bias checker question

[olgoat]

I built 4 bias checkers from Doug (2 for me, 2 for a friend) and ordered and installed the i ohm resistors in the banana plug so that I can  check both tubes by switching the meter back and forth and measure millivolts so I don't mistakenly blow fuses in the meter.

Tonight I see reference to a 1 ohm 3Watt resistor.  I ordered 1 Ohm 1Watt from Doug and that is what I installed. 

Did I screw up here?  Ie 1 watt insufficient?

Thanks for your thoughts as always. 

[sluckey]

Don't worry. 1 watt is plenty.

[olgoat]

Thanks for the quick reply. 

[PRR]

A really big hot tube might bias at 100mA, 0.1A.

Power = I^2*R
Power = 0.1A*0.1A*1r
Power = 0.01 Watts

1 Watt is ample.

It would take 1A (1,000mA) to hot-up the 1Watt part. No guitar-world tube can pass that much.

[olgoat]

I have an additional question.  As I mentioned I built 4 of the bias checkers and I just tried them out on my Deluxe Reverb.  A couple weeks ago, I had it over to a friends shop and we went over the amp. He checked the bias and it was high 30s to 40 ma measured with the shunt method (pin 3 on the 6v6 to the red/black leads off the OT (center tap?)).

When I test the amp using the bias checkers, I get 9.1 to 10 mv (With 1 ohm resistor) and about the same number of mA if I remove the resistor from the bias checker.  

The meter is a new Extecth EX330.  Meter was set to VDC and mA respectively.

All four bias checkers read the same when I put them into the circuit.  (so If I did it wrong, I was consistently wrong.)

I have not pulled the chassis to try the shunt method myself (admittedly a bit sheepish about trying that on my own as a newbie.)

Any ideas why the discrepancy between the readings?

Thanks for any thoughts on the matter.

Oh.  And I could hear hiss through the speakers with the amp in run mode (standby off). I did not try playing through it with the checker in line.  Both 6V6s were plugged in when I tested.

[sluckey]

Take it back to your friend's shop. Bring your meter and bias checkers. Set it up to check bias with your meter, then switch to his meter. How do the readings compare?

I would trust the readings with the bias checker over the readings with the shunt method.

[olgoat]

Sluckey.  I found one of your comments on a another thread and checked my meter with a 9V battery and a 100 Ohm resistor in series.  I came up with about 78 mA.   Does that sound right for that combination of Voltage and resistance. 

If right then the bias on this amp is set very cool as I thought it was supposed to be around 30mA. 

Thanks

[sluckey]

checked my meter with a 9V battery and a 100 Ohm resistor in series.  I came up with about 78 mA.

Theoretically, you would have 90ma, but I doubt the battery is really exactly 9V and I doubt the resistor is really exactly 100Ω. Your reading is 13% lower than the calculated 90ma. I'd say its probably OK.

Still think you should get a comparison with your friends meter.

[olgoat]

I'll do that.  Thanks.   

Is it common for a meter in the $60 range to be off that much?  It's brand new and unabused.  The battery measure 8.9V and the resister was 99.6 ohms. so that should be about 89.3 ma. 

Thanks again for the help.

TWC

[sluckey]

Your meter claims 0.5% basic accuracy, but the actual accuracies for different modes may be as high as 2%. I would not be concerned. You can always have the calibration verified/certified, but that cost $$$. I have access to certified test equipment and just check readings of my personal equip against the certified test equip.

When you go back to your friends shop, note what meter he uses and make several common checks with your meter and his. In addition to comparing bias readings (mv and ma) compare some other readings as well. Things like AC wall voltage, B+ voltage, check several resistors, check several caps, etc. It could just be that you have the 'good' meter, but I suspect the difference in your bias numbers and his, has more to do with method than accuarcy of meters. Like I said, I trust the cathode current method over the OT shunt method.