I have a working breadboard rebuild of the BeePre and am working on dialing in the filament regulator (LM1085 board). I have a bunch of questions that I'm hoping @PB and @PJ can help me with. Any directions/pointers you can provide will be greatly appreciated. I am taking my time with the BeePre rebuild and really want to find an implementation that works well with my EML 300B tubes.
In stock configuration the reg board is set up so that Vin - Vout = approx. 3.5V. Based on my best attempt at making sense of the TI datasheet for the LM1085 (attached), it looks to me like the LM1085 only needs about 1.5V difference between Vin and Vout, and even @1.5V there's should be at least a 200mV cushion at typical operating parameters. But the stock config has an additional 2V of difference - which would otherwise suggest to me that there is huge cushion to prevent reg dropout, but my past experience suggests that this isn't necessarily the case and that the reg can drop out even where Vin - Vout > 1.5V.
So my first question is: is some other part of the fil reg circuit dropping the additional 2V? As best as I can tell, the first half of the fil reg circuit is identical to the TI "ripple rejection enhancement" model found at 8.2.9 of the datasheet, except for the use of 33uF caps where TI has 10uF caps. But the last half of the reg circuit looks like its based on what I've learned is called a "Sziklai pair" - close cousin to the "Darlington pair" but with reported thermal regulation, impedance and possible lower voltage drop benefits over the latter -- I dunno about any of this: trying to read about it made my head spin. Related second question: What does the second half of the fil reg circuit do? / why is it there?
My next set of questions is about noise and ripple rejection. When the regulator drops out, where does the resulting hum come from - is it power supply ripple being passed to the filament? or is it generated by the reg circuit itself? If the former (passed power supply ripple), then my separate filament trafos with CLC filters that reduce ripple to a modeled (PSUD2) 8 mV pk-pk should help to keep things quiet if the reg drops out (for comparison, my PSUD model of the BP stock voltage doubler/rectifier shows about 2.25V of ripple being fed to the inputs of the reg board - all of which is eliminated by the reg circuit). But if the reg circuit is the source of the dropout hum, then no amount of ripple rejection before the reg circuit will make a difference.
To minimize the incidence of dropout, I'm thinking of increasing Vin to 14V instead of 13.5, while keeping Vout at 10V. I can't tell from the datasheet whether this decreases ripple rejection / increases noise. Does it? And if it does, do you think some or all of the increased noise would be offset by the increased ripple rejection of my CLC filter? If my basic arithmetic and use of an online dB calculator is correct, then 2.25V pk-pk (stock) : 8 mV pk-pk (CLC) is 281:1, or about -24.5 dB.
I anticipate that increasing Vin by 0.5V will put more thermal stress on the regulator. I plan to deal with heat stress to the LM1085 via (a) replacing the stock mica spacers with "kerafol keratherm" insulators that are reported to be more heat conductive than mica and comparable to a combination of mica and thermal grease; (b) forced air cooling @ 400 cfm. But this only takes care of the LM1085 (hopefully). Might other parts of the board - e.g., the transistors, need additional cooling if I bump up Vin?
Apologies for the numerous questions, but I really want to get this right.
cheers and many thanks in advance, Derek
