External Bias Question

[rolandwalks]

Hey folks,
I hope I'm in the right section because this is the only forum I belong to about guitars. Anyway I recently bought a PRS SE20 head and this model comes with external bias jacks. My problem is that I recently biased it both with a Eurotubes socket bias and the external jacks and then compared my results. What I found out was that the external bias jacks were approx 4 ma higher than the eurotubes bias probe! WTF? So my question here basically is which one do I go with? According to the PRS manual I needed to use Mv and according to the Eurotubes instruction sheet I used Ma. Thoughts?

Thanks in advance and I apologize if this post isn't supposed to be in recording gear!

 

[tazzboy]

I use the Ted Weber Bias Rite meter and calculator Weber Bias Calculator

The way bias works as Bob explain to me for EL34

The formula for biasing is the plate dissipation of the tube (25 watts for a 6L6 or an EL34 or KT77) divided by the plate voltage (let's use 480 for this example) times .7 will just get you out of crossover distortion and you can go as high as .8 percent.

25 divided by 480 = .0520 or 52 milliamps X .7 = .036 or 36 milliamps

 

[Charliep]

Hey folks,
I hope I'm in the right section because this is the only forum I belong to about guitars. Anyway I recently bought a PRS SE20 head and this model comes with external bias jacks. My problem is that I recently biased it both with a Eurotubes socket bias and the external jacks and then compared my results. What I found out was that the external bias jacks were approx 4 ma higher than the eurotubes bias probe! WTF? So my question here basically is which one do I go with? According to the PRS manual I needed to use Mv and according to the Eurotubes instruction sheet I used Ma. Thoughts?

Thanks in advance and I apologize if this post isn't supposed to be in recording gear!

I wouldn't worry about the difference because you have to try it at different settings to determine what sounds best to your ears. The initial setting is just a safe place to start. Adjust up or down until it sounds good to you and is not redplating the tubes! The ears are the final determinate!

 

[Lyle Caldwell]

The PRS is having you measure DC voltage across a 1 ohm resistor, so you will be reading mV which translates to mA.

I don't know what the Eurotubes circuit is exactly, though I would imagine it's just the same ground, plate, pin 8 probe points with another 1 ohm resistor between pin 8 and ground. Which means the tube itself has 2 ohms between it's pin 8 and ground, though that shouldn't affect anything really. But it might begin to explain the different readings you got.

Also, remember that the plate voltage will not stay constant over the first 15-20 minutes of the amp being on, so if any time passed between your two biasing methods you will get different results.

When I bias an amp I set it quickly to be in a safe range, say 60%, then wait an hour before fine tuning it, then I recheck about 15 minutes later.

I would trust the PRS measuring points.

 

[rolandwalks]

I see what your saying Lyle but I use my Eurotube bias probe alot and I'd hate to think that everytime I use it it's reading 4Ma higher than it should be. Is that the conclusion I should be drawing here?

Also I appreciate all the advice but biasing by ear is nutsofacto if everything I've read is correct. It's a great way to melt something, if I'm not mistaken.

Thanks again.

 

[Charliep]

I see what your saying Lyle but I use my Eurotube bias probe alot and I'd hate to think that everytime I use it it's reading 4Ma higher than it should be. Is that the conclusion I should be drawing here?

Also I appreciate all the advice but biasing by ear is nutsofacto if everything I've read is correct. It's a great way to melt something, if I'm not mistaken.

Thanks again.

You won't hurt anything once you have calculated a safe setting. You only adjust a little at a time and monitor each setting at idle. There is no magic right setting, that is where the ears come in.
As far as accuracy being different, resistor tolerance may explain that. If you are setting to 60% or 70% you are still in a safe zone and can adjust by ear with no problems. My bet is on the PRS being the closest in tolerance. Again monitor as you adjust!
I bias all my amps this way and I have never melted something. I am speaking from experience not what someone said or read in a forum! I do not bias my amps hot as some do. My tone that I like doesn't require a hot bias. Common sense is needed when biasing!

 

[Lyle Caldwell]

How are you getting the plate voltage? There is usually a 5mA drop across a screen. And +1 to Charlie.

 

[rolandwalks]

I got plate voltage through the Eurotubes bias probe. You can measure the plate voltage by setting the multimeter to read DCV. I called Eurotubes and got their take on it. I was hesitant to do this becuase after all it's there product and thought they'd spin it there way if they could. Anyway they said that when you use external bias jacks your not just getting the reading for the specific tube your measuring but other electrical components in the circuit as well which can throw off your reading.

By the way I misread your post Chuck and thought you were saying that you should just bias by ear. No disrespect intended!

 

[Soul Tramp]

Anyway they said that when you use external bias jacks your not just getting the reading for the specific tube your measuring but other electrical components in the circuit as well which can throw off your reading.