Utracer 3 and rectifier testing

[Dale Carr]

Hi again!

I just finished up my Utracer 3+ board this weekend, and the initial calibration and testing of an EL84 are looking great.  I plan on mostly using the Utracer for guitar amplifier tubes.

I was hoping to be able to use my external heater supply (cheap dc to dc converter like  others on this forum are using) to give the rectifier tubes such as 5Y3, 5U4 etc their 5V heater voltage but I am a little lost on the utracer and a direct heated tube. The external heater supply is powered by the same source as the utracer supply.

If I am not using the boards Cathode wire  for a rectifier tube test would the external heater power supply wired as the manual shows for utracer heater control (ext heater supply - volt to utracer supply - volt,  ext heater supply + volt to the tube socket heater pin) cause the issues noted with direct tubes or is that when the cathode wire is also used? 

My plan B is to use a separate ungrounded 5VDC supply that is not tied in any way to the utracer supply power.

Thanks,

Dale

[Martin Manning]

For rectifiers with indirectly heated cathodes (5AR4/GZ34 e.g.) I recommend you go with your plan B, and be sure to connect the uTracer's cathode terminal to pin 8. Using a DC supply for rectifiers with directly heated cathodes (5Y3, 5U4) requires two runs with opposite polarity on the filament pins and averaging the results. A better solution in this case is to supply the filament with AC, which you can easily do by using a 6.3VAC CT transformer and a Variac. Connect the 6.3VAC across the filament, and the CT to the uTracer's cathode terminal. Adjust the Variac to get 5V across the filament (or 6.3VAC if you are tracing a 6V rectifier). You can then trace both diodes in a single run. Of course this works fine for rectifiers with indirectly heated cathodes too. The uTracer has some difficulty with low voltage and high current, so when tracing a GZ34 resistors can be placed in series with the anodes to increase the voltage drop to produce a smoother curve of Ia vs. Va-k. You will have to calculate the actual Va-k by subtracting the resistor voltage drop (resistor value x indicated current) from the indicated voltage. Rectifiers with directly heated cathodes have a much larger voltage drop, so they can be traced without any added resistance.

[D.A.R Achterberg]

I never tested diode tubes, but testing with AC is better testing with DC?

With tube amps there should not be any difference with with the heater supply, i know that AC can be a hum source if you don’t twist the wires

And directly heated tubes, with Mains 220v AC i don’t get that why you use 6,3v AC with a variac, yeah to get 5v but that’s not directly though…

Are am I missing the point here?

Placing a resistor in series is the most important thing to test a diode, as far as i know…

But what about the current?

As far as i know you have to watch out with testing Diodes.

With the current around 5mA with standard diodes.

And around 50mA with power diodes.

Op 14 jun. 2022 om 13:05 heeft Martin Manning <mman...@fuse.net> het volgende geschreven:

For rectifiers with indirectly heated cathodes (5AR4/GZ34 e.g.) I recommend you go with your plan B, and be sure to connect the uTracer's cathode terminal to pin 8. Using a DC supply for rectifiers with directly heated cathodes (5Y3, 5U4) requires two runs with opposite polarity on the filament pins and averaging the results. A better solution in this case is to supply the filament with AC, which you can easily do by using a 6.3VAC CT transformer and a Variac. Connect the 6.3VAC across the filament, and the CT to the uTracer's cathode terminal. Adjust the Variac to get 5V across the filament (or 6.3VAC if you are tracing a 6V rectifier). You can then trace both diodes in a single run. Of course this works fine for rectifiers with indirectly heated cathodes too. The uTracer has some difficulty with low voltage and high current, so when tracing a GZ34 resistors can be placed in series with the anodes to increase the voltage drop to produce a smoother curve of Ia vs. Va-k. You will have to calculate the actual Va-k by subtracting the resistor voltage drop (resistor value x indicated current) from the indicated voltage. Rectifiers with directly heated cathodes have a much larger voltage drop, so they can be traced without any added resistance.

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[Dale Carr]

Thanks for the replies... I wasn't sure if we could use AC for the heaters after reading a different posting on the forum, but I think that instance was for an AC heater and its issue with the grid. In the case of testing a rectifier it doesn't cause any issues I guess?  I'm leaning towards using my variac and an extra 6.3V filament transformer, that sounds like a simple solution.

 If I did go with an external heater 5VDC 3A power supply (wall wart style) in addition to the 19.5VDC feeding the utracer... would that be considered isolated enough to not cause issues or would I need to grab a true isolated 5 volt supply? I'm having issues finding an isolated DC to DC converter that will get me 3 Amps. 

Sorry for all the new guy questions, wanting to make this tester as efficient as I can.

Dale

[Dale Carr]

Also would a potentiometer work as a filament center tap/cathode connection to let a 5VDC supply test a direct heated tube without needing 2 averaged tests?

Learning a lot on this forum, thanks!

[Martin Manning]

You need an isolated supply if there is any common heater-cathode connection, which includes all tubes with directly heated cathodes, and rectifiers with indirectly heated cathodes and a common h-k connection like 5AR4/GZ34. I wouldn't recommend using a pot to create a center tap for directly heated cathodes. All of the tube current would have to flow through the potentiometer, which would reduce the anode current for a given Va-k, and if you are using a DC supply, the average voltage on each cathode as well as the cathode connection at the wiper will be offset. An isolated DC supply is ok for indirectly heated cathodes, as there is no Va-k offset voltage. 

Below are some traces I ran using a DC supply and the AC CT transformer technique.

For the DC case, I made two runs tracing both sections simultaneously, reversing the filament polarity on the second run. The top two lines and bottom two lines go together. You can see the solid line is offset ~7.5V and the dashed line is offset ~2.5V with the polarity change, which is exactly what I would expect. 

For the AC CT case, the traces are virtually on top of each other (the sections are well matched), and they pass through 125 mA at 50V, which matches the Ia @ Va-k shown on the data sheet. Looking back at the DC case, you can see that averaging the two runs for each section will produce the same result.

[Dale Carr]

The AC with a center tap looks like the way I will go thanks! 

I am trying to keep my wire lengths nice and short... would there be an issue with the 6.3v filament transformer mounted external (a foot or so away) as opposed to inside the enclosure? I have room inside the box if the ct/cathode wire length would be a concern for readings. Would hate for a long wire run to affect the measurements.

Dale

[Martin Manning]

My tracer has a pair of banana jacks to connect an external heater source, so I just put a tube amp power transformer on the bench next to it and used clip leads to connect the heater winding to the heater jacks and the CT to the cathode terminal. There's no sensitive grid involved here, but keeping the leads short is still a good idea. Seems like a filament transformer with some banana plugs attached to its secondary leads placed next to the uTracer would be fine, and pretty quick to set up.

[Martin Manning]

A follow up on using this technique: I did not mention that the uTracer takes readings in its own time, and is therefore measuring current at varying Va-k as the AC heater supply cycles at 50 or 60Hz. This is not a problem, since the Tracer averages a number of readings to arrive at a result for each specified Va-k data point. The traces above show that averaging 16 readings is easily enough to produce a reasonable result as compared to the average of the DC measurements with opposite polarity. 

[Dale Carr]

Excellent... Thank you for that added info! 

Appreciated.