Vintage Sound Wokbench Amp Maniac

67plexi said:
Why my 1959 Fender Concert amp is running 507 VDC on the plates should be 456 VDC

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The bias is too cold.
When the bias is adjusted to around 70% the plate voltage will drop down.
Another couple things is that the screen resistors can be bad, or the speaker impedance will be incorrect.
Those 2 things will also affect the power supply voltages.

Without the proper load on the power supply, the voltage will climb up. That's ohm's law.
Cold bias means less load on the power supply, so higher voltage would be expected.

True, 110 volt or 115 volt primary running on 120 volts can affect the plate volts by a few percent.
But that alone does not explain the 50 volt discrepancy. Try the bias adjust first...
If you do not have a bias meter or a scope you should get something.
amp maniac.png

"Dramatic change in tone..."

Um....not really.
But we should keep in mind that 500+ volts DC is a bit high.
 
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Well the 1959 Triad power transformer was designed for 110 VAC input My input voltage is 125 VAC
And the filament voltage is high also that said it has the Tweed Bassman Output transformer
It's a killer amp paid $100.00 for it the harmonic Vibrato is to die for.
Thanks AMS for your input.

When I play out live I use a Furman power conditioner that holds the voltage at 120 VAC @ 120 VAC I set the bias cold 65%
And use the Output Transformer shunt method for setting bias.
 
That's neat. I need some sort of variac here. My line voltage in the house is usually 125 volts.
 
You guys got me digging in my old parts cabinet and I found a Variac I used when I was in Japan.

The input says 100 Volts with a variable output of 0 - 140 volts. Can I plug 125 volts into this instead
of the 100 volts rated? I'm sure it will change the output voltages on the dial, but will it damage the Variac?
 
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