Paramount soft start v1.1: low filament voltage

[dth31]

Hoping one of our Bottlehead experts can help.  I've measured the filament voltages on my Paramounts, and both are 2.30V.  That's NOT a problem (as far as I can tell) for the Sovtek or JJ 2A3s, which work and sound fine, but may be a problem for 1 of 2 EML mesh plates (one of which doesn't make any sound).

There is a sticky for how to increase the filament voltage for the 240V Paramount, if the filament voltage is less than 2.4V.  Place a 4,700-10,000 uF cap rated for at least 6.3V across FC-1 terminals 5 and 10 (2A3 filament terminals). 

Is this OK to try for low filament voltages on the 120V Paramount?  (Just want to be sure--don't like loud noises  )

Dave

[Paul Birkeland]

If I remember correctly, this is really only an issue on 240V Paramounts running well below 240V.

I'd add the cap to your amps for your situation. I would recommend a 4700uF 16V cap. The higher voltage cap will tolerate the ripple current a bit better.

[dth31]

Thanks PB.  Appreciate it.  As an FYI, I've measured the filament voltages on both amps on at least 5 different days, and multiple times during a day (trying to sort out the tube problem).  It was never higher than 2.32V, and never lower than 2.28V.  Both amps were remarkably consistent and never differed from each other by more than 0.01V.

For those who may be interested, I tested 1000 uF, 4700 uF and 10,000 uF caps across FC-1 terminals 5 and 10.  AC line voltage was 120.1 VAC.  With no cap, the 2A3 filament voltage was 2.28V.  With a 1000 uF cap, voltage increased to 2.29V.  With a 4700 uF cap, voltage increased to 2.54V.  With a 10,000 uF cap, voltage increased to 2.62V.  All measurements were taken on the tube base pins.

To get the filament voltages as close to 2.5V as possible for the EML tubes, I was going to use the 4700 uF cap.  Jac's comment was that there is a 0.1V voltage drop between the tube pin and the tube base, so he recommended using the 10,000 uF cap to deliver 2.5V to the tube filament.

Hope someone finds this helpful!

Dave

[Paul Birkeland]

I would agree that 2.54V is the place to be.

If indeed 0.1V of loss was incurred from base to filament with a given tube, the manufacturer would adjust the filament accordingly.

It's also worth mentioning that PJ may chime in here at some point regarding whether there's enough ripple present in the filament supply that it needs to be taken into consideration when considering what actual filament voltage is available.

-PB