What's the Yellow Wire?

[AgAuthority]

Hey Folks

Been quite awhile since building anything--back around early 2000's was super into building and playing with stuff but walked away from it quite awhile ago. Last couple of winters been going into the basement and tinkering with stuff and kinda got a bug to build a amp again and would like to use a set of transformers I have laying around but kinda wondering what I got going on.

Way back when bought a bunch of surplus Bedrock Royale Reverb power and output transformers. At the time had access to a Royale Reverb, the multi paged schem floating around the web was mine. Digging thru my notes but can't figure out what the yellow wire is for....

Primary
Black
Black/White Strip
Brown
Brown/White Stripe
Yellow

Found a picture on the web of the Royale Reveb's wired IEC socket with fuse. My googlefu couldn't locate that IEC socket

IEC.jpg

My notes show
Top Slot--Yellow
2nd Slot--Black/White--Brown/White
3rd Slot--have a line and arrow to switch
4th Slot--Ground

Then have Black and Brown tied together at the switch

Can't vouch for my accuracy here, sure I have lots of pictures on a old hard drive or a throw away camera that never got developed.....that was planned to explain this to my future self

Further messing around with my meter shows
Brown to Brown/White -- 2.9ohms
Black to Black/White -- 2.7ohms
Black to Yellow -- 2.2ohms
Black/White to yellow -- .7ohms

No continuity between either of the blacks to the browns

Any help/ideas/words of encouragement appreciated
brentw

[Phil_S]

I believe you are describing a universal voltage dual primary on the PT. My crude drawing is based on the information you supplied. I hope it is correct.

In series, they'd be appropriate for 230/240 mains or maybe 220/230. You'd use the yellow wire as the "end" of the winding for the lower wall voltage (220 or 230) and cap off the Black-White and leave it unused. For the higher wall voltage (230 or 240,) you'd cap the yellow wire and use the Black-White.

For use in North America 120V wall supply you'd wire the two windings in parallel, which is what you describe:
Black-White tied to Brown-White
Black tied to Brown

For North America (and maybe for Japan 100V, but not likely) you might be able to use the Yellow instead of the Black-White to cut 10V, but I'm unsure how well that will work and it might only be 5V for 115V or 110V wall supply.

The stripe, or no stripe is there to indicate phasing. For this to work, the windings need to be in phase.

My conclusion is that you should probably cover the yellow with heatshrink and tuck it out of the way. It isn't going to be used.

[AgAuthority]

Thanks for the reply, your post makes a whole bunch of sense. Have some time in a couple of days to test it out

Thanks!

brentw

[Phil_S]

Let's just say I have experience with it and was glad to share what I know. If you are testing, don't feel obligated to plug your PT into the wall socket. You might want to screw everything down to a wood plank and supply the primary with the 6.3VAC or 5VAC output you get from a filament winding on another transformer. A PT is a voltage multiplier. If you know the input and the output voltage, then you know the turns ratio. From there, a bit of ordinary math will tell you what to expect with 110VAC or 120VAC. Or, you can just hook directly to the mains, but I wouldn't do that without an in-line fuse on the hot leg. With no load, a low value fuse (500mA or 1A) is plenty big enough. Use slow blow type to allow for any initial inrush. The fuse is there primarily to protect your house and it's important to use one. Good luck.