Bigby - My Big BeePre Rebuild

[Paul Birkeland]

My best guess is that the hum is coupled onto the plate voltage supply.

Do you hear any hum if you run the BeePre with no 300Bs?

[Deke609]

I will check right now.  Thanks PB.

[Deke609]

No hum without the 300Bs.

[Deke609]

The schematic shows the design expectation, before thy example was built. The voltage check shows what was actually measured on the production prototype. Neither is critical; the usual +/-10% is applicable.

Many thanks PJ. I knocked it down from 200V to 189V, which based on your above advisement means it is close enough.

cheers, Derek

[Deke609]

Well, this is really strange. I wanted to take a break from working on the amp and finally listen to it, so I figured out a way to get in my rack and fired it up: almost no hum. But that's not the weird part. The weird part is that it seems like the hum is coming from me! If I just put my hand near the attenuators, but without touching them, the hum get louder. And when I do that, I can hear what sounds like a helicopter: ticka-ticka-ticka-ticka-ticka. It gets a bit louder still if I touch the knobs.

Another thing that's changed: I forgot to power everything through the variac. So everything is running off of mains which is probably 115VAC. I'll hook up the variac and test tomorrow.  Right now I'm enjoying listening. Life is good.

cheers, Derek

[Paul Birkeland]

That kind of hum usually means that the chassis plate is not earthed.

[Deke609]

The prototype is all wood, including the chassis plate which is 1/4 ply.  So I'm stumped.  But it sounds great so I'm pretty happy for now.

[Paul Birkeland]

Then every piece of metal that would otherwise touch a metal chassis plate needs to be wired together with a separate ground that will act as a chassis ground. 

A wood top plate is definitely a high risk for additional noise.

[Deke609]

A wood top plate is definitely a high risk for additional noise.

Thanks PB. That heartens me. Maybe the final build in the metal chassis will quiet things down even more.

[Deke609]

Well, Santa must have some audio elves, because the hum is now only faintly detectable with the attenuators fully open. If I wasn't listening for it, I wouldn't notice it. Super pleased!
 

[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). 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