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Bottlehead Kits => BeePre => Topic started by: Deke609 on November 23, 2019, 03:30:11 PM

Title: Bigby - My Big BeePre Rebuild
Post by: Deke609 on November 23, 2019, 03:30:11 PM
Here's my first stab at the layout of my BeePre rebuild.  Does anyone see any howlers?

The gigantic caps are 10uF 600V copper foils that I originally intended to install in the Kaiju - but they were too big, even with the planned bigger chassis.  The 600V is about 500V more than I need, but I can't find a 10uF 100-250V copper foil cap that isn't min $1K for a pair.  These cost me half that on sale. So in they go! 

cheers and thanks, Derek
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Deluk on November 24, 2019, 03:35:44 AM
With everything else being symmetrical, are lower middle transformer pairs at right angles to each other for electromagnetic field reasons? Asking as someone who knows virtually nothing about such things!
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Deke609 on November 24, 2019, 04:32:34 AM
With everything else being symmetrical, are lower middle transformer pairs at right angles to each other for electromagnetic field reasons? Asking as someone who knows virtually nothing about such things!

Hi Deluk - that's the idea.  Some other configuration might be better, but I'm just following the rule of thumb that coils turned 90 deg to each other are less likely to magnetically couple and generate hum.  All trafos and chokes will also have flux bands wired to ground that are made up of 3 layers: 30 awg copper sheet, a mumetal-like film called MCF5 that can be cut and bent, and then thin copper adhesive tape to hold the layers together.  The manufacturer of the MCF5 states that it results in -30dB attenuation from 10Hz to 200kHz. But it's so thin, I figure it must saturate really quickly - but I'm hoping the connection to ground takes cares of that.  But based on what I've read, the prevailing wisdom seems to be that shielding offers fairly minimal benefits, with spacing and orientation of magnetic parts being the best way of minimizing magnetically induced hum. But don;t quote me ... this is just my recollection of stuff I've read. I don't understand the science of it.

cheers, Derek
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Paul Birkeland on November 24, 2019, 06:22:55 AM
Hi Deluk - that's the idea.  Some other configuration might be better, but I'm just following the rule of thumb that coils turned 90 deg to each other are less likely to magnetically couple and generate hum.
Those coils are both pointing straight up, they are aligned.  As far as magnetic coupling goes, that's for an emitter (power transformers) and a receiver (signal carrying transformer).  There are no signal transformers in the BeePre.

You can also create some problems with high gain circuits picking this stuff up, but this shouldn't happen in the BeePre.
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Deke609 on November 24, 2019, 07:09:57 AM
Those coils are both pointing straight up, they are aligned.

In a bad way? I could make some aluminum L-brackets for mounting alternating chokes/trafos so that the center-line of the coils are at right angles.

cheers and thanks, Derek
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Paul Birkeland on November 24, 2019, 07:16:20 AM
I don't know, you'll have to do the experiment and see.
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Deke609 on November 24, 2019, 07:22:39 AM
I don't know, you'll have to do the experiment and see.

Well, I have a penchant for over-complicating things that I don't understand ... so why not?    L-brackets are now part of the plan.  ;D
Title: Re: Bigby - My Big BeePre Rebuild
Post by: caffeinator on November 24, 2019, 11:20:16 AM
Hi there,

I'm curious - what are all the various transformers (chokes, I presume) doing in there? I do recall there being an opportunity to sub in a couple of small chokes for 300 Ohm resistors, but how about the rest?

Where will the input/output jacks, switches, and input output wiring be accommodated? In the stock BeePre, those seem to occupy some space along the front and rear edges of the layout.

Those caps are impressive!
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Deke609 on November 24, 2019, 02:18:47 PM
I'm curious - what are all the various transformers (chokes, I presume) doing in there? I do recall there being an opportunity to sub in a couple of small chokes for 300 Ohm resistors, but how about the rest?

Where will the input/output jacks, switches, and input output wiring be accommodated? In the stock BeePre, those seem to occupy some space along the front and rear edges of the layout.

The two larger silvery ones between the power trafos are dedicated filament heater transformers for the 300Bs. I'm using EML 300Bs that draw 1.3A instead of a regular 1.2A.  The additional current dropped the voltage output of the stock heater windings a bit, and that combined with my wacky mains voltage over the summer was causing the LM1085 filament regulators to "drop out", resulting in hum. Plus, I wasn't able to hit my target of 4.9V dropped across the filaments - which would be within 2% of the ideal 5V.  The new filament trafos get me there.  All the others are chokes as you figured. There's a CLCLC filter for the HV and a CLC for 300B filament heater circuit.  Prompted by Deluk's question and PB's input, I'll be mounting half of them on L-brackets to get the coils oriented at 90 deg to each other (I based the current setup on on the typical way of orienting top-mounted PTs and OTs - forgetting that the center lines of their coils run parallel to the chassis, whereas my chokes are all perpendicular to it -- thanks Deluk and PB!   :)  )

I'll only be using one source, so I've eliminated the source selector switch.  Volume pots will be mounted vertically in the open space between the 300B sockets with extenders running to the front panel as shown in the attached. They'll be installed on a wider but shorter piece of aluminum sheet  so that the pots are closer together. It's going to be weird looking: PTs in front and tubes in back - but I like the idea of keeping the signal wiring well away from the magnetic bits.  Inputs are at the rear panel between the giant caps and outputs at the outer corners of same.

cheers, Derek
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Paul Birkeland on November 24, 2019, 02:20:02 PM
Personally, I would align all the coils with the power transformers, but that will be difficult.
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Deke609 on November 24, 2019, 02:25:43 PM
Personally, I would align all the coils with the power transformers, but that will be difficult.

Many thanks PB - these kind of pointers are really helpful.  Do you mean: align all coils so that their center lines are parallel with the top plate (like the PTs), and then orient them at 90 deg to each other? 

cheers and thanks, Derek
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Paul Birkeland on November 24, 2019, 02:47:14 PM
No, I mean I would have them all aligned with the power transformers. 
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Deke609 on November 24, 2019, 03:00:57 PM
Thanks PB.

But I still don't get it - sorry. Do you mean all in a single row? If yes, then also mount them vertically so that their coil center lines are parallel with those of the PTs, or just leave the coils standing upright?

many thanks, Derek
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Paul Birkeland on November 24, 2019, 05:51:37 PM
No, I mean all the coils pointing in the same direction.  The power transformers now point in the same direction.  I would suggest making all the other power iron also point in that direction.
Title: Re: Bigby - My Big BeePre Rebuild
Post by: caffeinator on November 25, 2019, 06:38:37 AM
Hi there,

Thank you for the thoughtful and detailed (with illustrations, no less!) answer to my question - looks like an ambitious project!

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. That would, at least judging from what I can discern as the bobbin orientation in each of them, have them pointing along the same line.

Hope this helps, and good luck with the BigBee!

caffeinator
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Deke609 on November 25, 2019, 07:46:54 AM
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
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Paul Birkeland on November 25, 2019, 07:51:16 AM
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. 
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Deke609 on November 25, 2019, 07:55:45 AM
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 (http://www.audioholics.com/loudspeaker-design/inductor-coil-crosstalk-basics)

I should be able to implement the orientation shown in figure 7.
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Paul Birkeland on November 25, 2019, 09:35:43 AM
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.
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Deke609 on November 25, 2019, 09:48:19 AM
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
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Deke609 on November 26, 2019, 01:03:31 PM
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
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Paul Birkeland on November 26, 2019, 01:08:41 PM
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?

Title: Re: Bigby - My Big BeePre Rebuild
Post by: Deke609 on November 26, 2019, 01:23:21 PM
I think you're completely missing the point. 

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

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

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

Quote
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  ;D  )

Quote
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

Title: Re: Bigby - My Big BeePre Rebuild
Post by: Paul Joppa on November 26, 2019, 02:00:55 PM
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.
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Deke609 on November 26, 2019, 03:05:29 PM
Many thanks PJ. I will explore this.

cheers, Derek
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Paul Birkeland on November 26, 2019, 06:08:56 PM
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  ;D  )
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.
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Deke609 on November 27, 2019, 03:41:02 AM
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.

Quote
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]
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Deke609 on December 22, 2019, 07:10:55 AM
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]]
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Deke609 on December 22, 2019, 08:54:49 AM
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
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Paul Joppa on December 22, 2019, 09:56:48 AM
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.
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Paul Birkeland on December 22, 2019, 10:22:19 AM

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?
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Deke609 on December 22, 2019, 10:28:36 AM
I will check right now.  Thanks PB.
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Deke609 on December 22, 2019, 10:32:14 AM
No hum without the 300Bs.
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Deke609 on December 22, 2019, 10:39:59 AM
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
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Deke609 on December 22, 2019, 01:53:26 PM
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
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Paul Birkeland on December 22, 2019, 02:14:59 PM
That kind of hum usually means that the chassis plate is not earthed.
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Deke609 on December 22, 2019, 05:12:13 PM
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.
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Paul Birkeland on December 23, 2019, 05:17:46 AM
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.
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Deke609 on December 23, 2019, 05:29:32 AM
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.
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Deke609 on December 25, 2019, 03:29:24 PM
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!
 ;D
Title: Re: Bigby - My Big BeePre Rebuild -- IS DONE!
Post by: Deke609 on July 06, 2020, 03:05:14 PM
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
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Paul Birkeland on July 06, 2020, 05:23:57 PM
Do you need a bigger chassis to fit those caps?
Title: Re: Bigby - My Big BeePre Rebuild
Post by: Deke609 on July 06, 2020, 06:16:50 PM
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