• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Checking voltages at pins in vacuum tubes

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apaulr

Member
Joined 2007
#1
Is removing the tubes and checking each pin for voltages an acceptable practice? If using the the negative speaker terminal as ground is it still necessary to have a load connected during this test? Other than the precaution for high voltages what are other concerns?
Thanks
Paul
 
#2
Tube amps should always be operated with a load. If you pull a tube, the voltages at the socket pins can change,
because the tube is gone. Here's an example of what you can use instead, if you can't get inside the chassis.

However, sometimes the speaker ground, or 0 ohm, output terminal is not actually ground. And be careful,
use one hand only, the other in your pocket. If no input signal is needed, use a shorting plug instead.
https://www.surplussales.com/Tube-Sockets/TubeSkts-1.html
 

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#6
You can buy or build just about any kind of tube test adapter from stuff off Ebay. Unfortunately the classic stuff (Vector, Pomona, ...) has sky-rocketed in price since the pandemic hit. Used to be like $7 for an adapter a couple of years ago. Making your own adapters from Chinese parts is still reasonable. Some test adapters can do pin current tests as well as voltage pin tests. (like the big right side capped tube in the pic below. A Vector T80C with current probe inserted in one pin. Some other Vector 7 pin, 9 pin, Novar, Compactron, and Octal current test adapters in the clear plastic boxes too.) One of these V and I test adapters can replace a big box tube tester easily for gm or mu tests, by the way. (with a signal gen, power supplies like Heath IP-17, and a couple of DVMs) You can also use them to calibrate a tube tester. So much cheaper than a real tube tester, and more accurate.


DSCF7176.JPG
 
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#7
Thank you all for your responses. I was able to test the voltages at the pins with all tubes removed and found the tube that was not getting the required plate voltages. An open 5 Watt resistor was the reason. But now I know whats available out there for tube testing.
Regards
P
 
#8
Pulling the valve and testing at the socket can be usefull to check if you have heater voltage, HT/B+ etc but if you've pulled an OP valve for instance,that may mean your HT/B+ goes up exceeding components max voltage specs. It is possible if your carefull to lift the valve a few mm or 1/8" or so and get to valve pins, but it's fiddly and your liable to probe slip. Lastly you can get little 4mm banana sockets to use as TP's or test points, for bias voltage etc,that and a handy ground point makes checking testing easier.

Andy.
 
#10
"Resisters don't usually fail for no reason".
I agree. I pulled the resistor from the board and it do not test at all. These resistors run hot, so hot that the pcb board is discolored in its vicinity. I swapped the same value resistor from the good amp and the amp worked. Based on comments I have seen so far it seems prudent to do more checking.
Regards
P
 
#11
Diabolical Artificer said:
Pulling the valve and testing at the socket can be usefull to check if you have heater voltage, HT/B+ etc but if you've pulled an OP valve for instance,that may mean your HT/B+ goes up exceeding components max voltage specs. It is possible if your carefull to lift the valve a few mm or 1/8" or so and get to valve pins, but it's fiddly and your liable to probe slip. Lastly you can get little 4mm banana sockets to use as TP's or test points, for bias voltage etc,that and a handy ground point makes checking testing easier.

Andy.
Click to expand...
Hello Andy. The component side of the board is very accessible and I will certainly and carefully check the voltages with the tube inserted from that end. The multimeter probe has an attachment that insulates it along its length only allowing about a millimeter of the probe for placement on the test spot. This amplifier also has a dedicated ground test point in which the negative probe is inserted. I will follow the one hand behind the back protocol.
Thanks for your concern , very much appreciate it.
P
 
#12
My building partner, a generation younger, married, and having something to live for, has introduced me to a cool modern concept:

DVMs are cheap and cheerful - use lots. Velcro or hot-glue or whatever a bunch of DVMs to a board. Connect said DVMs to everywhere you need to measure at one time. Not enough DVMs? Easy: use more. Don't apply power until you're well away, and after power down don't touch anything until your meters tell you in at least two ways that it's safe to do so.

Hands in pockets works as long as all assumptions are still true. Do you wear a ring or wristwatch? Then the assumptions aren't true. Just an example. There is an optimum level of paranoia about any dangerous activity. We're usually not paranoid enough about driving cars, or bad voting. But we're scared shirtless of bear attacks - yeah, that'll happen.

All good fortune,
Chris
 
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