Mainline biasing issue... [resolved]

[jbarnhardt]

Hi all. I just completed my Mainline build and it was a very smooth process - no hiccups at all. My resistance and voltage tests all went well (one caveat noted below) but when I got to the final biasing step I have run into an issue. Biasing against terminal 20 works fine - it started around 147VDC and I quickly adjusted it to 145VDC. However, biasing off terminal 30, my initial reading is 3.64V! I can adjust this voltage up or down by a minor amount by adjusting the trim pot but it's clearly not in the right ballpark to begin with. I've swapped the 6C45 tubes but get the same behavior (terminal 30 is still the problematic side) and have confirmed that all 4 LEDs on both C4S boards are lit.

Any thoughts on where I might look for issues? My voltage readings are very close to what the assembly instructions specify. The one caveat referenced above is that in the resistance tests, I get 220ohm readings at terminals 19 and 22. These are tagged with an asterisk in the assembly instructions so I expected fluctuating readings or higher resistance, but since they were non-zero I assumed they were okay.

Thanks very much for any assistance you can offer.

-John
Seattle, WA

[Paul Birkeland]

I would strongly suspect that you have a miswire on the 6C45 socket associated with that low reading.  If you have the correct voltage at OA/IB on that side (same as Breg, about 220V), then that really narrows things down significantly.

Do feel free to post some photos if the wiring error isn't apparent.  You're also local to us and you're welcome to make the trip to Renton (me) or Bainbridge (BHQ) to get some help.

-PB

[jbarnhardt]

Well turns out the error was very easy to spot and to fix - I was one pin off on the wire from the 6C45 socket to terminal 30. Moved it to the right socket pin and everything now works perfectly. I spent a blissful ~2 hours last night listening to tunes through this beautiful device. Thanks very much PB for your quick response and for guiding me to exactly the right place to troubleshoot.

-John