Tubelab SE Breadboard with 300Bs
by Russ Knize 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
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+.
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.
April 25th, 2009 on 11:58 pm
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!
April 26th, 2009 on 10:46 pm
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.