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Using tentlabs DC filament supply with EML 300B-XLS V4

PostPosted: Mon May 08, 2017 5:35 am
by musicpre
After reading Application Note AN-06 and reading the tentlabs-related threads on this forum, I wanted to ask about the best practices for using the tentlabs DC filament power supplies <http://www.tentlabs.com/Components/Tubeamp/Tubefilament/index.html> with a center-tapped filament EML 300B-XLS V4. Looking at the EML 300B-XLS V4 data sheet, the filament draws ~1.5 Amps when a 5 Volt DC potential is applied across pins H1 (pin 2) and H2 (pin 7). My question is whether an alternative would be to apply a 2.5 Volt DC potential between CT (pin 8) and H1 (pin 2) while at the same time applying a 2.5 Volt DC potential between CT (pin 8) and H2 (pin 7). The DC filament power supply would now have to supply twice the current at half the voltage in this alternative configuration, with the same ~1.5 Amps running through both halves of the filament. My question is whether this is even supported or considered a best practice when using the tentlabs DC filament power supplies with a center-tapped filament EML 300B-XLS V4?

Re: Using tentlabs DC filament supply with EML 300B-XLS V4

PostPosted: Tue May 09, 2017 12:44 pm
by Jac
Hello,

the way to use the V4 tubes is hopeflly described in the data sheet in such a way it makes sense to you, and you don't get stuck with a feeling not really knowing what this is.
The AN-06 is just interesting stuff for people who want to know details.

In short, you should just connect the cathode of the -V4 tubes the same way as you would connect the cathode of any indirectly heated tubes. The same way, there is no difference whatsoever. So you connect anode, grid, and cathode, and you are done.

What remains, is how to heat the -V4 tubes. Also that is not any different, with the only exception the heater supply of the -V4 must be floating. Whereas for indirectly heated tubes it can be floating or not.

Best is DC heating, so you have zero hum. The Tentlabs should be checked for exactly 5V on a warm tube. Then, the tube will divide that in 2.5 and 2.5V because the tap in the middle.

The issue with DC offset inside a DHT tube, can not be solved with any kind of DC heating. However there are no practical problems resulting from that. Alternatively you can use 50Hz AC heating, use ultrasonic heating, or subsonic heating like 20Hz. The thing you could do, is parallel the two heater halves of the 300B-XLS-V4 so you get a 2.5V 2.8 Ampere heater. That would reduce the DC offset, but it becomes a normal 4 pole DHT then, with no more cathode center tap. This can be done only for the 300B-XLS-V4.