Fender Twin Reverb 65 Reissue Bias Question

[TomCarlos]

Friends....

I bought two different sets of 6L6GC tubes for my Fender Twin Reverb 65 Reissue. I am using a combination of the Bias Scout Probe and a trusted VM to take some measurements. I double checked the problem for resistance readings- it looks spot on. There are some puzzling results with what I am seeing- I think.

I started with the Ruby Tubes 6L6GC set (quad matched). After cranking the bias adjustment fully clockwise (from inside the chassis), the best I could do is get 37ma of Cathode Current at a Plate Voltage of 436 volts.

I then inserted a set of JJ 6L6GCs. Again, a quad matched set. With the bias cranked fully clockwise (from inside the chassis), I can get 44.5ma with a Plate voltage of 428 volts. I am getting -42.3 volts at the grid Pin 5 (across the 1.5K resistors).

I checked all 4 tubes on both sets and getting the same results across all four tubes.

I was expecting that I can get above 44.5ma at 428 volts. But it seems like that is "all she got."

Is this normal for this amp?

Thanks in advance. Tom

[Jazz P Bass]

No clue how or what you measured but -50 volts is a 'normal' bias voltage.
(it appears you are baking the tubes!)
I set 6L6 tubes to 25 ma's & play it.
If it sounds good, out the door it goes.

[TomCarlos]

Hi John...

I was using the Weber calculator and info from other threads. So maybe I got it wrong? The Weber calculator has you entering the plate voltage and it then tells you what the bias should be for the 6L6GC.

As I adjust the bias control, the plate voltage seems to be changing. I didn't think that would happen (unless you put the tube in absolute saturation). See below.

If I throttle down to lower currents, here is what I am seeing:

I=35ma, PV=432v, Grid=-44.97v

I=30ma, PV=436v, Grid=-46.7v

I=25ma, PV=438v, Grid=-48.5v

Tom

[g1]

They show the plate voltage at 440V. The more current the tubes idle at, the lower the plate voltage will drop. Fender usually biases quite cold, the 20mV JPB mentioned seems pretty stock value.
The weber calculator uses a "magic" number (70% ?), which many manufacturers do not follow. The most basic guideline for longest tube life is to bias as cold as possible with out objectionable effect on the tone.

[Jazz P Bass]

[QUOTE=TomCarlos;376804
As I adjust the bias control, the plate voltage seems to be changing. I didn't think that would happen (unless you put the tube in absolute saturation).[/QUOTE]

Try to keep in mind that the power supply is not at a 'fixed' voltage.
So, if the tubes are pulling more current, it follows that the B+ must drop.
And vise versa, if the tubes are pulling less current, the B+ must rise.

I hope this helps.

As to the bias setting, I am a firm believer in the crossover method.
Increase the bias to remove it and add a touch more for the Gipper.

[TomCarlos]

Got it.... I think we're good to go. Thanks again.