Elementary Fender Twin Bias adjustment

[kdawg]

Hello - I have a Fender twin reverb Master 100w that I've black faced and now I'm setting bias. I have an inline socket with pin 8 resistor in series and a tap to plate voltage. Question is, I've got about 427 plate voltage so 70% is about 40mA according to 6L6GC chart. Now since each pair have pin 8 connected, should I be setting to 40mA or 20mA?

Thanks
-kdawg

[Jazz P Bass]

If both tubes are connected to a sense resistor & then ground you would be reading the current through both tubes.
No?
Then the reading should be divided by the number of tubes.
Personally, I think 40 ma per tube is a tad high.
Leo rarely ran them over 25.

[kdawg]

If both tubes are connected to a sense resistor & then ground you would be reading the current through both tubes.
No?
Then the reaing should be divided by the number of tubes.
Personally, I think 40 ma per tube is a tad high.
Leo rarely ran them over 25.

My 1 ohm resistor is in series at the arrow, so inline with a tube pin 8 to ground... so no dividing?


I was going by Weber Bias Calculator, which shows a 6L6GC around 42ma at 60%.

Thanks
-kdawg

[The Dude]

To clarify (and according to your red arrow) all of the other cathodes go directly to ground? The reason I ask is because if that arrow is moved a centimeter on your drawing past the junction of the two cathodes, then current from both tubes will flow through it. That said and assuming your drawing is correct, you would NOT divide your reading.

Personally, I would not go above 35ma and probably shoot for somewhere around 30, but that's my preference. As Jazz P says, "Leo rarely ran them over 25".

Also, you might consider adding 3 more resistors to the other cathodes so that you can check current for each tube and see how well they're matched. You've already modded the amp, so why not?

[kdawg]

Yes its a tube socket adapter I made, so it is inline with pin 8 not between the junction to ground. I'll bring her down a bit, thanks.

-kdawg

[The Dude]

So your talkin' a separate external bias socket/probe? If that's the case ignore my comment about adding 3 more resistors. You can check each tube with the probe.