Bigby - My Big BeePre Rebuild

[Deke609]

I think what PB is saying would mean to turn all the transformers, apart from the power transformers, so that their channel frames are parallel to the extension shafts you added to the photo. Then lay them down so that the channel frames would be parallel to the plane of the chassis plate.

Many thanks caffeinator!  Yes, now I see. That might indeed be difficult. I think I'd have to totally rejig the layout, including chassis orientation - which I will do if I get hum and can't find a solution with the present orientation.

I think for now I'm going to move the filament transformers to where the big chokes currently are, and vice versa. I'm wary of having four power transformers in a row like I do now. And I'll mount all parts so that the center lines of the coils are at least perpendicular.

Question - @PB or anyone else who has played around with using an oscilloscope to test for interaction between inductors (transformers or chokes): to test between adjacent inductors, is it enough to send a signal through the first (e.g., mains power through a loaded PT) and "listen" with the scope to the unpowered second? Or do I need to send a (different) signal through both?

Many thanks, Derek

[Paul Birkeland]

Power up the power transformer, then you can listen to the other coil.  I don't even use a scope for this a lot of the time, a multimeter that can resolve AC mV will usually do the job.

I would suggest doing some reading on the right hand rule and properly aligning coils in a speaker crossover to better prepare you to answer some of these questions on your own. 

[Deke609]

Many thanks PB.

I would suggest doing some reading on the right hand rule and properly aligning coils in a speaker crossover to better prepare you to answer some of these questions on your own.

Yes,  good advice. I was reading about crossover inductor layout last night.  I'll look at the right-hand left-hand rule again - I recall that from the NEETS materials. 

cheers and thanks, Derek

[Edit: wrong hand!]

[Edit 2] I just came across the following helpful link that shows scope shots of the effects of different coil orientations: http://www.audioholics.com/loudspeaker-design/inductor-coil-crosstalk-basics

I should be able to implement the orientation shown in figure 7.

[Paul Birkeland]

Just keep in mind that all of your inductors are carrying 60Hz AC, so crosstalk is completely meaningless.  My suggestion to align all the coils is to avoid spewing that 60Hz magnetic radiation in more directions than is absolutely necessary.

[Deke609]

Right! Didn't think of that!  I guess this goes back to your original point about interaction with a signal (music) carrying coil like an OT (or a speaker cross-over). Many thanks again PB. That suggests to me that protecting the last filter choke from interference might be worthwhile, but otherwise, I should be ok. But it's probably a good idea for me to take a stab at some minimal approximation of layout "best practice" - so I'll do what I can to keep the magnetics from interacting.  And it'll give me a chance to try out a new 6" metal brake that works with a bench vise.  I bought it and a modestly priced 8" metal shear after seeing the prices that Lundahl wants for transformer enclosures - all in for both, less than the price of 4 enclosures.  And 1/16 steel and aluminum sheet is pretty cheap.

cheers and thanks, Derek

[Deke609]

I tested the interaction between filament transformers for myself today by powering one fil trafo and listening with the scope to the primary of the unpowered other. Results attached. Best: spaced with center lines of coils at 90 deg and not crossing - about 2mV.  Worst: stacked on top of each other - about 160mV.  Note: in the scope shots, the purple trace is the induced signal on the second trafo. The scope shots are taken at different scales to make the traces equally visible: 500 mV per vertical division for the worst, 100 mV for 2nd worst, 50 mV for third worst, and 20 mV per division for the 3 best.

I also tried looking for interaction when both were powered, but couldn't detect anything.

cheers, Derek

[Paul Birkeland]

I think you're completely missing the point. 

What would the negative consequence be of having your power transformers all aligned? 

How about your filter chokes aligned with your power transformers? 

What would the positive consequences be of having your power transformers and chokes all aligned?

[Deke609]

I think you're completely missing the point. 

Sheesh. My point was to see for myself how coils interact!

What would the negative consequence be of having your power transformers all aligned?

Speculating: could be bad news for any wiring/components between them.

How about your filter chokes aligned with your power transformers? 

I don't want a final filter choke aligned with a PT - no sense in going to the trouble of adding a CLC or CLCLC filter to remove ripple, only to have some ripple put back in the power supply via magnetically induced current in a choke (even if it's tiny, the idea of it offends me    )

What would the positive consequences be of having your power transformers and chokes all aligned?

Can't think of any that involve the chokes also being aligned.  What are the positives of this?

cheers and thanks, Derek

[Paul Joppa]

For learning purposes, you could also check the AC current induced in the unpowered coil, and calculate the impedance of the equivalent source. That would tell you how much effect it would have in a real circuit, or at least the things you would need to do that.

[Deke609]

Many thanks PJ. I will explore this.

cheers, Derek

[Paul Birkeland]

Speculating: could be bad news for any wiring/components between them.

I would put this under the category of being a more likely issue when you have magnetic fields directed in many different directions (IE coils not aligned)

I don't want a final filter choke aligned with a PT - no sense in going to the trouble of adding a CLC or CLCLC filter to remove ripple, only to have some ripple put back in the power supply via magnetically induced current in a choke (even if it's tiny, the idea of it offends me    )

Who is to say that it will be in phase with the power supply ripple.  What if the interaction partially cancels some of the ripple.
I would put this under the category of being a more likely issue when you have magnetic fields directed in many different directions (IE coils not aligned)

Can't think of any that involve the chokes also being aligned.  What are the positives of this?

With everything aligned on the power transformer, you are taking advantage of the fact that we have built and evaluated the BeePre under very similar conditions, and you're improving your odds of not running into issues.  If you start firing magnetic fields in new directions, then you are giving up on that.  I would also mention that the grid of a 300B is technically a vertically aligned coil.

[Deke609]

Many thanks again PB. I appreciate your patience - I *may* finally be getting it!

Who is to say that it will be in phase with the power supply ripple.  What if the interaction partially cancels some of the ripple.

The possibility of cancellation may explain why I found that minute adjustments to a "noisy" configuration could make the interaction disappear, but nudge one trafo every so slightly and the noise/interaction reappeared.  I briefly thought of installing the PTs and chokes on plates with a single central mounting bolt and rotating the PT/chokes to find the least noisy position before tightening things down - that's still a possibility for the final build on the metal chassis.

With everything aligned on the power transformer, you are taking advantage of the fact that we have built and evaluated the BeePre under very similar conditions, and you're improving your odds of not running into issues.  If you start firing magnetic fields in new directions, then you are giving up on that.  I would also mention that the grid of a 300B is technically a vertically aligned coil.

I think I finally see your point. I don't need to worry about the interaction between power supply coils: provided the PTs are mounted in phase they are operating with and producing the same 60 Hz sine wave; and the interaction of 60Hz interference with rectified 120 Hz ripple would only slightly augment the ripple waveform (partially cancelling one hump and boosting the next). So my new plan is is to have all four PTs (two stock plus 2 new filament) and the two first HV filter chokes  aligned in a row along the long edge of the chassis, and the remaining final chokes (2 for HV, 2 for LV) aligned in an adjacent row. The alignment will have the center lines of all coils running parallel with the long side of the chassis and away from the 300Bs.  I can space the final chokes at least 5-6 inches way from each other (center to center) - based on yesterday's experiments I know that that yields decent results.

cheers, Derek

[Edit - Oh shoot. Regarding in phase or out of phase - I may need to read up on this. I recall something about time constants with caps and inductors, with voltage either leading or lagging current depending on impedance/capacitance?  In which case different cap/inductance values will produce different phases? No need to correct if I'm wrong - I'll read up about this tonight]

[Deke609]

My Beepre lives and hums again!     

The hum is quieter but still there. But the real kicker is the wood frame adds 1 and 1/2 inches of extra width and length over the metal chassis that I spec'ed to fit my rack. So [the prototype] doesn't fit, and I can't listen to it in my main system.  That is very disappointing.  Not sure how i am going to solve this one.  [Even without hum, I am at least a month away from doing the final build in the slightly smaller metal chassis] The Beepre has been out of commission since August, and I've not enjoyed listening without it. So I'm jonesing to listen to music through it again - even with a bit of hum.

Re hum troubleshooting -- The B+ is about 20V high, so I will fix that today. I have no idea whether that could contribute to hum (e.g., by overworking the EL84 shunt reg), but it needs to be fixed in any event.  I haven't yet mounted the transformers/chokes on L-brackets to get their cores perpendicular, so I can try that too. And if that doesn't do it, there's still a couple of grounding paths that I can play around with.

My best guess is that the hum is coupled onto the plate voltage supply. The signal wiring is well away from the magnetics, so I don't think that's were the hum is getting in.

On the upside: mounting the BeeQuiet internally on a L-bracket worked well, and I'm now so familiar with the BP circuit that I could probably draw the schematic with the right component values from memory (other than the solid state bits)!

Worst case scenario: I will rebuild it stock in the larger chassis that will accommodate the gigantic 10uF copper foil caps.  But I'm still a long way off from giving up.

cheers, Derek

[bracketed edits added to clarify that the wood build is just a prototype, but I wanted to listen to it for the month or so it will take me to complete the metal work on the final build]]

[Deke609]

Question for PJ or PB: the schematic shows B+ of 180V, but the voltage checkout calls for 186V - which should I shoot for, or does anything between these two values work?

many thanks in advance, Derek

[Paul Joppa]

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.