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Using an unused filament winding to buck voltage on power transformer

Hello All!

I am in the process of building the power supply for a 6P15P-EV to 45/46 tube amp. The transformer I am using is huge and was originally in an organ amplifier. The organ was from the late 1950's, so I'm guessing the amp was designed to run at 110-115 Volts, while the voltage at my house is usually 120-125 Volts. The power transformer is grossly overspecified for my intended use, having powered dozens of tubes originally. It has two filament windings. One is 6.3 volts center tapped, and was used for two 6L6 power tubes, and the other is 12.6 volts center tapped, and was used to power the heaters of all the other tubes, so it is likely at least 5 amps. I am using Coleman filament regulators for the power tubes, and the 6.3 Volt winding for the heaters of the 6P15P-EV. This leaves me with an the unused 12.6 Volt center tapped winding. Can I use this extra 12.6 Volt winding to buck the transformer in order to lower the B+? While I am pretty sure that I can, I am not sure if there are there any downsides, so any advice would be appreciated. Additionally, I assume that the 12.5 volt secondary (or half of it for 6.3 Volts) would be used to buck the primary, but is there any way to use it to buck just the B+ winding? Typically, I need to use dropping resistors on the filament windings of these sorts of transformers to get them down to the proper voltage, so bucking the primary should not be an issue, but I would appreciate any input on the possibility of bucking just the secondary if there is a way to do it.
 
The downside is that the mains winding will now connect directly to a secondary winding, so you are pretty much bypassing whatever insulation and hipot testing was applied to that transformer to allow it to pass whatever safety standards were relevant. Transformers have 'primaries' and 'secondaries' as a way of managing electrical safety. So you now wear a safety related risk.

Using an independent, isolated transformer for just the 'bucking' function avoids that risk.
 
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Yes you can.
Buck the primary, where it will be more effective, 12.6VAC is perfect to buck down 110-120 class Mains,some 10% nominal.
It will have minimal effect on hundreds of Volts HV secondary, not worth it.
Thanks for your reply. The transformer is on the border of providing too much voltage for the circuit. The 12.6 Volt winding may make the required difference if used to buck the secondary.
 
Bucking the secondary is indeed the only safe solution. Whether it is possible or not depends on the winding and rectification arrangements
Thanks for your reply. The power transformer has three separate filament windings. A 6.3 Volt, which will be used for the 6P15P-EV preamp tubes, a 5 Volt, which will be used for the rectifier tube, and the extra 12.6 Volt winding. I have not seen a schematic that shows how a secondary is best used to buck another secondary. Any advice/guidance would be appreciated.
 
Thanks for your reply. The transformer is on the border of providing too much voltage for the circuit. The 12.6 Volt winding may make the required difference if used to buck the secondary.
No, not the secondary.
If you buck the primary, all voltages, included secondaries will drop by 10%
If you only buck HV secondaries result will be minimal and only on those windings.
 
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It will work but you will lose the 6kv isolation margin provided by the insulation tape used between the primary and secondary. The full insulation is now the paint in the magnet wire.
NO, repeat NO factory made transformer relies on just wire enamel (what you despisingly call "paint" :rolleyes:) , there is always at least 1 sheet of mylar/nomex/pressphan between them.

Maybe not the doubled/tripled layer between primaries and secondaries, or not tested for HiPot, that´s something else but not as flimsy as you want to imply.

That said, if you insist on Lawyer Proof answers, then bucking should be forbidden, and only acceptable one is:"buy a new transformer, properly tested by all International Agencies", period.
 
Actually a transformer rated for 110 volts AC RMS input will probably be running in slight saturation and increasing the input voltage may not proportionally raise the output voltage. It will heat up the transformer a bit more, but that should not be an issue.

So you should actually measure the unloaded output voltages first with a peak reading rms reporting meter. Not a true RMS meter!
 
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Gentlemen,

The late 50s Organ power transformer was Not Hi-Pot tested to 6,000V.

Do not buck each and every of the secondary windings (5V rectifier filament; 6.3 / 12.6V filaments; High voltage [perhaps 350-0-350V].

Let's think this one out carefully.
The transformer has a nominal 110V-115V primary.
We have a nominal modern power mains of 120-125V primary.
So . . .
Take a High Pot Tested new transformer, 120-125V primary, and a 10V secondary.
You connect the Hi Pot Tested new 10V secondary in series with the old transformer 110-115V primary. Be very careful to connect the new 10V secondary out-of-phase with the old 110-115V primary. Careful, you test the phase after the primary is already connected, you better be able to test 10V phase, and 120-125V phase, or have a professional do it.
Now, the old transformer sees 110-115V all over again (de ja vu all over again).

Example: If the old Organ transformer had a 2-1/2A primary fuse, then if your new 10V transformer secondary is rated for 5 Amps, you are OK.
And, do not forget to fuse the new transformer's primary with an appropriate fuse (now you have 2 separate primary circuits with separate fuses).
If your power mains is 3 wire, and has ground, neutral, and hot, be sure to wire the fuses in series with the hot line, not the neutral line.

Wow! Was that easy, or what?
Caustion: it is only as safe as the excellent wiring techniques that you use, Right?
If you are not knowledgeable about these things, do not do it, have a professional do it.

Our US government used a boost transformer in the same way (but boost, not buck) to get the 208 three phase Y / 110V two phase, to be 120V two phase for a 1kWatt transmitter. They were either correct in their engineering, or if it was incorrect, they could do it because they were the US government, so they did it anyway. You decide if you trust your government or not.

Safety First!
Prevent "The Surviving Spouse Syndrome".
 
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Gentlemen,

The late 50s Organ power transformer was Not Hi-Pot tested to 6,000V.

Do not buck each and every of the secondary windings (5V rectifier filament; 6.3 / 12.6V filaments; High voltage [perhaps 350-0-350V].

Let's think this one out carefully.
The transformer has a nominal 110V-115V primary.
We have a nominal modern power mains of 120-125V primary.
So . . .
Take a High Pot Tested new transformer, 120-125V primary, and a 10V secondary.
You connect the Hi Pot Tested new 10V secondary in series with the old transformer 110-115V primary. Be very careful to connect the new 10V secondary out-of-phase with the old 110-115V primary. Careful, you test the phase after the primary is already connected, you better be able to test 10V phase, and 120-125V phase, or have a professional do it.
Now, the old transformer sees 110-115V all over again (de ja vu all over again).

Example: If the old Organ transformer had a 2-1/2A primary fuse, then if your new 10V transformer secondary is rated for 5 Amps, you are OK.
And, do not forget to fuse the new transformer's primary with an appropriate fuse (now you have 2 separate primary circuits with separate fuses).
If your power mains is 3 wire, and has ground, neutral, and hot, be sure to wire the fuses in series with the hot line, not the neutral line.

Wow! Was that easy, or what?
Caustion: it is only as safe as the excellent wiring techniques that you use, Right?
If you are not knowledgeable about these things, do not do it, have a professional do it.

Our US government used a boost transformer in the same way (but boost, not buck) to get the 208 three phase Y / 110V two phase, to be 120V two phase for a 1kWatt transmitter. They were either correct in their engineering, or if it was incorrect, they could do it because they were the US government, so they did it anyway. You decide if you trust your government or not.

Safety First!
Prevent "The Surviving Spouse Syndrome".
When getting phase right is a concern while connecting transformers, I usually use a dim bulb tester, and also test with 6.3 or 12.6 volt on the primary. If that goes well, I power the circuit with mains voltage. I have not bucked a secondary with another secondary before, but have doubled up identical transformers on a few occasions.
 
Now you can see why I recommend using a Buck transformer to Buck the Power Mains voltage.
Bucking all 3 of those secondaries is not recommended (with the high voltage windings center tapped, you need 2 (two) high voltage capable buck windings, one per original high voltage secondary lead).

Any transformer company worth its salt, Does Hi Pot testing from primary to frame/laminations; And primary to secondary.
Please do not tell me of a company that does not do complete Hi Pot testing. I get high blood pressure, and a worry streak just thinking about that.

Anybody heard of UL, CSA, etc.? I used to work with them on measurement issues, including single ended high voltage, differential high voltage, etc.
 
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Now you can see why I recommend using a Buck transformer to Buck the Power Mains voltage.
Bucking all 3 of those secondaries is not recommended (with the high voltage windings center tapped, you need 2 (two) high voltage capable buck windings, one per original high voltage secondary lead).

Any transformer company worth its salt, Does Hi Pot testing from primary to frame/laminations; And primary to secondary.
Please do not tell me of a company that does not do complete Hi Pot testing. I get high blood pressure, and a worry streak just thinking about that.

Anybody heard of UL, CSA, etc.? I used to work with them on measurement issues, including single ended high voltage, differential high voltage, etc.
6A3sUMMER: I want to buck the B+ secondary down with the 12.6 Volt secondary. Both are centre tapped. 12.6 Volts may seem nominal, but it makes the difference between achieving the desired operating point for the power tubes (45s) while using a soft starting rectifier tube.