are real and observable.
I make amps and I sold both of my tube testers. I rig up a real gain stage and test tubes for distortion. I save the good ones for building....AND the bad ones for designing and testing my circuits. Many of the distorted tubes test just fine in a tube tester, even a Gm tester.
I once retubed a guys amp head and he was back the next day saying that I killed his tone. WTF? The old tubes were still on my workbench, so we started swapping and cranking. Guess what? The old half dead rectifier tube was giving him the sustain that he liked. It went back into the amp and he was happy.
Most circuits were designed to operate in a reasonably linear manner even as the tubes lost Gm. The operation outside the linear range will change considerably as the tubes age.
Consider your 6EU7 in a typical guitar amp preamp stage. The tube might have a 100K plate load resistor operating from a 250 volt supply voltage. With a 1.2 mA plate current the plate voltage is 130 volts. Overdriving this stage slightly will cause the tube to saturate on the bottom half of the wave first since there is plenty of headroom for the plate to swing upwards. This is usually a soft rounding of the wave.
If the tube is weak, drawing say .5 mA, the idle plate voltage will be high, near 200 volts, so clipping will occur on the top of the wave first, and it will me much sharper. This changes the "tone" of the amp.
Granted this is an over simplified explanation since overdriving a tube will cause grid current, thus shifting the bias and therefore the plate voltage, but the effects I describe are real and observable.
I make amps and I sold both of my tube testers. I rig up a real gain stage and test tubes for distortion. I save the good ones for building....AND the bad ones for designing and testing my circuits. Many of the distorted tubes test just fine in a tube tester, even a Gm tester.
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