RobRob bucking transformer

Lowerleftcoast

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How can you tell if a piece of equipment would do better with the lower voltage?
One would measure the voltages in the amp.
The tube heater voltages are important for longer tube life.
The B+ can also be addressed as long as the heater voltages are within range. On occasion the B+ will be above the ratings of the filter capacitors.
Solid state amplifiers can also recieve too much voltage for their components.
 

King Fan

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How can you tell if a piece of equipment would do better with the lower voltage? Just by age?

Vintage amps (mid to late ‘60s and before) were likely built to a 115 standard. But I go by ear, for starters. Since you’re bucking *down* from wall voltage, it’s almost always safe for the amp to try it. My ‘67 VC sounds better — to me — bucked. My faithful ‘64 PR clone sounds better at 120.

As LLC says, heater voltages are a great barometer. Merlin notes tubes last longer if heater voltages are at or below 6.3V, but this also tells you (if your PT is a decent clone, or vintage), you’ve arrived in vintage voltage territory.

But the most common reason, as he notes, is that so many vintage amps and clones end up with high B+ or hot bias or both on modern power. Seems like 3 out of 4 new builds here come in hot — some, like the BF/SF Champ family, started out that way even on vintage power.
 

Blue Bill

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Thanks, LLC and K-Fan, that's some very useful information.
 

muscmp

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Very cool.
Great idea, I think when I make mine I'll add a four gang outlet on it as well just as an extension cord... I have a bag full of 12ga 12' cords with just the plug. Perfect!
doesn't seem like a problem but thought i'd ask. i'm planning on building one for a 75 vibro champ and 76 champ that i run in stereo. rather than using 4 outlets, couldn't i use a power strip in between and plug both into the power strip? that is what i'm currently doing.

play music!
 

Jazzman1955

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Vintage amps (mid to late ‘60s and before) were likely built to a 115 standard. But I go by ear, for starters. Since you’re bucking *down* from wall voltage, it’s almost always safe for the amp to try it. My ‘67 VC sounds better — to me — bucked. My faithful ‘64 PR clone sounds better at 120.

As LLC says, heater voltages are a great barometer. Merlin notes tubes last longer if heater voltages are at or below 6.3V, but this also tells you (if your PT is a decent clone, or vintage), you’ve arrived in vintage voltage territory.

But the most common reason, as he notes, is that so many vintage amps and clones end up with high B+ or hot bias or both on modern power. Seems like 3 out of 4 new builds here come in hot — some, like the BF/SF Champ family, started out that way even on vintage power.
It is instructive, (and frankly sort of fun) to have a bucking transformer unit that displays AC incoming line voltage, then bucked down voltages in real time.

I note tonight, that without my bucking transformer, my Tweed Princeton would be toasting with 124V AC , which I have previously tested to be about 7 VAC on the filaments.

To me this is not so much about sound, but protection of vintage amps designed for lower input AC voltage.

I frankly don’t notice a whole lot of sonic difference; if anything my older amps sound less “brittle” with the more appropriate input voltage
B9A18E3A-3124-42A2-8F82-CC46C467FE30.jpeg
 
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I have been looking for a supplier for the transformers and havent been able to source out a few obviously Hammond.
this is a cool thread thank you
 

Jazzman1955

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doesn't seem like a problem but thought i'd ask. i'm planning on building one for a 75 vibro champ and 76 champ that i run in stereo. rather than using 4 outlets, couldn't i use a power strip in between and plug both into the power strip? that is what i'm currently doing.

play music!
Absolutely. You only one need outlet to provide the lower input AC voltage. You can run a power strip off that. Be sure to not exceed the amperage of the bucking transformer- usually 4A.
 

Jazzman1955

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I do want to acknowledge GoKart Mozart for the digital LED voltmeter idea.
And again thank Rob Rob for the amazingly informative and comprehensive website that he has kept up for us.
 
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chas.wahl

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It is instructive, (and frankly sort of fun) to have a bucking transformer unit that displays AC incoming line voltage, then bucked down voltages in real time.

I note tonight, that without my bucking transformer, my Tweed Princeton would be toasting with 124V AC , which I have previously tested to be about 7 VAC on the filaments.

To me this is not so much about sound, but protection of vintage amps designed for lower input AC voltage.

I frankly don’t notice a whole lot of sonic difference; if anything my older amps sound less “brittle” with the more appropriate input voltage
Since you have the setup to do this instantaneously, I'd be interested to see what the differences are, in the "bucked" voltages, between being unloaded (nothing plugged in), and then with the amp plugged in. If you could also report what the amperage of the connected amp is (recommended fuse amps) that would be a plus.

It seems to me that not only the ratings of the step-down transformer being used (in primary voltage [115-117 V for Hammonds], secondary voltage [divided by 2 for use of the center tap] and its amp rating), but also the actual current load of the amp being supplied are affecting the amount of voltage reduction -- along with the actual wall voltage being supplied, of course.

As an example, in @robrob's website article on this (search for "Buckminister"), he says that with wall voltage 121.25 and a Hammond 166N6 (6.3 V CT 115-117 V 4A 25.2 VA) he got reductions of 3.7% and 7% (or 4.35 V for use of CT and another 4 v for using the full secondary winding to buck, 8.35 V). That's more than the rated 3.15 V or 6.3 V (33–38% more), and also more than just factoring in the difference in voltage rating (115–117) of the transformer and the actual wall voltage (121.25/116 = 1.045), and that times 6.3 is only 6.58 V. So I'm wondering if putting a load on the transformer results in an additional drop in the voltage reduction -- and if that can be estimated in advance.

I note the prices for the Hammond transformers that might be considered in the 4 A range (enough for a Bassman, say, at upper limit: 166N6 (6.3 V) is roughly $15, while the 166N12B (12.6 V) is ~$25 and the 166N8 (8.5 V) in the middle is $30. My own opinion is that I'd just as soon buy one that on the "max" reduction setting would reliably get me from 121 to 112 V, and have a "half" setting that would give 116 V or so. Yeah, they're pretty cheap, all things considered, but if doing everything empirically and iteratively, the costs add up.
 

King Fan

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I can't shed any light on that, Chas. FWIW, mine seems to drop the same voltage no matter if I'm running the little Micro or the big 5G9. Sadly, I can't test it out on a Twin or Super Reverb -- unless someone has an extra to loan out?
 

chas.wahl

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Ha Ha — A Twin or Super (reverb or no) is way outside my amp universe! I'm strictly an apartment-bound aficionado. Don't know if I will ever build a LTPI amp, or one bigger than 5Y3 w/6V6. My big issue is always going to be getting a full sound out of a low-volume situation. Thanks for the input though.

I'll probably start with the cheaper 166N6, with the 7-percent solution being the greater reduction; but I'd still like to know why @robrob's initial build dropped considerably more than the rated secondary.
 

muscmp

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Jazzman1955

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Since you have the setup to do this instantaneously, I'd be interested to see what the differences are, in the "bucked" voltages, between being unloaded (nothing plugged in), and then with the amp plugged in. If you could also report what the amperage of the connected amp is (recommended fuse amps) that would be a plus.

It seems to me that not only the ratings of the step-down transformer being used (in primary voltage [115-117 V for Hammonds], secondary voltage [divided by 2 for use of the center tap] and its amp rating), but also the actual current load of the amp being supplied are affecting the amount of voltage reduction -- along with the actual wall voltage being supplied, of course.
Fantastic idea Chas. Fellow Prine fan? Accept my disclaimer that I am a paint by numbers self trained dilettante. Enjoy electronics but no formal training. I would be disinvited from Audiokarma.

I did entertain adding an amperage meter to the setup as well, but did not do so.

Over the years I have sold off my multiple big hitter 50-100 W tube amps. But I can certainly trial my remaining amplifiers - 5 to 35 watts. By weekend will add the data regarding rated fuse listing + baseline vs under load led voltage meter photos for a sampling of my amplifiers.

To be clear we’re talking about the same thing: I’m going photograph the unloaded bucking unit voltage -with nothing plugged in- then with various specified amplifiers?
 
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chas.wahl

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To be clear we’re talking about the same thing: I’m going photograph the unloaded bucking unit voltage -with nothing plugged in- then with various specified amplifiers?
That's exactly what I suggested. Thanks! I think that unloaded, one lower-powered amp and one higher-powered one, as available, would tell the story easily enough. And since "voltage" is typically measured at quiescence, I don't think that there's any need to take measurements with those amps playing or "dimed". The basic question is whether the addition of amp load makes a significant difference in the voltage reduction effected by the bucking transformer.
 

Jazzman1955

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Here’s one example. Tweed Princeton, single 6V6. 1A fuse rating. When I first flipped on the amplifier, the outlet voltage (yellow meter) sagged to 94 VAC for about one second, then promptly resumed baseline 114 V, unchanged from the no load situation.

E5EF1CA9-26CF-42D1-ADB3-16E61538E4DC.jpeg
5DB849FC-2952-469A-B333-CDD08EA4E1A4.jpeg
 

King Fan

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Seems like a good trial -- reflects what I've seen. I don't know the physics here at all, but if our little amps could *measurably* drag down the output voltage, given wall voltage must be pretty stable, where would the 'missing' voltage be going, and what would it be doing?
 

chas.wahl

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Well, that's a revelation! The voltage reductions appear to be unaffected by whether the bucking transformer is loaded or not, and also appear to be consistent with a transformer rated for 115 V primary being given 121 V (that is, the reductions of 7 and 14 V are about 121/115 x 6.3 and 12.6 V).

Not to be perseverative, but I still wonder why @robrob's initial trial with 121.25 V through a 6.3 V (or 3.15 V for half secondary) transformer gave him reductions of 8.35 and 4.35 V respectively. That transformer (Hammond 166N6) is also rated 115 V, and seems to be "over-performing" compared to what you'd expect based on the ratio of actual vs rated primary voltage. Interesting.

Thank you for doing this.
 




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