I have an early 60s Philco tube tester. It's a beautiful piece of industrial art if you ask me. The quality of the components is top notch. It looks really steam-punk too.
However.....I was fixing a friends guitar amp. Kind of a hybrid - a solid state reverb driver and effects loop implemented with op-amps, but all the rest is tube and circuit board mounted everything to my chagrin. (The Chinese have figured out how to make a tube amp inexpensive)
By the third stage the gain dropped off dramatically. In fact, on ten, the amp whispered. This was apparent using my o-scope.
This thing was giving me da bizness!
I removed all tubes and tested them - all good according to trusty Philco.
Finally I decided to measure bias voltages.
The third stage plate read 1.3 volts - ahah ---I measured the plate resistor in circuit- infinite R, it had gone open I ASSumed. I replaced the resistor and still a whisper. I measure the plate resistor it read 150k properly...what? Ok, you can't measure resistance in circuit - I should know better.
Finally in desperation I replaced the tube and voila all was good.
I took the tube (12AX7) and tested in my Philco. All measurements were good. The gain ws as good as other 12AX7s. I put the tube back in the amp - no sound. I put the tube in another amp - no sound. I put the tube int he tube tester again - measure good.
My world is rocked!
So much for my Philco.
Anyone care to comment on tube testers?
It is now industrial art only and has no other value.
I will hang it on my lab wall.
However.....I was fixing a friends guitar amp. Kind of a hybrid - a solid state reverb driver and effects loop implemented with op-amps, but all the rest is tube and circuit board mounted everything to my chagrin. (The Chinese have figured out how to make a tube amp inexpensive)
By the third stage the gain dropped off dramatically. In fact, on ten, the amp whispered. This was apparent using my o-scope.
This thing was giving me da bizness!
I removed all tubes and tested them - all good according to trusty Philco.
Finally I decided to measure bias voltages.
The third stage plate read 1.3 volts - ahah ---I measured the plate resistor in circuit- infinite R, it had gone open I ASSumed. I replaced the resistor and still a whisper. I measure the plate resistor it read 150k properly...what? Ok, you can't measure resistance in circuit - I should know better.
Finally in desperation I replaced the tube and voila all was good.
I took the tube (12AX7) and tested in my Philco. All measurements were good. The gain ws as good as other 12AX7s. I put the tube back in the amp - no sound. I put the tube in another amp - no sound. I put the tube int he tube tester again - measure good.
My world is rocked!
So much for my Philco.
Anyone care to comment on tube testers?
It is now industrial art only and has no other value.
I will hang it on my lab wall.
Fix it!...let me guess, it has tubes inside
What a vicious circle.
------------------------------------------------------------------------Rick...
What a vicious circle.
------------------------------------------------------------------------Rick...
I don't know about your tube tester, but in general, they are not foolproof....😉
Why do vacuum tube testers often falsely indicate a tube is 'good' as often as they do? Why hasn't someone invented a better tube tester? - Quora
" 1. La mayoría de los probadores de tubos de vacío no prueban el tubo con precisión en condiciones de trabajo.
La mayoría de los probadores de tubos que existen son probadores de emisiones de producción económica que no prueban con precisión el tubo en las mismas condiciones de trabajo que experimentan en el circuito. De hecho, ni siquiera prueban la medida más básica de la funcionalidad de un tubo: la conductancia mutua.
Estaban destinados a proporcionar pruebas rápidas de “Pasa / No Pasa” para las personas que realizan reparaciones y los aficionados. Simplemente prueban si las emisiones del cátodo cuando se aplica una pequeña cantidad de voltaje.
Cuando un probador de emisiones prueba un tubo, los resultados deben venir con una advertencia: "Hasta donde nuestra prueba limitada es capaz de revelar" .
Por lo general, puede saber si un probador de tubos es de uso limitado si el medidor que indica los resultados de la prueba no tiene unidades de conductancia mutua (µmhos o micromhos), sino que tiene un medidor que simplemente va de "malo" a "bueno" que parece esto: "
Why do vacuum tube testers often falsely indicate a tube is 'good' as often as they do? Why hasn't someone invented a better tube tester? - Quora
" 1. La mayoría de los probadores de tubos de vacío no prueban el tubo con precisión en condiciones de trabajo.
La mayoría de los probadores de tubos que existen son probadores de emisiones de producción económica que no prueban con precisión el tubo en las mismas condiciones de trabajo que experimentan en el circuito. De hecho, ni siquiera prueban la medida más básica de la funcionalidad de un tubo: la conductancia mutua.
Estaban destinados a proporcionar pruebas rápidas de “Pasa / No Pasa” para las personas que realizan reparaciones y los aficionados. Simplemente prueban si las emisiones del cátodo cuando se aplica una pequeña cantidad de voltaje.
Cuando un probador de emisiones prueba un tubo, los resultados deben venir con una advertencia: "Hasta donde nuestra prueba limitada es capaz de revelar" .
Por lo general, puede saber si un probador de tubos es de uso limitado si el medidor que indica los resultados de la prueba no tiene unidades de conductancia mutua (µmhos o micromhos), sino que tiene un medidor que simplemente va de "malo" a "bueno" que parece esto: "
Attachments
Even your test equipment has to get tested.
Look up calibration instructions if you can.
Also use other known good tubes if unsure.
Look up calibration instructions if you can.
Also use other known good tubes if unsure.
You can assume the tester sees good when the tube is bad, But it is also quite possible the tube is making good contact in the socket of the tester, but not in the amp. A different tube into the amp might touch slightly different surfaces within th socket.
Tube testers are not good at telling you a tube is GOOD, they are good at telling you a tube is BAD.
If you want to learn something, put the bad tube back in the amp and find out HOW it is misbehaving. Is there zero ccurrent through it? Is one side not heating?
Tube testers are not good at telling you a tube is GOOD, they are good at telling you a tube is BAD.
If you want to learn something, put the bad tube back in the amp and find out HOW it is misbehaving. Is there zero ccurrent through it? Is one side not heating?
A channel on my amp started to distort a long time ago.
Alternating the same with others of the same type, I detected a faulty tube simply by listening.
There were no doubts.
A friend has a vintage tube tester so I took it. It didn't detect anything wrong so I resold the pins (they may have cold welds) but it didn't fix it. I bought a KT 88 Shuguang which is the same brand my amp uses and the sound returned to normal.
I wonder if modern tube testers, with more functions, would have detected what is the fault that was causing the distortion ......
Tube tester - Wikipedia
Alternating the same with others of the same type, I detected a faulty tube simply by listening.
There were no doubts.
A friend has a vintage tube tester so I took it. It didn't detect anything wrong so I resold the pins (they may have cold welds) but it didn't fix it. I bought a KT 88 Shuguang which is the same brand my amp uses and the sound returned to normal.
I wonder if modern tube testers, with more functions, would have detected what is the fault that was causing the distortion ......
Tube tester - Wikipedia
You probably did it but just in case did you test both halves of the 12AX7?
I also tested some old tubes from a 1934 radio, one of the 45 output tubes tested good but would osscilate. On close inspection there was a hairline crack near the base of the tube.
I also tested some old tubes from a 1934 radio, one of the 45 output tubes tested good but would osscilate. On close inspection there was a hairline crack near the base of the tube.
Enzo: "Tube testers are not good at telling you a tube is GOOD, they are good at telling you a tube is BAD."
Exactly right! I have heard of many tubes that tested good on a tube tester but didn't work properly in circuit. I don't think I've ever heard of a tube that tested BAD and still worked fine in circuit!
Exactly right! I have heard of many tubes that tested good on a tube tester but didn't work properly in circuit. I don't think I've ever heard of a tube that tested BAD and still worked fine in circuit!
Thanks for the input folks.
Indeed I have swapped the bad tube into another amp and it's bad there as well. Both sides test good on the tube tester. I've inspected/burnished/straightened the pins on the bad tube - still bad.
By bad I mean little or no sound.
No wonder - tests with a meter show the plate voltage at ~ 3 volts. The plate is supplied with current via a 150K resistor from a 350 volts source. It's operating point has shifted dramatically apparently yet my tube tester says it's good.
Indeed I have swapped the bad tube into another amp and it's bad there as well. Both sides test good on the tube tester. I've inspected/burnished/straightened the pins on the bad tube - still bad.
By bad I mean little or no sound.
No wonder - tests with a meter show the plate voltage at ~ 3 volts. The plate is supplied with current via a 150K resistor from a 350 volts source. It's operating point has shifted dramatically apparently yet my tube tester says it's good.
BTW there is no calibration procedure. There's a good/bad test that involves looking at the voltage at each tap of the multi-tap transformer. A typical tube test procedure has you varying the primary current of this transformer via a big precision wire wound rheostat.
I kind of suspect that the amp in question has an issue with the tube socket itself.
are the connectors tight on ALL pins of the "bad" 12AX7?
Because another tube might work if its pins are slightly differently angled/bent.
Perhaps replacing the tube socket would be wise, or "pinch" the connectors tighter.
are the connectors tight on ALL pins of the "bad" 12AX7?
Because another tube might work if its pins are slightly differently angled/bent.
Perhaps replacing the tube socket would be wise, or "pinch" the connectors tighter.
The Philco is probably an emission tester that checks for electrons boiling off the heater/cathode, essentially cathode/anode resistance. It doesn't check for gain etc. It may have a leakage/short test. The more premium/exotic testers check for gain.
However I read in a book from the end of the commercial tube era on getting reliability from tubes that you should only check tubes that checking in circuit performance shows may be bad. The probability of the tube getting degraded by testing (physical damage/electrical stress) causing premature failure is higher than the benefit of testing what is a working tube stage.
The old testers in grocery stores were great at generating sales but not at evaluating tubes. Same for service tube testers. They were good profit machines.
There are many tube circuits (e.g. Tektronix deflection amps) that will work to spec with marginal tubes. Its just good design.
However I read in a book from the end of the commercial tube era on getting reliability from tubes that you should only check tubes that checking in circuit performance shows may be bad. The probability of the tube getting degraded by testing (physical damage/electrical stress) causing premature failure is higher than the benefit of testing what is a working tube stage.
The old testers in grocery stores were great at generating sales but not at evaluating tubes. Same for service tube testers. They were good profit machines.
There are many tube circuits (e.g. Tektronix deflection amps) that will work to spec with marginal tubes. Its just good design.
Does it have a model name/number?
Do you know how it works? If not, this isn't "faith", this is Steve Winwood, Eric Clapton, Ginger Baker, and Ric Grech (Blind Faith).
As said, an emission tester will "OK" a resistor or a ripe banana (if you can fake the pinout). The Philco 9100 would actually test a tube for gain, but real testers cost more and have never sold well.
It was common, with a lot of those "self service" store tube testers to find a "bad" tube, indeed, to boost sales.
They were in drug stores, markets, etc.
In some stores, like a high-fi shop, the counter person even had a hidden switch behind the counter, to make a tube "look bad" and promote a sale.
Evil as it may sound, it was just another marketing ploy to make tube manufacturers some extra money.
Like today, it's all about "cash flow".
Yes, the OP shouldn't have any faith to start with.
There're 3 categories of tube testers according to their theory of operation:
- "Emission":
Cook the cathode and see if any electron come out of it. These may kill your tubes. - "Dynamic" or "Transconductance"
In the US, the Hickok brand was popular even though they used a most barbarous method: apply arbitrary AC to the grid and the plate, and show what they think is the Gm thanks to a big fudge factor. These may also be hazardous to your tubes. I own a Weston 981 and a Precise 111, but I wouldn't recommend them. - "Laboratory"
The only good answer: basically 2-3 variable DC voltage sources so you can set the parameters by the (data) book. Problem is, they're well into the K$ region, unless you DIY. I own a Metrix U61B.
Note that there're a number of suspects like the AVO: they're similar, but they use AC instead. Avoid.
As regards obsolete technology instead of checking up tube websites I went to big business ones and while old SS technology is being dispensed with I did find (surprisingly ) a comment on watching the rising market in tube technology .
As regards "avoiding " AVO VCM.s ---- have you read the technology details on why it was designed to be used with AC and the good accuracy ?
I myself have renovated an ex government AVO VCM MK4 and the high wattage resistor used for determining accuracy I have reduced from 0.5 % down to 0.1 % including fitting a much larger wattage to lessen any heating effect.
At the time of the designs making a fully DC unit would have required 1950,s power supply technology which apart from being very large would cost a fortune .
FYI-- my tube tester is now selling for £1500 online , I originally paid £500 for it.
But please read the design engineers comments on the quality and reasoning .
Laboratory ??--have you seen the prices ?-- I have visited a UK valve manufacturer and their testers had a dial for every conceivable test you could think of but its price was miles outside the general public's reach.
As regards "avoiding " AVO VCM.s ---- have you read the technology details on why it was designed to be used with AC and the good accuracy ?
I myself have renovated an ex government AVO VCM MK4 and the high wattage resistor used for determining accuracy I have reduced from 0.5 % down to 0.1 % including fitting a much larger wattage to lessen any heating effect.
At the time of the designs making a fully DC unit would have required 1950,s power supply technology which apart from being very large would cost a fortune .
FYI-- my tube tester is now selling for £1500 online , I originally paid £500 for it.
But please read the design engineers comments on the quality and reasoning .
Laboratory ??--have you seen the prices ?-- I have visited a UK valve manufacturer and their testers had a dial for every conceivable test you could think of but its price was miles outside the general public's reach.
The Metrix U61 really is a big box with 4 stabilized DC supplies for G1, G2, G3 and A, including metering and patching. Its last known list price was 3100 French Francs in 1967, about €4000 today; so I guess it's a weight class or 2 above the VCM. I paid €306 for mine, non working, but devoid of any terminal ailment: just a deep wash and replacement of all the 'lytics with MKP's.
It'd be interesting if some brave souls start a cloning project, possibly with SMPS's. A few things that can be improved upon:
It'd be interesting if some brave souls start a cloning project, possibly with SMPS's. A few things that can be improved upon:
- A dedicated circuit to measure Gm directly
- A regulated heater supply, can be off the shelf