Bias through 1 ohm 1 watt resistors

[daveyajd]

I installed resistors from pin 8 to ground so I can measure bias on my Fender amp. Up until now I have measured bias using a tester I got from Eurotubes. Are both of these methods measuring bias in the same way? In other words, should the numbers I get using each way be the same? Also, what is the best way to add a bias balance to my bias adjust? A small trim pot? Where is it installed?

Thanks.  

[sluckey]

I installed resistors from pin 8 to ground so I can measure bias on my Fender amp. Up until now I have measured bias using a tester I got from Eurotubes. Are both of these methods measuring bias in the same way? In other words, should the numbers I get using each way be the same?

They are slightly different methods. The Eurotubes bias probe measures real plate current in milliamps by actually breaking the plate circuit and inserting an ammeter. With the 1Ω resistor on the cathode you are actually measuring the voltage drop across that resistor in millivolts. This voltage drop represents the cathode current which is the sum of the plate and screen grid current. The numbers should be approximately equal, but the Eurotube probe gives a true plate current.

Also, what is the best way to add a bias balance to my bias adjust? A small trim pot? Where is it installed?

Just look at the schematics for some of the CBS era schematics to see how to implement a bias balance. Many people prefer dual bias adjust pots to the bias adjust plus balance.

[daveyajd]

Thanks sluckey. I assume that the resistor measurement method is higher by a couple mA. How much does the screen current usually add? My amp is actually a CBS amp that has been "blackfaced". I usually use matched pairs but I had tube break so I had to mix them up. I have heard about adding a trim pot to get the balance but I'm unsure where to add it.

Thanks.

[tubeswell]

Thanks sluckey. I assume that the resistor measurement method is higher by a couple mA. How much does the screen current usually add? My amp is actually a CBS amp that has been "blackfaced". I usually use matched pairs but I had tube break so I had to mix them up. I have heard about adding a trim pot to get the balance but I'm unsure where to add it.

Thanks.

If if 6V6, screen current can be estimated about 2mA at idle. If is 6L6, screen current can be estimated at about 5mA at idle

[sluckey]

And if you want to know exactly how much screen current you have, just measure the voltags across the 470 ohm screen resistor (usually mounted on the tube socket). Divide that voltage by 470 and the answer is the screen current for that tube.

My amp is actually a CBS amp that has been "blackfaced".

Your amp likely had a bias balance pot before it was blackfaced. Look at the original schematic for your amp.

The attached drawing shows the circuit for the dual bias circuit I put in my Sunn Sceptre. Look at the last two pics on this web page for a photo. It's an easy mod...

http://home.comcast.net/~seluckey/amps/sunn/sunn.htm

[12AX7]

Yes, dual bias is the bee's knees. I would never build another amp without it. I've had tubes drift as much a 10 or more mA and with dual it's no problem. I can just grab any 2 tubes out of my drawer and doesn't matter if they were a matched set of from 2 very different sets.  I have 2 test lead jacks on the back, one ground and one cathode and a switch to select the cathode for either tube. 1% precision dales on each cathode. I can't see why every PP fixed bias amp on earth doesn't have this ! (well, i do see the manufacturer's reasons but you know what i mean) I wish i had this during my gigging days. With the change in mains power from venue to venue i'd check the bias at every gig, a 1 minute operation.

[daveyajd]

Wow, the dual bias looks pretty cool. May have to work up to that. So I want to bias according to my plate current right? Subtract my screen grid current from the cathode measurement and bias accordingly? How much difference in mA can there be between tubes before you start to notice issues in the sound quality? My tubes are currently about 10mA different.

[PRR]

> How much does the screen current usually add?

6V6, 6L6, 6550.... assume 5%.

EL34, EL84.... assume 10%-20%.

[daveyajd]

So I measured my bias using both my adapter and the cathode method. I am getting the same numbers.