Bias probe vs. OT shunt method - which is more accurate?

[mike80]

I ask because I got totally different readings on the same tube. Using the OT shunt method, I got 45mA on a set of E34Ls. Using a bias probe, I got 53mA.

My guess is that the OT shunt would be a more reliable method and/or I have a faulty bias probe.

[hasserl]

It depends on what you mean by the shunt method, but if you are measuring the resistance of the OT center tap to each leg, than measuring the voltage drop across those same points and dividing that by the resistance, that is the most accurate method. If you're setting your DMM to read current and connecting it in parallel to the OT center tap and each leg, that is not.

[Jade]

Mike, I agree with what hasserl said. One other thing to consider when using a bias probe, is that you're actually measuring cathode current as opposed to Plate current which is really what you're trying to determine. The difference is that when measuring Cathode current, you're seeing the current being drawn by the Plate, AND the screen grid, so your readings are going to be 2-4 mA higher than what the plate is drawing by itself. So you need to take that into consideration when you're deciding what the max reading you want to see on your probe is.

Jade

[mike80]

Thanks guys.

It depends on what you mean by the shunt method, but if you are measuring the resistance of the OT center tap to each leg, than measuring the voltage drop across those same points and dividing that by the resistance, that is the most accurate method. If you're setting your DMM to read current and connecting it in parallel to the OT center tap and each leg, that is not.

That helps to know. I was doing the latter.

Mike, I agree with what hasserl said. One other thing to consider when using a bias probe, is that you're actually measuring cathode current as opposed to Plate current which is really what you're trying to determine. The difference is that when measuring Cathode current, you're seeing the current being drawn by the Plate, AND the screen grid, so your readings are going to be 2-4 mA higher than what the plate is drawing by itself. So you need to take that into consideration when you're deciding what the max reading you want to see on your probe is.

Jade

Also good to know.

I'm new to this whole biasing concept since I've owned mostly Mesa amps. I recently added a bias pot to my Mesa, and I'm trying out different settings.

[phsyconoodler]

Well,whatever bias method you are using,45ma and 53ma is way too hot in a mesa amp.They have a B+ of about 450v and the safest current for modern EL34's would be about 38ma.

[mike80]

Well,whatever bias method you are using,45ma and 53ma is way too hot in a mesa amp.They have a B+ of about 450v and the safest current for modern EL34's would be about 38ma.

That's what I thought, but it's been running that way for over 2 years now with no issues.

:dunno

[donnyjaguar]

I test using the cathode voltage internally myself. This way you can check on the screen grid resistors which, in my experience, are whipped about half of the time on older amp's.

[phsyconoodler]

If you have a mesa amp with mesa tubes,there is no way they are biased at 45ma.You'd be lucky to see 25ma with mesa tubes.They set them up cold on purpose.The STR rating governs how much current they draw.
I suggest re-examining how you are checking the bias.What amp is it?

[mike80]

If you have a mesa amp with mesa tubes,there is no way they are biased at 45ma.You'd be lucky to see 25ma with mesa tubes.They set them up cold on purpose.The STR rating governs how much current they draw.
I suggest re-examining how you are checking the bias.What amp is it?

It's a 2 channel Dual Rectifier. I'm not using Mesa tubes though. I have a set of JJ E34Ls that I bought on Ebay a few years ago.

I checked the voltage, and I'm getting 486 volts on diodes and bold power setting. When I was in the amp the last time, I checked the current draw by reading the current from each side of the OT to the center leg and was getting 45mA per tube...that's after already running the amp that way for nearly 2 years. It has sounded great during that time also.

When I checked with a bias probe yesterday, I was getting 53mA. I also have a set of Yellow coded Mesa STR420 6L6s that I stuck in there briefly, and with the bias probe I was reading 36mA on those...which didn't sound quite right to me. I thought they should be normally right around the 20mA range. Which leads me to believe that either the meter is off (which I can't see it being off THAT much), or the bias probe is inaccurate.

[phsyconoodler]

Two things that happen with meters is the battery gets low and they read wrong,and the cheaper meters can't take the transformer shunt and it scrambles the guts and you get eronius readings.
Try a fresh battery in your bias probe.The JJ tubes are tough and may be able to take 45ma for a while,so you may not have to worry.

[mike80]

Two things that happen with meters is the battery gets low and they read wrong,and the cheaper meters can't take the transformer shunt and it scrambles the guts and you get eronius readings.
Try a fresh battery in your bias probe.

:jo

I forgot that meters don't read right with low batteries. I'll stick a fresh pair in tomorrow and give it another shot.

The JJ tubes are tough and may be able to take 45ma for a while,so you may not have to worry.

They've been taking it for a couple years without problems, so I'm not all that worried about it.

It is getting time for a new set though.