EVH 5150 Stealth Bias Points Inaccurate

[Rising Farce]

My apologies if this is old news (or if no one cares), but I just checked and set the bias on my 5150 Stealth using a meter, then compared that setting with the readout from the factory test points. The results were a bit of a surprise: With the power tubes set to an actual, meter-verified 32 ma, the test points showed roughly 40 ma.

Has anyone else had this experience? I know Peavey's external test points are famously inaccurate, but I expected more from Fender/EVH.

 

[PatF]

Yes, I just got into trouble with this the other night. It's because the 1 ohm resistor on those test jacks is not exactly 1 ohm. Even 10 percent off will lead to the inconsistency you are noticing. My Bassman has 465 VDC B+. It runs 6L6GC's. The resistors measure .9 ohms. I thought I'd just under shoot the number a bit. Wrong. The resistor method relies on the resistors being close in tolerance so we can translate the voltage drop across the resistor into current. The bias probes are cool because it lets you plug in an easy way to measure current directly.

For 6L6GC Class AB at 465 VDC the bias calculator says 45ma for 70% dissipation.

http://www.tedweber.com/webervst/tubes1/calcbias.htm

Quick Ohm's Law review. Easy as it gets. Example is reading 45mv on the resistor and hoping for 45ma current draw. It's actually 50ma. A bit hot.

I = V / R 50ma = 45mv / .9 ohms
V = I x R 45 = 50ma x .9 ohms
R = V / I .9 ohms = 45ma / 50ma

I = current
V = voltage
R = resistance

 

[PatF]

Note that I am still a bit of a rookie, so anyone please correct any misinformation.

 

[SpiderWars]

Using the numbers from the op: you measured 40mV when there was really only 32mA current flowing. So using V=IR, or rather R=V/I we get R=40/32=1.25 ohms. So theoretically you could just use the bias points but instead of using 1 ohm as the resistance use 1.25 ohms.

 

[Rising Farce]

Using the numbers from the op: you measured 40mV when there was really only 32mA current flowing. So using V=IR, or rather R=V/I we get R=40/32=1.25 ohms. So theoretically you could just use the bias points but instead of using 1 ohm as the resistance use 1.25 ohms.

I see what you're saying, but can we safely conclude from comparing only two data points (indicated and actual) from one bias setting that the discrepancy will be consistent throughout the amp's bias range? In other words, might that discrepancy increase as the bias itself increases, and vice versa?

My apologies if my thinking is unclear and/or completely off base here. I'm a writer by trade, so this sort of thing is slightly (OK, extremely) outside my wheelhouse.

 

[SpiderWars]

Using the numbers from the op: you measured 40mV when there was really only 32mA current flowing. So using V=IR, or rather R=V/I we get R=40/32=1.25 ohms. So theoretically you could just use the bias points but instead of using 1 ohm as the resistance use 1.25 ohms.

I see what you're saying, but can we safely conclude from comparing only two data points (indicated and actual) from one bias setting that the discrepancy will be consistent throughout the amp's bias range? In other words, might that discrepancy increase as the bias itself increases, and vice versa?

My apologies if my thinking is unclear and/or completely off base here. I'm a writer by trade, so this sort of thing is slightly (OK, extremely) outside my wheelhouse.

Yes, check it at several settings and see if they agree.

 

[Rising Farce]

Using the numbers from the op: you measured 40mV when there was really only 32mA current flowing. So using V=IR, or rather R=V/I we get R=40/32=1.25 ohms. So theoretically you could just use the bias points but instead of using 1 ohm as the resistance use 1.25 ohms.

I see what you're saying, but can we safely conclude from comparing only two data points (indicated and actual) from one bias setting that the discrepancy will be consistent throughout the amp's bias range? In other words, might that discrepancy increase as the bias itself increases, and vice versa?

My apologies if my thinking is unclear and/or completely off base here. I'm a writer by trade, so this sort of thing is slightly (OK, extremely) outside my wheelhouse.

Yes, check it at several settings and see if they agree.

Thanks, I'll give that a shot.