I got my new tubes and attempted to bias my K-Tre.

[ZEEGLER]

I got my brand new set of JJ EL34 IIs and went about biasing the amp. I haven't had to do this for maybe 10 years. Most of my amps don't get used enough that they need the tubes replaced, and The Mullards that I put in my KII 10 years ago are still fine. (although I'm now thinking I might get a new set for it too).

Anyway, I dug out my bias probe, and realized that I couldn't remember for shit how to do this. I got my plate dissipation voltage. Rivera says it should be around 460v. Sure enough, pretty close to it. I plugged that into the formula, and I'm looking at 38mV. So I plug in my bias probe, and I just can't get a proper reading. I have it set to mA, but all I'm getting on the display is 0.02. WTF is that? Adjusting the bias trim pot does nothing to the reading. I figure maybe my multimeter is fucked so I try my backup meter. Exactly the same.

At this point I'm just really frustrated, and I've had the worst head cold for 5 days now, so I have zero patience. I ended up just playing the amp, and adjusting it to where it sounded the best, which turned out to be WAY lower than what it was set at previously. I specifically remember that when I biased the KII, it also ended up being set much lower than it was with the previous set of tubes.

Anyway, I'm going to have to figure out what I'm doing wrong so I can get a proper reading, but in the meantime, the amp sounds absolutely incredible. I have never heard an amp with such an authoritative low end thunk for palm mutes. Sounds absolutely massive!

The only problem I have now is that the Tre sounds better than the KII. So now I gotta try to dial in the KII as close to the Tre as possible, and maybe grab another set of the same tubes for it.

To sum up: I'm retarded and the amp sounds killer!

 

[Dave L]

Have you taken your amp off standby when taking the reading?

 

[mnemonic]

If your bias probe is reading voltage drop across a 1 ohm resistor between cathode and ground you should set the meter to volts. The start of you post mentions you’re looking for 34mV but later says your meter is set to mA.

 

[ZEEGLER]

Have you taken your amp off standby when taking the reading?

Yup

 

[BABMusic]

They are expensive, but I recommend the Eurotubes bias probe with wattage option. All you have to do is connect it to a power tube and you'll get the plate voltage, mA and the watts. Nothing wrong with a cooler bias as that's how some amps sound great.

 

[ZEEGLER]

If your bias probe is reading voltage drop across a 1 ohm resistor between cathode and ground you should set the meter to volts. The start of you post mentions you’re looking for 34mV but later says your meter is set to mA.

Man, I've watched at least a dozen YouTube videos on this, and I don't know, maybe because I'm sick, my brain isn't functioning but that's how I remember it.

Plate voltage = 460V
Tube wattage = 25w

25 / 460 X 70% = 0.038A (38mA)

Yeah, I guess it's DC milliamps I'm looking for.

 

[shar-vell Dan]

Your first mistake was buying jj tewbs. Your second mistake was buying jj tewbs.

 

[311boogieman]

^ You're a fucking idiot

Don't listen to ShitvelleDan. He has a tech take the Zap for him

1. Current production JJ tubes are just fine despite what people say. I've been buying EL34s, EL34Ls and EL34 II for years from Eurotubes and have had zero problems including preamp tubes. Even if they did suck, they wouldn't cause a reading like you are getting.

2. @ZEEGLER - your formula and assumptions are correct. I have a bias tutorial I wrote if you want to double check. Usually, when I get wonky readings like that it is either the meter itself or the battery in the meter. Usually, I would swap out the battery regardless, and then I try with a second meter to double check. After that, I'd try a second set of tubes. I've also had probes go south on me. I know have a 4 tube probe I like. Lastly, make sure you are off standby and NOT running any signal through the amp. You can have the guitar plugged in just turn all volumes off. Obviously make sure the speaker cab (load) is plugged in. Those are not all related to your wonky reading - just reminders. Good luck

 

[fusedbrain]

You need to set your multimeter to VOLTS.
As mentioned above, you are measuring the voltage drop across a 1 ohm resistor, therefore mV = mA.
The 1 ohm resistor is inside the bias probe.

 

[fusedbrain]

And yeah, JJ E34L are great tubes.

 

[311boogieman]

Here is the subset of the tutorial I was referring to (strictly as an example). The mV to mA comment is correct and often times, these resistors fail.


9.) At this point you should get a similar reading in milliAmps as you did before. This is your Cathode bias which is variable by adjusting a variable resistor that you'll find soon enough. Let the amp warm up as it may take a bit to climb and settle in. Here is where we introduce a simple formula to determine a 'rough' bias measurement that is healthy on paper. In my amp, I've found it sounds better on the 'cool' side of the bias range (60%). Many people do. We will learn in a minute how to bias to tone, but it is always good to start with a safe range and general idea of where you think you want to be. Here is where plate dissipation comes into play. Tube amp power tubes dissipate a certain amount of electrons during operation. Generally, anywhere from 60% to 75% is an acceptable range. If you like things on the 'hot' side you may want to use 70%. If you like things on the 'cool' side you may want to use 60%. Running tubes 'hot' should not be confused with better or 'hot-rodded' or anything like that. Running tubes too hot will create a muddy and 'warbly' sound. It will also cause you to burn through tubes more quickly. Many amps are run on the cool side out of the factory. Peavey for example is notorious for having a cold bias. Splawn HQ also recommends a cooler setting. The numbers we will be focusing on is our plate voltage, cathode bias in milliAmps, plate dissipation %, and the wattage of the tube in question.

MilliVolts is a constant that we captured earlier. (475)
Cathode bias is a variable that we will be adjusting.
Plate dissipation % we decide on up front but only as a guide. (60%)
Tube wattage is a constant. EL34s are 25 watts each. (25w)

If you take your tube watts and divide it by your plate voltage you will get your cathode reading in milliAmps. You then take this number and multiply it by your desired plate dissipation %. This will give you your cathode bias 'ball park' starting point. We will adjust the variable resistor up or down from this number but more importantly - we will adjust it to the best tone.

Here is the formula with my readings:

------------
25 watts per tube / 475 plate voltage = .052 amps
so 52 mA x .60 plate dissipation = .031 or 31 mA

bias amp to 31 mA to start
-----------

 

[shar-vell Dan]

You need to set your multimeter to VOLTS.
As mentioned above, you are measuring the voltage drop across a 1 ohm resistor, therefore mA = [MV]
The 1 ohm resistor is inside the bias probe.

FIXED.