Do you have an early 1950's Fender High Power Tweed Amplifier? Ever notice that your power tubes run hot 🔥 🥵 maybe discolor the rear panel and burn the silk screening off the pentode power tubes? It's because the bias circuit is set up for 110 VAC and the line voltage is 122 or 123 causing a rise in the output voltage of the secondaries of the power transformer.

In the pictures you'll see me biasing a 1951 Fender Tweed Pro with the 250 ohm 10 watt resistor with a 25mfd @ 25volts parallel capacitor. When I checked the amp the bias voltage drop across the resistor was 29.55 volts. Using ohms law  E ÷ R = I  29.55 volts divided by 250 ohms equals .1182 Amps. Or 118 mA... divided by two tubes is roughly 59 mA per tube.

After installing the updated bias kit the voltage drop across the new bias resistor was 37.6 volts divided by the new 500 ohm bias resistance equals .0752 Amps total. Divide by two tubes is 37.6 mA per tube if the tubes are fairly close to being matched. This was done at a line voltage of 122 VAC, (which gives me 36mA bias at 120 VAC line). At 110 VAC line the bias was still above 30mA.

The last two photo's are of the math.

Don’t forget that the plate voltage has increased to 441 vdc via the higher AC line voltage. This compounds the bias issue.

While designing this kit I also took the liberty of increasing the working voltage of the capacitor to 50 vdc. The polarity of the kit is also clearly marked.

I don't suggest using 6L6G's in these 1950's amps because today's wall voltage makes the plate voltage exceed the ratings for 6L6G. You can, however use a 5R4WGA rectifier.

*NOTE* This kit needs to be installed by a professional as electron tubes that are present operate on high voltages and can be lethal.

**Rechecked voltages today 441 VDC plate 37.6 vdc drop across the bias resistor with 122 vac line.

So 37.6 volts (E) ➗️ 500 ohms (R) = .0752 mA (I) divided by 2 tubes gives you 37.6 mA @ 441vdc..