How to properly bias an express......
Moderators: pompeiisneaks, Colossal
Re: How to properly bias an express......
HI, everyone.
Can someone help me? I don't know where the problem is, V4 pin1=24.9mv Is this normal?
use 240v
Re: How to properly bias an express......
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.
Mind you, it is unusual to find a small voltage there.
Yours Sincerely
Mark Abbott
Mark Abbott
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Re: How to properly bias an express......
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
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
tUber Nerd!
Re: How to properly bias an express......
Thanks man, this is an incomplete measurement. I supplemented them and published another post. Thank you for your advice and help!pompeiisneaks wrote: ↑Fri Jul 19, 2019 4:15 pm 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
Re: How to properly bias an express......
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....
Most people stall out when fixing a mistake that they've made. Why?
Re: How to properly bias an express......
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.
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.
Last edited by Winder on Sun Aug 04, 2019 8:45 pm, edited 1 time in total.
Re: How to properly bias an express......
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?pompeiisneaks wrote: ↑Fri Jul 19, 2019 4:15 pm 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
Yours Sincerely
Mark Abbott
Mark Abbott