Bigby - My Big BeePre Rebuild

[Deke609]

Finally finished!  Lots of little... err, "unintentional design compromises" (mistakes), some fixable, some permanent. But overall, I am quite happy with it.  Voltages all good, no hum, and the fans work. 

The fans are suspended between two right angle aluminum cross bars from thin silicone hangers crudely cut from a sheet  This for damping vibrations from the fans, and unlike my silicone washers on the 4 pin sublates, the thin silicone fan hangers actually work.  When right-side-up, the fans hang level with the bottom of the chassis - and with feet, about 1 inch above the table. The fans are externally dc powered (12V). Since I have separate filament transformers feeding the 300B filament reg boards, I could have run the fans off of the 6.3V secondaries of the main transformers (with voltage doubler, rectifier, and smoothing), but I didn't want to risk the dc fan motors injecting weird high frequency noise (no idea if they would -- but I've read that dc motors can generate all kinds of weird stuff - so why risk it?). So there's a separate dc jack beside the mains power iec. Front power toggle switch turns on/off both mains power and the fans.

I decided to leave out the huge Miflex KPCU 10uF 600V copper foil caps in favour of much smaller ODAM 10uF 250V "bypassed" by 0.1uF CUTF. I use a similar combo in the Kaiju and am very happy with it. The CUTFs are already burned in, so only the ODAMs need some hours to find their stride - but they burn in much faster than the CUTFs, so I shouldn't have long to wait.

Here a few pics. Sorry for the blurry pic of the fans. Just by happenstance, the fans (Silent Wings 3) are labelled "Be Quiet" - so now there are 2 types of "Bee Quiet" in the amp!

cheers, Derek

 

[Paul Birkeland]

Do you need a bigger chassis to fit those caps?

 

[Deke609]

Big chassis was not for those caps.  [Edit: they were orginally intended for the Kaiju that I had ordered but not yet built -- but they wouldn't fit. So they found their way into the BeePre prototype. The amp was going to be rebuilt to get rif od the hum problem and allow me to use EML 300Bs]. Here's what my (humming) BP looked like even without separate filament trafo and CLC filter for EML 300Bs.  No room to work or measure.

The extra filtering of the raw filament supply knocks the dropout voltage of the fil reg down to around 110 VAC mains (tested it last year with a variac).  That's a plus.  I still don;t understand how the reg works, but based on my observations I'm guessing that part of what it does is "shunt"  the AC ripple. So the greater the ripple the higher the required difference between raw POS IN and desired POS OUT.  So reduced ripple = reduced required incoming voltage. If that translates into less heat, which I think it does (assuming that less voltage needs to be dropped by the regulator), I might even be able to dial down POS IN and try EML 300B mesh tubes that require 1.4A filament current.  Current would go up, but that might be offset, power/heat dissipation-wise, by a drop in incoming voltage.  Anyway, that's my present working hypothesis/wondering.

To anticipate: no I didn't need to make any of the mods I made to the BP, including changes to make use of the EML 300B possible -- buying Takatsuki's probably would have been cheaper and certainly much less effort.  But not even a 1/100th as much fun (even taking into account last summer's maddening futile attempts to hunt down my hum problem). ;D I try not to think of "need".  I'm more a "Hey, this might be cool to try" kinda guy.  Every amp I have is an ongoing perpetual experiment.  I tinker, get tired of tinkering so instead listen, then get a hankering to tinker again, ... and so it goes, ad infinitum (or rather, ad the finite limits of me).  I enjoy it.  And, notwithstanding that I am still totally confused about so much of the electronics theory, I am learning a lot and find it all fascinating.

cheers, Derek