Deluxe AB763 bias

[Leevi]

This question has probably been handled here before.


My question is concerning Deluxe AB763.



http://el34world.com/charts/Schematics/files/fender/Fender_Deluxe-AA763-schematic.pdf



What is the safe maximum level in mV measured on 1 Ohm resistor on the cathode when the plate voltage is 410V.


My assumption is ~24mV/tube but I think it can run even hotter?


/Leevi

[HotBluePlates]

What is the safe maximum level in mV measured on 1 Ohm resistor on the cathode when the plate voltage is 410V.

For 6V6, 14w/410v = 34mA -> 34mV.

It probably won't even redplate at that current at idle.  It may redplate when signal is applied.  You'd watch the plates under actual use with maximum signal applied to determine "max safe bias".

You cited 70% of max dissipation.  That's probably safe-but-conservative.  If this is a repair you'll not see again, choose 70%.  If this is your amp, you can likely idle hotter if you want, depending on the characteristics of the actual tube you have in the amp.

[sluckey]

That's a very subjective question. Most tube charts list 14 watts as the maximum plate dissipation. Based on that, you could run 34mV. .034a x 410v = 13.94w. But, if you play guitar through the amp, the power will increase and exceed that tube chart max dissipation. Tubes would probably die quickly. May not even make it through a 4 hour session.

I would feel safe biasing for 70%. That means .7 x 14 = 9.8w. Plate voltage of 410 would yield 23.9mV. This is my personal safe zone.

Note... All these calculations assume that plate current is equal to cathode current, ignoring screen current. So dissipation numbers err on the side of caution. Actual plate dissipation is slightly less than calculated dissipation.

[Leevi]

You cited 70% of max dissipation.  That's probably safe-but-conservative.

Yes, I calculated 70% of max dissipation and the reason for question was how much the old data sheets
correspond today's tubes. E.g. what comes to maximum plate voltages many amp manufacturers exceed those.

I would feel safe biasing for 70%. That means .7 x 14 = 9.8w. Plate voltage of 410 would yield 23.9mV. This is my personal safe zone.

Yes, follow it or not?


/Leevi