Problem with heater voltage direct heated triodes

[Okhan Vatansever]

Hello
I have a external dc heater for my utracer 6 and when testing direct heated triodes the manual says that when using external heater, it is ok to connect the cathode to one of the heater terminals.
The problem I have encountered is that when connecting to one of the heater socket I get about 17V DC instead of 5V. This has unfortunately made me break open 2pcs 300B tube filaments.
If I connect the cathode to the other heater my external DC heater source shuts down. 

What am I missing here? 
Don't want to blow more 300B or my utracer. 

[Martin Manning]

I only have experience with uT 3+, but for directly heated cathodes the filament supply must be isolated. If it is a DC supply, the average filament voltage will be Vf/2, so the test results will not be consistent with the nominal supplied Vf. 

A transformer-derived AC supply is another option. If I recall correctly, Ronald devised a circuit to synchronize the measurement pulse with the supply crossing of nominal Vf, but I have had success measuring directly heated rectifiers without that, as seen in the trace below. Presumably, the normal averaging of multiple measurement pulses produces a result very close to nominal Vf. Note the Ia-Va-k trace is relatively smooth, about as smooth as expected when measuring this type of tube using a DC filament supply.

[Nebojsa Tomic]

you will need isolated external power supply
check if your AC ground of the external power supply is connected to
DC minus (ground)

чет, 11. сеп 2025. у 14:45 Okhan Vatansever
<okhan.va...@gmail.com> је написао/ла:

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[Ihor Smal]

Indeed you need a fully isolated/floating external heater supply. From the picture it is not clear how you feed that Chinese DC-DC converter, I'd should not have any common grounds with utracer circuit, so it is better to have a separate transformer or power supply to that DC-DC converter. I have exactly the same setup on utracer3 and can test 2a3 tubes without any problems. 

--

[Davo]

Verstuurd vanaf mijn iPhone

Op 11 sep 2025 om 17:23 heeft Ihor Smal <iste...@gmail.com> het volgende geschreven:



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[Davo]

Sorry wrong one lol

Op 11 sep 2025 om 17:43 heeft Davo <djda...@gmail.com> het volgende geschreven:



[Davo]

Verstuurd vanaf mijn iPhone

Op 11 sep 2025 om 17:45 heeft Davo <djda...@gmail.com> het volgende geschreven:

Sorry wrong one lol

[David Hanslip]

I use a Chinese supply too. I power it from a 19V laptop power pack. No earth. 

On 11 Sep 2025, at 11:48 pm, Davo <djda...@gmail.com> wrote:



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