Quick case study -- bucking transformer for the win...

King Fan

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I've been a fan ever since I built my version of Rob's bucking transformer, but I just had a chance to open up my hot-from-the-factory '67 Vibro Champ and compare voltages with and without. I usually run at the (approximate) -7% setting. Here's the comparison:

1659842936673.jpeg


It seems many/most BF/SF Vibro-Champs (and Champs, and Broncos) ran crazy hot voltages -- sometimes over 400 on B+. I felt lucky to get my plate voltage down to 380 with a vintage 5Y3.

The amp sounds fantastic, and I have every hope the lower heater and plate voltages will help it live another 55 years. The -12% setting gets heaters down to a perfect 6.29V, but bias on my NOS RCA 6V6 drops to 93%, and the amp doesn't sound quite as awesome to my ears.

If you have high B+, hot bias, or simple curiosity to hear how your amp sounds on vintage voltages, I strongly recommend this simple build.

Rob describes his version (with several nice variants) on his 5E3 mods page:

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

The whole concept is detailed in this classic 1998 piece by Mr. Keen about his 'Vintage Voltage Adapter':

http://www.geofex.com/article_folders/vintvolt/vintvolt.htm

There've been some nice builds here in recent years, several in this thread:

https://www.tdpri.com/threads/robrob-bucking-transformer.1091943/
 

Kevin Wolfe

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Nice summary @King Fan
I’ve got to get back to the bench and do one of those. I’ve not been playing my ‘66 AB165 for those very reasons. Heater voltage is around 7.3 VAC.
When I restored/converted it, I was able to tame the B+ with a fat sag resistor, but the heater voltage never even crossed my mind. I’ve considered just dropping the heater volts, but I feel like the entire circuit would benefit from a reduced primary.
 

King Fan

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Nice summary @King Fan
I’ve got to get back to the bench and do one of those. I’ve not been playing my ‘66 AB165 for those very reasons. Heater voltage is around 7.3 VAC.
When I restored/converted it, I was able to tame the B+ with a fat sag resistor, but the heater voltage never even crossed my mind. I’ve considered just dropping the heater volts, but I feel like the entire circuit would benefit from a reduced primary.

Good one. That's actually the long version of this story; I restored the VC before I built the 'brown box', so ran a sag resistor, not considering the heater voltages.
 

schmee

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I bought I think 5 transformers for these a couple years ago. Thought I would pass on the savings, but never even got around to making my own! :lol:
So if anyone wants a transformer, I'll take a pic of the model and let's make a deal!
 

muscmp

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I bought I think 5 transformers for these a couple years ago. Thought I would pass on the savings, but never even got around to making my own! :lol:
So if anyone wants a transformer, I'll take a pic of the model and let's make a deal!
i'd be interested in one.
thanks,
play music!
 

King Fan

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There've been several discussions here recently about how much to buck, whether one level of bucked voltage is enough, and how to know when bucking might be right for you. :)

Briefly, it can't hurt to have two levels; if you're intent on the simplest design, though, something in the roughly -7% suits most of my amps -- by ear and DMM. Utility companies tell us that delivered power in many parts of the US had already reached 115V by the 1950s. But manufacturers often continued to rate their amps for 110 (ratings are a regulatory thing). So you may find vintage amps that are happier down 10-12%. Plus, some supposed clone PTs come in *way* too high on B+, and some vintage and clone amps run heater voltages north of 6.9 or 7VAC, which is definitely hard on tubes.
 

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schmee

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Here's another thread on making one. Or visit Rob Rob's site. I guess I need to "get 'er done" after a few years of procrastinating now!
 

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joulupukki

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I wonder how well using a thermistor does a similar thing? On my 18W TMB build where I’m currently using an Antek AS-1T300 toroid as the power transformer I added an Ametherm SL15 22102 thermistor to control the inrush current issue I was having and discovered it drops the voltage by about 5 VAC so instead of running around 122 VAC it’s now at 117 VAC. That part wasn’t necessarily desired but I wonder if that solution would work for people trying to accomplish the same thing for vintage amps? Those thermistors are very inexpensive.
 

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@King Fan, for people who want to understand why a bucking transformer might be useful and/or prudent, I would add this thread:
It seems only peripherally related, but has some interesting discussion about how a bucking transformer works, and design issues.
 

King Fan

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It seems only peripherally related, but has some interesting discussion about how a bucking transformer works, and design issues.

Thanks for the 'citation.' In fact, I started that thread with the bucking transformer in mind, and I’d argue “how low to go” isn't peripheral. If a lot of 1950s amps placarded for 110 were actually running on 115, it might explain why most of my tweeds are happiest / bias best at Rob's ~7% reduction.
 

robrob

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Even though the -14% gives you a lower than perfect voltage it will give the amp a softer "browner" tone so it's a worthwhile option.
 

King Fan

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Even though the -14% gives you a lower than perfect voltage it will give the amp a softer "browner" tone so it's a worthwhile option.

Excellent point — I sometimes run mine at the ~107V setting for just that reason. Browner isn’t just for vintage; witness EVH.

For sound comparison, I kinda like my “low” and “lower” on a switch; really great for A:B listening, and as Rob notes, safe and quiet to hot switch. But the two-outlets approach would work too; unplugging to switch outlets isn’t hard. I have a Luddite friend who claims he doesn’t need a bucker, that to change B+ he can just switch rectifier types and re-bias. Um, yeah, A:B that….

It may also be smart to have the lower setting in case you end up with a clone PT (weirdly common) that *way* exceeds clone B+, even at vintage voltages. And if you run spendy tubes, old time engineers with big tube installations, like radio stations, mainframes, etc, apparently always kept heaters at or below 6.3V. The +/- thing is tolerance for operation, not tube life….
 

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schmee

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OK, I gotta finally build mine. My BF Deluxe's run at ~460 plate volts. The fear is, ......they sound fantastic now! Will they be too clean, polite and sparkly with the voltage down?
I guess I'll find out.... I haven't used the Variac in years, I suppose I should hook it up and see what I think....
 

schmee

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OK, after parts sitting for years I finally built my Bucking Tranny rig. I have to say, it's a big klutzy thing. although only a 4.75" cube it seems huge.

What I really wanted was just a little thing like a largish wall wart. I may redo it. All I need is a pigtail single AC outlet at 7% reduction and a tranny really.
QUESTION: Do I really need a fuse if the amp has a fuse, and the wall has a breaker anyway? Extension cords dont have fuses.
Ordering 3" diameter shrink tube now ... :lol:

005.JPG
 




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