"Buckminister”® vintage voltage device mods

[chas.wahl]

While thinking about building this voltage “browner” devised by @robrob (via R.G. Keen)
https://robrobinette.com/5e3_Modifications.htm#Bucking_Transformer
the following mods occurred to me, and I’d like advice on whether they’ll work, or not:

• Rather than having one outlet of the duplex fixed at “full voltage” and only one outlet, build it with two channels, each independent from the other
• Use two sp3t switches so that each outlet is variable 3 ways, between full line voltage, half-reduction (‑3.15 V), and full-reduction (‑6.3 v).

The big question is whether I can “mix-and-match” the outputs from the bucking transformer, powering one outlet from the middle of the bucking winding, and the other outlet from the whole winding. Here's the most apposite of @robrob's schematics, along with my proposed build:

I’ve laid this out in an aluminum Hammond 1590X box (5.7 x 4.8 x 2.2 inches) with C&K rotary switches and a couple of these LED voltmeters to show what’s what, and where

(thanks to @GoKart Mozart in this thread).
If it’s confirmed that the schematic here probably won’t electrocute me or burn my house down, I’ll finish a physical layout and publish that.

Thanks!

 

[mountainhick]

Cool idea if you need such a thing. I don't know when I'd have the need to run two things on separate voltages.

The schematic looks fine.

Make sure the TF will handle the current load.

1A or 4A fuse?

Did a little double take with the "ground foot" arrow. I assume you mean the AC ground wire to chassis.

 

[2L man]

Clever idea and to me it looks to work!

However both circuits primary-secondary phasing look like they will increase the voltage? It is woth measuring before outting in use that voltage drop if lower voltage is wanted!

Switching primary voltage current flowing cause very short break which in theory 50% times make higher surge current when phase come back opposite what current did flow when switching began. Its because transformers iron core "magnet bars" keep orientation for a while and that can blow mains fuse!

 

[chas.wahl]

@mountainhick: Good catch -- I had copied that 1 A note from another schematic, and neglected to make the fuse 4 A, which is what the transformer in question is rated for, and @robrob's advice was to use a 4 A, slow-blow. He used a 12 V / 4 A model, but I don't think I need that much voltage drop -- plan to use the 6.3 V, or the 8 V, which are almost the same size.
As to the need for two channels: I just thought it was a waste of an outlet to have it fixed at wall voltage -- a power cord will do that.
And, yes, conductive box grounded.

@2L man: I do understand that getting buck or boost out of such a transformer depends on the relative wind polarity (beginning and end) of the primary and secondary. My (small) understanding is that the dots on a schematic indicate the beginning of winding, and that even if shown by xfmr mfgr in data, they're often not "aligned" as shown -- which is why the instruction to swap secondary leads (similar to squealing NFB) exists.
And: one of the reasons I include the on/off power switch (not strictly necessary if one is willing to unplug power) is to turn the unit off before switching to a different power level. It would probably be a good idea to turn the amp off too.
This is the poor man's variac after all . . . the sister to the light bulb current limiter.

 

[Ten Over]

I consider the dots to be phase designations. Phase relationships are not that important with output transformers, but they can be absolutely critical with power transformers. Accordingly, I have never had any power transformer of any size that had incorrect phase designations.

Here is a diagram that sort of detangles different ways to connect a buck/boost transformer:

 

[King Fan]

Having built one of these and strongly evangelized for them here and elsewhere, my $.02...

I see no reason you couldn't make two different bucked outlets -- but is the game worth the complexity candle? As @mountainhick says, it's hard to see how often it'd be useful.

That's doubly true if you're just dropping ~3 or ~6V. A few volts just doesn't make that much difference, in either sound or heater voltage. For sound, more than one user has said they have a hard time hearing the difference even with Rob's ~6V or 7% drop. I can hear it, but it's not huge. And for heaters? If your wall is at 123V and your heaters are at, lets's say, 6.9V, bucking down to 119-120V will only drop the heaters to 6.75V.

By contrast, I often find it handy to have both a 'full' and a 'bucked' outlet. I have lots of amps that *can* run at any of those voltages, but may sound better or run happier heaters at a particular level. For testing, I'll plug into the 'hot' outlet, listen or measure, then just move the cord to the 'bucked' outlet and try both the 50% and 100% cut with the flip of a switch. For everyday use, I plug my 'vintage voltage' amps into one power strip, and my 'full voltage' amps into another, and just run the two power strips off the two outlets.

I will admit I seldom use Rob's bigger ~12% cut, so for me a simple useful bucker could just do the nominal 6V thing. But I've *never* wished I had a smaller cut available between 6 and 0. If you want two levels, I'd for sure use at least the 8V transformer

It's not 'needed' but I strongly endorse both a pilot light and an on-off switch -- especially if you have multiple amps on a 'bucked' power strip. More information, more control, more clarity.

Finally, I know *you* know all about the difference between a bucker and a LBL, but new builders, who need LBLs the most, might get confused if they thought a bucker could do what an LBL does or vice versa.

 

[chas.wahl]

@Ten Over: Unfortunately, Hammond doesn't seem to provide a diagram with the polarity dots; hence @robrob's instruction to see what happens and swap leads if necessary to buck. And he shows the connection between primary and secondary crossing diagonally; so that's what I did too. If I have the dots in the wrong places, compared to how I've wired it (if what I've shown would boost and not buck), let me know -- the diagram "detangling" buck & boost doesn't seem to indicate which ones are buck and which boost either; so the "correct" way to wire them is still escaping me.

@King Fan : I did think about this, and if you've noticed, the circuit proposed allows full wall voltage as one of the three settings. You're probably right about -3.15 V not being an increment worth worrying about. The "simpler" version for me would be to eliminate the intermediate drop, not the full or -6.3 V. I did, in layout, allow for moving to the 8 V transformer if 6.3 doesn't seem like enough. The 10 and 12 V transformers are a slightly larger size.

 

[King Fan]

I did think about this, and if you've noticed, the circuit proposed allows full wall voltage as one of the three settings. You're probably right about -3.15 V not being an increment worth worrying about. The "simpler" version for me would be to eliminate the intermediate drop, not the full or -6.3 V. I did, in layout, allow for moving to the 8 V transformer if 6.3 doesn't seem like enough. The 10 and 12 V transformers are a slightly larger size.

Good ideas. You've thought this through.

BTW, although I've had good luck trusting phase dots on OTs, I suggest you verify whatever hookup you choose is a bucker and not a booster. IIRC, Rob had some clue based on the physical location of the Hammond 2°s, and on mine that clue was 'backwards.'

 

[Emster]

So you say you want a Buckminster Fuller tone? Geodesic mon

 

[Ten Over]

@Ten Over: Unfortunately, Hammond doesn't seem to provide a diagram with the polarity dots; hence @robrob's instruction to see what happens and swap leads if necessary to buck. And he shows the connection between primary and secondary crossing diagonally; so that's what I did too. If I have the dots in the wrong places, compared to how I've wired it (if what I've shown would boost and not buck), let me know -- the diagram "detangling" buck & boost doesn't seem to indicate which ones are buck and which boost either; so the "correct" way to wire them is still escaping me.

I guess I should have known that the 166N6 doesn't give you the phase relationship since I used a pair of them on a buck/boost box a while back. I probably determined phase with a dual trace 'scope.

#1 starts off with 120V and delivers 114V to the amplifier PT, so it's bucking.
#2 starts off with 120V and delivers 114V to the amplifier PT, so it's bucking.
#3 starts off with 120V and delivers 126V to the amplifier PT, so it's boosting.