Ye old wall voltage of yesteryear.

[Axtman]

I am working on a 1974 Fender Silverfaced Champ amp. I noticed that all the voltages I check were really high as compared to the schematic. When I put the amp on a variac and turned the AC voltage down to 105 volts the amp's voltages matched the voltages on the schematic.

I know that old amplifiers were designed for lower AC input voltages such as 110 or 117. My house is typically checks out at about 123 volts AC.

What do you guys do in case of higher voltages in the amp?

[Enzo]

Accept them as simply part of the environment. These are just guitar amps, not lab gear.. If the mains voltages are 10-15% high, everything will survive. Most old Fender schematics have a note printed on them that voltage readings can be within +/-20%.

[Leo_Gnardo]

What do you guys do in case of higher voltages in the amp?

Like Enzo says, live it or live with it. For those who are sensitive to line voltage there are such things as multi tapped autoformers, or you can build a line-bucking circuit for reasonable money. I was lucky to inherit a couple of huge (like 2 KVA) buck/boost GE autoformers built like tanks, and I run old Dynaco stereo gear on one at about 112VAC which keeps them happy. The line-bucker is made easily enough with a filament transformer that handles a couple of amps. Primary connects to your AC line, and secondary is run in-series but reverse phase to the AC power you bring to your prehistoric prizes. 6.3 and 12.6V transformers can be easily found and not terribly expensive. With one of these you could knock down your line voltage by 6.3 or 12.6V, or whatever other voltage transformer you happen to find, and make life more comfortable for your legacy gear.

[g1]

If I think it's too far out, I look to the heater voltage, and the high voltage vs filter caps voltage rating.

[pdf64]

It seems reasonable to assume that the PT is receiving its intended mains voltage when the heater voltage (amp at normal idle loading) is 6.3VAC.

[Leo_Gnardo]

It seems reasonable to assume that the PT is receiving its intended mains voltage when the heater voltage (amp at normal idle loading) is 6.3VAC.

I'm with ya on that. The amp doesn't so much care what the line voltage is, it DOES care what the internal voltages are. An easy indicator is the filament voltage. If that's spot on, everything else should follow.