DC Filament Voltage Too High [resolved]

[skygazer]

Rather than simply adding to a previous topic, I heeded to the "this is an old post, do you really want to reply" warning and started a new one.

I completed a Stereomour II last weekend.  Sounds great through Buschorns (FE83 drivers), but there was of course a constant very low level hum (sounded like 120hz) noticeable in silent passages when listening to Stax SR-Lambdas/SRD-7.

Today, I just finished building the DC Filament power supply.  Voltage across the tube filaments are 2.627 VDC on A, and 2.638 on B.  Line voltage at the time was 121.5 VAC. I used a trusty BK Precision 2940 meter.

Since these voltages are just barely over the 2.625 VDC spec, I'm running the amp anyway.  It sounds fine.  Of course, the low level hum is completely gone in the headphones.  Very sweet!

So, the key question is:
Am I shortening the life of my valuable 2A3's by running them with the filament voltage so high? 

Please let me know what you think.

[Paul Birkeland]

You could pop out the 0.13 ohm resistors and try 0.15 ohm ones instead.  I would probably start by doing that with just one of the 0.13 ohm resistors on each side.

You could also remove the red wires leaving terminal 48 and going to the power transformer, then substitute a pair of 1 ohm 1W resistors in their place to drop your primary voltage just a touch.

[Paul Joppa]

A few caveats:

* It's a good idea to make sure the power line voltage is typical of the intended location and time of day (and of year).

* The voltage will drift a little as the amp warms up; it can take an hour or two for the internal temperature of the power transformer to stabilize - this affects the wire losses and the rectifier voltage drops.

* Ideally the filament voltage is measured at the tube pins, not the socket's solder lugs.

The low voltage and high current of the 2A3 filament makes these effects more significant than one might expect.

[skygazer]

Thanks, Paul.  Looks like my line voltage actually gets up to 122.5VAC during the day.  I have solar, but I'm not sure that makes much of a difference... my power company can get pretty flakey.

Warm-up doesn't seem to change the voltage at the filament pins much... still 2.63-2.64VDC.  I've ordered 0.15Ω resistors to replace the 0.13Ω as Paul suggests.

Since you specifically said the effect is significant, I'll not use the amp until I make these mods.  I don't want to risk shortening the life of the 2A3's.

[skygazer]

Update: replaced the 0.13Ω resistors with 0.15Ω, and filament voltage is just barely in spec.  I'm calling it done.

I also finished & installed the Shunt Regulator mod, and of course all voltages for it are fine.   Enjoying it as I type this.