Twin Reverb II Bias Help

[ncusack]

Hey Everyone,

I have an 80's twin reverb II and im in the process of biasing the power tubes and would like to make sure I'm getting things right. Im measuring ~495VDC on the plates and with a bias probe on a single tube I'm reading 40mA. I would like to know if that reading is actually the current through a single tube or if its actually split between the parallel pair. The probe I'm using is the 1 ohm shunt type.

[AZJimC]

If you're using the type of probe with a 1ohm resistor, on a single tube, then that 40mv, thus 40ma, is the draw of that one tube. Measure each tube the same way.

[ncusack]

OK that being the case 40mA @ 495VDC seems a bit hot especially since the matched set of tubes has each one matched for 25mA Ip. I would imagine a set of JJ 6L6 tubes would be a better choice in this environment. My guess is that the alternative would be to add a bias control to this amp since it only has the output matching control.

[Willabe]

OK that being the case 40mA @ 495VDC seems a bit hot especially since the matched set of tubes has each one matched for 25mA Ip.

That's not what that value is about. That doesn't mean they can't handle more output.

Any given output tube is rated for a max power output dissipation. It is listed on the tube data sheet.

When they test a large group of the same output tubes to match them up they test them for output current in a setup that remains the same for all the tubes tested. All power tubes will give a different output even when their the same tube, from the same run, made with the same materials, on the same machines.

So, lets say for a large batch of 6L6GC they want to match they will test them in an amp with say 450dcv on the plate with 440dcv on the screen grid at -50dcv on it's grid. They have a 1 ohm R on the K to ground and measure across it for mV's which transfers directly to mA's and write it on the tube. After they test 50 or 100 or what ever amount they have they put them together in matched pairs, quads, etc.



                         Brad     

[Willabe]

I should add;

Some test for other things too, which they don't always say what those tests are.

6L6GC is rated for 30w max power dissipation.

Power dissipation is dependent on how much plate (anode, A) dcv and how much current. Higher Adcv then lower current, lower Adcv then higher current.

So lets take the 6L6GC's tube max A dissipation value 30w, now we divide it by the dcv on the A, then we multiply that result by .7 for 70% of max dissipation or .6 for 60% or .5 for only 50%.

(Max A dissipation/Adcv) x .7 = 70% of max mA output dissipation.

Here's from a note I have on an amp I was biasing 6L6GC's;

30/396=.0757x.7=53mA.

On a higher Adcv amp, like a Fender BF AB763 (~440dcv to ~480dcv) I'd start around 25mA to 30mA, ~420dcv and lower I'd probably start around ~35mA.   

When you let more current flow through the tube the Adcv will drop so you have check the Adcv and do the math a second time.


             Brad