Built a bucking transformer.

Tallying up the cost I probably spent around $90 but it was a fun / frustrating project.





Drops the wall AC by around 7.4 volts.

Why frustrating? Fairly straight forward project ...

Great addition to any tube system though, considering today's wall output.



PS ... mine cost me next to nothing as it was all "crap" that had been collecting dust in the bins.  

Been there, done that. Got the desired reduction in voltage.

Also, some of us have found used, but good condition US made 15-20 amp variacs on craigslist for $100 or so.....

I already have a variac. I just wanted to build one of these for fun and thought I'd show it off.

Congrats... What application is the bucker being used? The Hammond transformer used here is rated 37.8VA on the primary. The transformer core would saturate with the current draw of an ST-70 or ST-120 amp.

arledgsc wrote:Congrats... What application is the bucker being used?  The Hammond transformer used here is rated 37.8VA on the primary.  The transformer core would saturate with the current draw of an ST-70 or ST-120 amp.  

Hmm.. I believe this was the one that Skizo recommended. It's similar to his, 6A secondary. I built it for my ST35.

How do solid-state devices (CDs, TVs, DVDs etc. react to the lower voltages?

I built a simplified version of this one.

https://robrobinette.com/5e3_Modifications.htm#Bucking_Transformer

At first I tried to use a smaller box but it just wasn't worth the hassle. This case is approx. 7" x 5" x 3".
Right now I'm getting 123.3 out of the wall and 115.8 out of the bucker. Close enough for ('60's) rock & roll.

(This is the first one I built, the second was neater. I use one for my VTA ST 70, VTA SP 14, PH 14 and ancient Audio Control 520 EQ. I use the other for what I consider more "delicate" '60's guitar/keys gear, like a 1967 Silvertone 1482 amp and a '67 Vox Jaguar organ. I generally don't worry about pro level gear like '60's Fender amps.)

corndog71 wrote:

arledgsc wrote:Congrats... What application is the bucker being used?  The Hammond transformer used here is rated 37.8VA on the primary.  The transformer core would saturate with the current draw of an ST-70 or ST-120 amp.  

Hmm.. I believe this was the one that Skizo recommended.  It's similar to his, 6A secondary.  I built it for my ST35.

I think you are OK.  Sorry for the alarm bells.  Only the current needed to buck 6V is carried through the bucker primary.  The main current for the amp circuits is pulled directly from the wall plug (and carried through the secondary).

corndog71 wrote:

arledgsc wrote:Congrats... What application is the bucker being used?  The Hammond transformer used here is rated 37.8VA on the primary.  The transformer core would saturate with the current draw of an ST-70 or ST-120 amp.  

Hmm.. I believe this was the one that Skizo recommended.  It's similar to his, 6A secondary.  I built it for my ST35.

Correction ... 6 VOLT secondary, just to head off any potential confusion.

And yes, it's also a 6 AMP rating, so it can handle the amplifier easily ... both windings should be the same rating unless otherwise noted on the label. Main thing you lil bucker builders gotta remember is to TEST before using, as it is possible to hook it up backwards and BOOST what the amp sees.

Also, solid state isn't as vulnerable to out of spec wall voltage as tube equipment - you should be good up to around 125vac even on the classic equipment, but don't quote me on that. I just use the bucker on the ST120 and the Eico HF12a that I use to power a center channel. Everything else is straight out of the wall ... with protection of course.

sKiZo wrote:Main thing you lil bucker builders gotta remember is to TEST before using, as it is possible to hook it up backwards and BOOST what the amp sees.

Right. That was the frustrating part as the first time I connected the wires I got 129 volts!

Here's a basic schematic for a bucker with a 10V secondary at 120Vac. 

I use a battery to figure out polarity of the secondary to avoid 50/50 chance of wiring a booster instead.  Touching (on/off) a 1.5V battery to the secondary produces voltage pulses on the primary.  Mark which end of the secondary is touching the battery (+) terminal produces positive going voltage spikes on the primary and mark it.  In the example below these same polarity sides of the primary and secondary are noted with "dots" near there respective ends.  Then wire up accordingly.  But good idea to smoke test with no load!   Using a battery at the secondary steps up the primary voltage that is easy to measure versus the other way around.  

thanks for the schematic arledgsc, for those of us that can read them, they are worth 100,000 words!

I would still test your wiring because I tried doing it like that first pic and got a boost in voltage.