Bias Question - Hot Rod Deluxe

[evco]

Recently got a Weber Bias Rite, just checked my HRDx and it read 30.5mA/426 plate voltage on one tube and 30mA/423PV on the other. Using the bias test point on the amp I get 68mV. Accoding to http://studentweb.eku.edu/justin_holton/ the mA & mV readings should be in sync but I'm coming up with 60.5mA vs. 68mV. I recently had my BF Princeton in for some work at Mission Amps and compared my Bias Rite against a Compu Bias & the readings were exactly the same so I'm relatively sure my Bias Rite is working fine, just replaced the battery to make sure that wasnt the issue. Any ideas why there would be such a difference between the mA/mV readings?

2nd question...according to Weber's bias table, 60-70% plate dissipation for 6L6GC's (I have GT 6L6GE's in the amp) would be about 43-50mA.....that sounds really high based on the readings I'm getting above, any idea what a safe range would be in the HRDx?

Thanks!

Eric

[Blue Strat]

Forget about the 60 to 70% thing. 70% is considered maximum safe, which is really too high. Stick with 50 to 60%.

As for the difference in readings, the current sensing resistor in the amp may be out of tollerance. 10% high would give you 66mA if the actual reading is 60. You can try measuring the resistance of that resistor by setting your DMM to ohms and measuring between the test points. Then measure the resistance of your meter's test leads by touching them together. Subract that .x number from the resistor reading you get.

[Pearly Gator]

Blue Strat is correct. Resistor tolerance will throw off your bias reading. I like to replace the 1 ohm resistor with a 1% or better precision resistor for more accurate readings.

PG

[evco]

Thanks for the feedback Mike and PG. I was guessing that the Bias Rite would be the more accurate of the two readings, I'll check the ohm reading on the resistor and see what I get and look at replacing it if it's off. Sounds like 35-40mA is what I should shoot for on the Bias Rite reading.