Tubelab SE Breadboard with 300Bs

by on Apr.15, 2009, under Electron Tube Audio

With a custom power transformer from Edcor, I could finally try this amp with some borrowed 300Bs. This transformer’s primary is tapped so that it can be used with either 300Bs or 45s:

XPWR131 - 330-260-0-260-330 @ 175mA, 6.3 V CT @ 4A, 5V @ 3A

Tubelab SE - Testing Edcor 45

Voltages came in a bit high, even under load. The top row is the Tubelab SE in “45 mode”, with about 25mA per tube. Meters are reading: 660VAC winding, 520VAC winding, B+ at the filter cap. Bottom row is in “300B mode”, running Shuguang 300Bs (not mine) at about 28mA per tube. First two meters got swapped: 520VAC winding, 660VAC winding, B+.

Tubelab SE - Testing Edcor B+

With the 300Bs biased to 80mA, the 660VAC winding gets pulled down exactly to spec: 660VAC. B+ sags to around 370-375 VDC. Supposedly this is the ideal spot for the 300B. These tubes are Shuguang’s aptly-named “300BS”, which have mesh plates and globe-shaped glass. They have a weird blue glow on the glass, presumably from electrons flying through the gaps in the mesh.

Tubelab SE - Testing 300BS Dark

2 comments for this entry:
  1. Avatar

    How’s the Edcor power transformer working out? Is it as good of a fit for the 45’s too? Is it running cool? I’ve been looking for a better option than a Hammond power transformer and I think I may have just found it. Thanks for posting this!

  2. Avatar

    Yes, it works well for 45s. The B+ is a little high…about 320V…but this is what a lot of folks end up running these amps at with 45s due to the “hot” Hammond/Allied iron. With 45s, it barely gets warm. With 300Bs its pretty well loaded (B+ and heaters) and gets warmer…typical of what other trannies run at. I can keep my hand on it for as long as I want. All in all it turned out well.

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