Testing Directly Heated Rectifiers on VCM163

[cathoderay57]

Hi folks, I have tested a number of directly heated rectifiers in recent years using my VCM163 and all seem to read on the low side - rarely better than 60% and most scarcely above 50%. All of the directly heated rectifiers have been used ones, and would therefore be expected to have lost some emission; however, indirectly heated rectifiers seem to test significantly higher at between 75-85% and signal diodes often 100%. I understand how the diode/rectifier test works having read Page 8 of the Avo manual, and that testing on lower current ranges will inevitably lead to higher readings; moreover, yesterday I completed the calibration check using a BY127 (=DD058) silicon reference diode with 150R series resistor and all current ranges were approx mid-spec. Are the readings I am getting while testing directly heated rectifiers normal, i.e. do other people get similar results? And is it the case that any meter reading in the green area means the valve is good (higher the better), while the actual percentage on the 0-100 scale is merely a guide and should not be taken as highly accurate? Hope you get my drift... Cheers, Jerry

[mark-jacobs]

There is an imbalance between which heater pin is used when using a directly heated tubes which will effect the results of the valve under test. The a discussionin the attached link explains why you see a difference in the good/bad measurement. So it depends on which pin is used for the cathode. This also applies to directly heated triodes, so I use a center tapped heater supply for those applications, I also can null/0 the meter (non AVO testers) with no signal as this can cause a significant error depending on the hester supply AC/DC and test circuit design.
https://tubesound.com/testing-a-bala...a-tube-tester/

[Leon Crampin]

A directly heated full wave rectifier is a fully symmetrical device. With an AC energised heater, there can be no difference as to which heater connection is used for the rectified output.

An indirectly heated rectifier with the cathode connection tied to one end of the heater is asymmetric. A small difference in output voltage and ripple form will result if the output is taken from the heater pin not actually connected to the cathode. With a low voltage heater, the difference is very small.

A directly heated triode similarly AC heated is also fully symmetrical. The cathode connection was usually made from an actual or artificial heater centre tap to minimise hum.

Leon.

[cathoderay57]

Thanks for the responses - food for thought. One thing I learned is that the filaments in at least some directly heated rectifiers are connected in series - visually inspecting a U10 indicates that appears to be the case. I had always assumed that a single filament failure would cause the rectifier to fail to half wave, same as an indirectly heated valve. Examining a 5V4G (indirect) shows the heaters are in parallel. Every day's a school day. Cheers, Jerry

[cathoderay57]

I've just bought a brand-new Mullard 5Y3G (and I'm convinced it IS brand-new). At 60mA on the VCM163 it tests 64% and 62%. The rectifier cal check was last done in September 2022 (see Post #1). Therefore, I still think that on directly heated rectifiers the VCM163 is giving a 40% under-read and so any testing down to 40 or so on the meter scale, and outside the green "good" region, are pretty certain to be still useable with considerable life left. Jerry

[CambridgeWorks]

Don't know if relevant to your experience, but I have a couple of NOS Mullard AZ41 and 2 or 3 used ones. All test at around 50% on my CT160.
5Z4, 5v4 used ones test OK.
I meant to ask about this on here maybe 2 or 3 years ago,
This post jogged my memory.
Rob
Rob

[Chris55000]

I don't know whether it applies to the "all–dry" type of battery set using B7G "90" series valves, as these were always operated from purpose–mads batteries with polarised plugs, (members who use these sets more regularly than me who have to adapt them for today's available LT & HT supplies will know!) but in the old pre–war days of accumulator–heated valves, it wouldn't be unknown for the h.t. consumption to double if you accidentally reversed the polarity if the l.t. filament supply, as this effectively reversed the average grid–bias voltage to those valves that didn't have a separate negative GB lead – R.H. Warrings's book "Instructions to Radio Constructors" from the World Radio History site makes this point!

Chris Williams