Less gain than SEX?

[Jameson]

I finished the Stereomour II today and started testing it (very limited tests so far). With everything the same, it seems to have a good amount less gain than the SEX with C4S. Is that to be expected or is there a possible issue?

 

[Jameson]

It seems like the SEX has a gain of 15 and the Stereomour has a gain of 8, so that seems to track with what I’m seeing (as long as I’m reading that right and understanding it).

 

[Paul Birkeland]

Are the impedance taps set the same between both?

I see 15dB of gain for the SEX on the 8 ohm OT configuration, which IIRC doesn't go up a whole ton with the C4S upgrade.  I roughly calculate that the Stereomour would be a few dB below that.  If you'd like to increase the gain of the Stereomour a bit, you can install cathode bypass caps on the driver cathode bias resistors.  2200uF at 6.3V would be my recommended starting point.

 

[Jameson]

I believe I had them both wired for 8 ohms. I’ll double check when I get home.

I might do the cathode bypass caps after some listening time. If I do I’ll 100% need some help understanding what that means and how to do it.

 

[Jameson]

If I go that route, I assume these caps are sufficient. Any concerns?

https://www.mouser.com/ProductDetail/647-UKL0J222KHD

 

[Paul Joppa]

The cathode bypass caps will make a very small difference - I calculate 0.7dB.

My calculations show a gain of 7.08 (17dB) for SEX with C4S, and 4.5  (13.1 dB) for Stereomour II, both set for 8 ohms. Note that they use the same output transformer, but at different impedance levels, so the 8-ohm wiring is different between them.

 

[Jameson]

I think it’ll be loud enough for my current room. I’ll revisit in the future if it becomes an issue.

I have a handful of those caps on the way just in case, although it sounds like they won’t make much difference.