DC Filament Voltage Check

[Urbansandz]

I recently upgraded to the DC filament. It's a big upgrade and I'm glad I did it. I have a question about the DC voltage check. On one side I was getting up to 4.86 or thereabouts, but on the other side I maxed out around 4.72 and that's after letting it warm up a bit. What could account for that difference? I'm really wondering if there are simple factors that could readily account for one side not reaching that 4.86? And is this something I should be concerned with? 

I currently have the upgraded Kaiju in use and I don't perceive any difference in volume on one side and I did not need to change the balance on the preamp to bring it to pre-upgrade balance. I do have the balance quite skewed as it is, so maybe that explains that. Other than the voltage differential it's a great upgrade that has resulted in very quiet speakers. I highly recommend it for anyone bothered by the hum/buzz of the stock build.

 

[Paul Joppa]

The regulator sets the voltage with the resistors 237 and 1000 ohms. These are +/-1% resistors, and the chip's internal reference has a similar variation. And the series 0.51-ohm/3-watt resistors near the sockets are probably 5% (i.e.1% of the total voltage).

Plus the accuracy of your meter.

Plus, the variation in the resistance of wire and soldered connections.

You may just have an unlucky combination of those variations.

In any case, those 0.51-ohm resistors can be adjusted. To increase the voltage, parallel one or both with a suitable resistor. In most cases, including yours, it's close enough to not worry about.

The exception is with EML tubes. The manufacturer  specifies a very tight +/-4% tolerance on filament voltage. That's 4.8 to 5.2 volts. If you are using them you'd want to increase that 4.74 to match the 4.86. At that level of precision it's best to measure the voltage at the physical tube pins rather than at the socket terminals. Fussy, but you only have to do it once!

 

[Paul Birkeland]

Along with what PJ wrote, it would be helpful to know the DC voltage across each pair of regulated output pins on the PC board as well. 

 

[Urbansandz]

Quick update. I switched out the stock PSVane tubes with a pair of Electro-Harmonix the discrepancy completely disappeared. Both sides are now measuring at 5.086.