Measuring plate voltage on Fender Prosonic

Discussion in 'Amp Tech Center' started by JDB2, Oct 18, 2020.

1. JDB2Tele-Holic

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I'm getting ready to set bias on my Fender Prosonic. I have read that generally you can measure plate voltage off of pin 3 (plate) and pin 8 (cathode). Any reason that won't work on a Prosonic? Schematic attached. I'm going to use a bias probe to measure plate current. By the way, the amp will operate in either class AB fixed bias or class A cathode bias, per a switch on the rear panel.

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2. Jon SnellTele-Holic

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Makes no diference which amplifier you are setting up.
The maximum output from an 6L6 is about 30 watts plus 2 for the screen grid. So measure the anode voltage of the amp, for this exercise we will assume that to be 465 volts. Divide the 32 by the 465 then multiply by 0.7 the answer in ma. is the 70% plate load bias setting required for that amplifier.

The 0.7 means 70% Hot anode / plate dissipation. Therefore you can use 0.5 (50%) as a Cool AB setting, 0.6 (60%) as a Warmer AB setting the one I would prefer. Or go for the 0.7 or even 0.75 (70 - 75%) if you like it hot.

Example: 32 / 465 X 0.7 = 48.2 ma. Bias
Measure the current as voltage in mA across a 1R 1W resistor temporarily placed between points d and the ground point of d.
Or as you suggest, use your bias probe.

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3. JDB2Tele-Holic

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Thanks for your very thorough and concise explanation! So I can measure the anode voltage from pin 3 to pin 8, is that correct?

4. Jon SnellTele-Holic

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Anode voltage is measured from the anode pin to the cathode pin, correct.

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JDB2, I measure plate voltage from the anode to chassis ground. In the Pro Sonic, there is a switching circuitry for the cajoles. In the two fixed biased modes, using the cathode as a reference for that chassis ground is fine since in fixed bias the cathode goes directly to ground. However, in the cathode biased mode, one would do better to take the ground probe to chassis ground. And yes, I do want to know what the plate dissipation is in the cathode biased mode. In the Pro sonics I have had, that dissipation was at 100% or a bit higher, which is what one would expect. You will find two different sets of numbers for those two fixed biased modes. The plate voltages differ and therefore the current draw and the plate dissipation numbers differ as well.

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