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I have a pair of monoblock tube amplifiers that were designed to run on 117VAC. My wall voltage is about 121-123V.

What is the best way to drop the voltage down to an appropriate level?

Option 1: Install a 12V bucking transformer before the primary of the power transformer.

Option 2: Install some type of dropping resistor before the primary, such as 6 ohm 50W chassis mount.

The amplifier currently draws 2.5A from the wall at idle. It is a class A audio amplifier with 4 1614 (6L6GC) power tubes in push-pull parallel configuration.

The filament voltages of the 6L6 tubes are measuring 7.2VAC where they should be 6.3VAC, and the B+ is 418VAC where I think it should be 390V, but the schematic is quite hard to read.

I did some math and came up with a 6 ohm dropping resistor, which would drop the voltage down to 107.7VAC and provide 6.3VAC at the filaments, but 107 seems low when the original specification was for 117VAC line voltage. This resistor would dissipate 30W of heat too! which I don't think would be good.

Attached schematic and pictures of amp underside.

Underside of amp chassis

Full schematic

Thank you.

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    \$\begingroup\$ Anything designed for 117 VAC will be designed to work for anything within about about 10-15 VAC of that. 121-123 VAC is definitely an expected value that the designers anticipated. Not even a small question in my mind. Why are you worrying, exactly? \$\endgroup\$ Nov 16, 2023 at 1:15
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    \$\begingroup\$ The bucking transformer sounds a much better approach than a resistor. What's the voltage at the center of the output transformer - the schematic says 440V (which can be adjusted in the regulator). \$\endgroup\$ Nov 16, 2023 at 1:27
  • \$\begingroup\$ @periblepsis I'm worried because the filament voltages are 7.2VAC when they should be 6.3VAC, so it is running the tubes extremely hot. \$\endgroup\$
    – Jon
    Nov 16, 2023 at 14:58
  • \$\begingroup\$ @Jon I don't see how you got 7.2 VAC from 6.3. And I've run tubes, myself, from 6.3 and 12.6 filament voltages driven off of lines that vary from place to place where I've operated them. Keep in mind that Tungsten also has a natural opposition to increasing temperature, by increasing resistivity with increasing temperatures. It's fairly self-regulating. I'm not getting your figure from 121-123 VAC vs 117 VAC. \$\endgroup\$ Nov 16, 2023 at 23:44
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    \$\begingroup\$ @Jon I suspect your measurement may be off. Do you have a true-RMS meter? Or a cheap one that uses an approximation? I guess I'm questioning something in the numbers you are citing. 122/117*6.3 isn't anywhere near 7.2. Just doesn't work out. So either the 121-123 is wrong, or the 7.2 is wrong, or the transformer has a problem or the designers used the wrong transformer. \$\endgroup\$ Nov 17, 2023 at 20:08

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Someone linked you over on Audio Karma. I am uploading what I have for Scott 265a bucking transformerenter image description here

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  • \$\begingroup\$ That's awesome thank you. I found a similar setup on TheRadiolaGuy's website and he was kind enough to give me some tips via email. Unfortunately the transformers I bought are too tall to fit under the chassis :( so I will be making an external housing for them. \$\endgroup\$
    – Jon
    Dec 1, 2023 at 17:41

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