Western Electric Model 91A (WE91A)
watts per channel stereo, single ended directly heated triode, fixed biased,
2x 300Bs, 2x 6J7s, 1x 5Z3
Completed on 3/10/11
Built on 17x10x3 aluminum chassis reinforced with aluminum angle pieces under
heavy irons. The chassis was sanded then finished with several layers of clearcoats.
20W James 6123HS OPTs are hard to come by these days.
I had to directly import them from Taiwan.
These meters are for convenent biasing of the output
tubes. I dialled in at 62 mA with 410 VDC on the plates of those EH300Bs to hit the "sweetspot".
These knobs control bias to the output tubes.
There are not a whole lot of "Boutique" parts
in this amp with the exception of the coupling caps used. Those Jensen PIOs do sound very nice!
Bias circuit, if you can figure it out...
The wiring was a bit more complicated in the fixed bias version, mostly
because I've never built a fixed WE91A-300B circuit before. I had to put in some hours designing a suitable bias circuit and
it worked pretty well for most part. As far as sonic characteristics, the fixed bias version of the circuit seems to have
a cleaner bass than the cathode bias version. It was apparent when listening to classic rock and watching action movies. What
I like the most about this amp is its ability to adapt to different brands and types of 300B tubes. I don't believe that
every 300B is created equal, therefore it needs some minor adjustment for optimal performance. This amp allows just that.
I would love to try some EML300BXLs to see if I can pump out more of that SET magic watts!