# Bias Probe Help

Discussion in 'Amp Tech Center' started by Askwhy, Oct 29, 2020.

1. ### LowerleftcoastFriend of Leo's

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@Ten Over
As referred to earlier, just trying to explain why the tech may have different numbers than that of the bias probe. I do not want to be a pedant with Askwhy.

2. ### Ten OverTele-Holic

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The tech may not have subtracted the screen current, but then again, neither did Askwhy. Once they both subtract screen current, there is still a huge discrepancy between the two readings. If the tech did subtract the screen current, then the difference in readings is even larger.

3. ### Ten OverTele-Holic

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I took a random sample of fifteen EL34's of various manufacture and age. The plates were at 450V, screens at 445V, and the control grids at -41V. The average screen current was 16% of plate current.

4. ### Ten OverTele-Holic

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No matter how I interpret this, I keep getting a negative number.

If you want 60% of max dissipation at a plate voltage of 475V, then you first need to know how many watts that is.

25W x 60% = 15W

Watts is current times voltage, so you now need to know how much current.

15W / 475V = 31.6mA

If you go with screen current being 13.3% of plate current, then you need to adjust your probe reading target.

31.6mV x 13.3% = 4.2mV
31.6mV + 4.2mV = 35.8mV

So your meter should read 35.8mV to get 60% dissipation at 475V.
____________

What kind of bias probes do you have?

Paul G. likes this.

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Oops i mean .6 So 25/475=.0526 x .6 = .0315 or the same answer you got then -.008 = .0235 or set my bias so the probe reads 23.5 ma. Sorry about my carelessness with decimal points. SRS dual probe and then also the plate voltage reader. The screen current part, though i'm sure its correct does not make sense relative to my tech getting 28ma while my probes were reading 20.
Thx!

6. ### Ten OverTele-Holic

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The 8mA you're subtracting never existed and even if it did and it was screen current, you would add it to your meter reading because your meter reading is plate plus screen. But don't do that because the 8mA you guys were talking about is just a misinterpretation of the data sheet.

I don't know why your tech said 28mA--I wasn't there. I trust your probes way more than I trust the tech.

Lowerleftcoast likes this.

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Interesting. I do trust the tech, been around forever and knows an amazing amount about everything we have talked about so i would be shocked (get it?) if he got simple biasing wrong. Also that would be under 40% which i think would sound very sterile and it sounds great. Certainly possible though. Also when i first got the amp it had the original power tubes, the factory biases at 28 and my probe read 20. Much more of a mystery than i thought, but thanks for all the thoughtful replies.

Last edited: Oct 30, 2020

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Ha! Normally i very much appreciate that and was actually banned at TGP for railing against pedantism as a method for not actually addressing questions or issues (happens in tons of threads over there, especially in the pub), but when talking about electronics, being precise is critical so if i can provide any more info, let me know and i will try my best.

9. ### LowerleftcoastFriend of Leo's

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From your first posts on this thread you were just asking *How could this be?* so I was simply trying to give reasons as to why the numbers may be different.

The thread seems to have changed to *How do I bias with this probe?*. This needs to be answered much more specifically.

I did a search for the SRS dual probe unit. It is said to measure the cathode, so yes, the screen current should be figured in when measuring.

Ideally you can measure the screen current but that would defeat the purpose of using the bias probe so the screen current is estimated.

Basically @Ten Over went through a procedure to estimate the screen current in post #24.

Rob Robinette has a site with a bias calculator. You can choose tubes and plug in your numbers to calculate the bias. His site uses 5.5% to estimate the screen current.

As you can see, the estimates differ by about 2mA.
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You will need the "plate to cathode voltage" to calculate the bias. This voltage changes as the bias changes. If you adjust the bias the "plate to cathode voltage" has to be checked again.

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