How to properly bias an express......

[fugezii]

HI, everyone.
Can someone help me? I don't know where the problem is, V4 pin1=24.9mv Is this normal?
use 240v

[Mark]

Nothing is connected to pin 1 of a 6V6, why are you measuring it?

Mind you, it is unusual to find a small voltage there.

[pompeiisneaks]

I now note you posted two posts with differing information but the same basic data. Was the other post asking this same question?

You need to know how to calculate the tube current to get the right data for this question, these voltages only help in knowing what rating it 'should' be at.

Since the express doesn't have 1 ohm cathode resistors you'd need to use the transformer shunt method, or add in some precision 1% 1W 1 ohm resistors to the cathodes to ground so you can measure mV drop and assume that's mA due to ohms law. Then once you have the current reading, you can compare to any off the online bias calculators and see what's the appropriate negative voltage that gets you there.

~Phil

[fugezii]

I now note you posted two posts with differing information but the same basic data. Was the other post asking this same question?

You need to know how to calculate the tube current to get the right data for this question, these voltages only help in knowing what rating it 'should' be at.

Since the express doesn't have 1 ohm cathode resistors you'd need to use the transformer shunt method, or add in some precision 1% 1W 1 ohm resistors to the cathodes to ground so you can measure mV drop and assume that's mA due to ohms law. Then once you have the current reading, you can compare to any off the online bias calculators and see what's the appropriate negative voltage that gets you there.

~Phil

Thanks man, this is an incomplete measurement. I supplemented them and published another post. Thank you for your advice and help!

[rooster]

Wow, this was an interesting reread. Funny, too, because I'm running original Mullards and not biasing on the hot side, maybe 30mA. And then there's Hi Test Guitars Gregg Levy who was tight with Ken who said I should run these tubes @ 20mA. Crazy world out there, keep an open mind....

[Winder]

I'm all in on this. Been running what I thought was 70-71% dissipation, but my plates are about 398VDC and I'm dropping 5.6mA and 5.8mA across the Screen Resistors so actual was more like 62-63%

Amp was good there ... is much better at 70-71% actual. It's like having a Chewy Goodness knob, and diming it. Glad I read through this! Thanks all!

FWIW, Amperex preamp tubes, and close matched xf2 Mullard power tubes. I use a 4-Probe Bias Rite BR-4 that I originally purchased to use with my 100W Marshalls. Certainly not as cheap as 1-Ohm resisitors, but measures cathode current and plate voltage (toggle selectable). If I didn't already have it, I probably would have gone with 1-Ohm cathode resistors.

[Mark]

I now note you posted two posts with differing information but the same basic data. Was the other post asking this same question?

You need to know how to calculate the tube current to get the right data for this question, these voltages only help in knowing what rating it 'should' be at.

Since the express doesn't have 1 ohm cathode resistors you'd need to use the transformer shunt method, or add in some precision 1% 1W 1 ohm resistors to the cathodes to ground so you can measure mV drop and assume that's mA due to ohms law. Then once you have the current reading, you can compare to any off the online bias calculators and see what's the appropriate negative voltage that gets you there.

~Phil

I definitely would be putting the 1 ohm resistors on the cathodes to ground. That seems like small concession. Is there some reason to omit them?