Evaluating Signal Level Capacitors in Vacuum Tube Amplifiers

The final of four videos made to help evaluate signal level capacitors typically used in vacuum tube audio amplifiers.

[UncleDoug]

Another excellent video, David. I really like the simple, in-series-with-ammeter, cap testing method, and will definitely try this in the near future.

When I created the capacitance-to-ESR data and chart for non-polar caps, as shown in your video, it was mostly out of curiosity. I first needed to create a data base for "normal", i.e. relatively new and fully functional, capacitors, and then determine if faulty caps would deviate from that norm. To date, as you noted, I have not found the deviation (if indeed any was noted) to be adequate for diagnostic purposes. As you found, a bad cap will often deviate a bit from the expected value, but generally not more than the deviation seen between good caps.

So it appears that, although it would be nice to be able to test caps in-circuit quickly and with no voltage applied, there are currently no reliable ways to do this. You can measure the leakage in circuit or (as you demonstrated) out of circuit, but in both cases, they must be charged to their working voltage.

I would add only one observation. To me, the implications of leakage are far more compelling with coupling caps than with tone caps, which generally reject or pass different frequencies (often to ground) and operate at very low (signal) voltages.

[crazyuncleduke]

Thank you for this series of videos....Very helpful.
I really enjoyed viewing all your snazzy test equipment and the data sheet Uncle Doug sent your way.
Glad I subscribed.

[kgsalvage]

You are right about the years passing faster as you get older. For me a year goes by like a month did 40 years ago. I think I'm going to make a unit just to test coupling caps out of a smaller v/a transformer and rectify and filter it some so I can just plug my analog VM and cap into . Pretty simple. Great!

[ohmedarick]

Thank you for a great video I did watch your previous on capacitors A++. I do a use an ammeter for checking leaking DC capacitors. I'm old but new in the electronics field.

[retrotechandelectronics]

I used the uA meter because ultimately it was to be my reference for setting up the IT-11. The switch settings allow for 3 different criteria. 2ma, 15ua, and 2 ua. at the 2ua setting full open was less than 1/10th of 1 microamp. which is a lot more specific than measuring the voltage across the resistor

[salazarich]

Very interesting I have a amp 1969 I think the preamp capacitors are original I would like to check them now

[channel-rjqt]

Really enjoyed this, thanks for posting. Do you have any good methods for evaluating HV electrolytic filter caps for leakage and potential shorts?

[MichaelLloyd]

I really enjoyed this series. It was very informative and entertaining. I have a number of different ways to test caps. I restored my Eico 950. It was one of the first tube devices that I restored so I always haul it out first. I have a couple of Heathkit IT-11's that I also restored. And I need to finish a little work on a Sprague TO-6. Can you tell I like to work on vintage test equipment? I keep everything I take out and test it. I don't know why... probably something related to the last part of this vid :) 73 Mike NE5U

[llamatone]

I just love your Videos. Can i use a DMM set to DC volts to perform my test, or do i need something with a larger input impedance?

[simonkormendy]

Normally, capacitors should be virtually an open-circuit to DC, there should be very minimal DC leakage flowing through them, if there is some DC leakage that indicates that the dielectric between the two plates of the capacitor has suffered a breakdown, or punch-through, or even a partial short-circuit.

[aircooledcamper]

Ah yes, you and I may think alike. I always do the straight forward mental thing first..."I need a microamp measurement so I need a microamp meter..." Then later, I think of or find out an easier way. : )

[kgsalvage]

I've had half decent luck using a good analog ohm meter. If the reading doesn't go to infinity, the caps bad.

[keithschreiter]

Can you email a copy of the Excell file of the ESR of the CapsThanks Keith