Biasing a Marshall JCM 800 2205 Amp with a simple multimeter! Pin 1 and 3??

[jalexquijano]

Can you bias and amp just with a simple Radio Shack digital multimeter measuring directly with the red probe on pin 1 and pin 3? I had bought a weber bias rite to do the bias on the JCM 800 2205 amp and at the moment of pulling the heads out of the EL34 power tubes, one of them broke off completely.

I saw this video and wonder if it could be done without soldering a resistor between pins 1 and 8. Just directly with the probes of the multimeter?

https://www.youtube.com/watch?v=qLz94mgC4tY

[Dave H]

Can you bias and amp just with a simple Radio Shack digital multimeter

I saw this video and wonder if it could be done without soldering a resistor between pins 1 and 8. Just directly with the probes of the multimeter?

It can be done with just a DMM without a resistor by using the transformer shunt method to measure plate current but it is a lot safer to add the resistor if you are not sure what you are doing. Scroll down on Randall Aiken's site for the shunt method, note the warnings.

[Jazz P Bass]

Can you bias and amp just with a simple Radio Shack digital multimeter measuring directly with the red probe on pin 1 and pin 3?[/url]

First off: Pin #3 is the HIGH VOLTAGE Pin.

To read it, one probe must be referenced to ground.
Do not assume that pin #1 is attached to ground, as it is an unused pin (internal to the tube itself).
Some manufacturers will use that pin for various things.

Check it with a meter.

[lowell]

In short... no. Measuring from Pin 1 to pin 3, on your meter's voltage setting, will only tell you the voltage between those pins. Bias is current through the tube. If there is a low tolerance 1ohm resistor from pin8 to chassis ground, then you could measure millivolts across that resistor to get millamps thru the tube. Since V=I*R, voltage across a 1ohm will be equal to current THRU that resistor. It's a good idea to check that the 1ohm is indeed 1ohm with your meter as well, though this may not be accurate with a cheaper meter. The transformer shunt method is accurate, but VERY DANGEROUS, as DaveH mentioned. I forget, does this Marshall not have the bias measurement pins on the rear of the chassis? Some Marshalls have 3 rca looking jacks. They are setup so millivolts can be measured, revealing correct bias settings....which if I remember correctly, and don't quote me, should be 50mv.

[jalexquijano]

Well, i had one tech come to my house yesterday and taught me a weird way of doing bias. I explain as follows:

1. Open head and remove from cabinet

2. turn multimeter on and place it on DC voltage measurement

3. Attach alligator clip on the screw that is besides the filter capacitor and is labeled with a ground sticker. The other end of the clip attach black probe.

4. Turn amp on and leave it on stand by.

5. Place red probe on the metal film resistor and Green wire that is soldered to one of the power tubes pin and take the measure aiming to 40vdc

6. Place red probe on the metal film resistor and red wire that is soldered to the other power tube pin and aiming to 40vdc.

I really dont know where did he got this 40 vdc measurement as to correct bias? and how without taking the standby switch off any voltage can flow around.

Anyone??

[The Dude]

That measurement is meaningless and mostly irrelevant. Bias voltage can vary immensely from tube to tube. You need to actually measure the idle current. I'm not going to belittle your tech friend, because he might be good at some things, but he doesn't know how to correctly bias a tube amp.

[pdf64]

There is a much better method than the OT shunt; just simply measure the OT primary resistances (from CT to either plate) and then the voltage across the those points at idle.

The Last Word On Biasing

The plate current can also be measured by first measuring the resistance across each side of the output transformer primary (it will usually be different on each side) with the power off. Make a note of the resistance on each side, and then, with the amplifier on, measure the DC voltage drop across each side of the output transformer. Divide this number by the previously measured resistance, and you end up with the plate current for the tubes on that side. Again, if there is more than one tube on each side, you must divide the total current by the number of tubes. This method is extremely accurate, and much safer than the shunt current measurement method, because a slip of the probe won't short anything out due to the high resistance of the voltage measurement setting on the meter compared to the very low resistance of the current measurement setting. You can also make a safer measurement by clipping the negative side of the voltmeter on ground, and measuring the center-tap voltage of the output transformer and the voltage at the plate of each output tube. Subtract the plate voltage from the center-tap voltage and you have the voltage drop across each side, and can then use this to calculate the current in each tube, again dividing by the number of tubes on each side.

The bias voltage is the negative voltage applied to the tube's control grid; so the 40V should have been -40V, with respect to chassis 0V.
However, such a measurement provides no indication of how much current the tube is drawing / power its plate is dissipating, at idle.
Various different individual power tubes may draw widely differing idle plate currents at a particular operating point.

[nickb]

Personally, wherever I can, I add a flameproof 0.6W 1 ohm 5% resistors between pins 1 & 8 of each power tube, as has been suggested. I find the few minutes it takes is repaid in time saved in doing the measurement. Once in place I can very quickly go down all the tubes and very instantly see how well they are matched as well as measure the bias current.

A bonus is if I have to change the tubes in that amp, whoopee! It's as easy as pie to check.

Yet another nice thing is if a tube goes South, it acts as a fuse.

It's also great as it allows you to scope the currents flowing in each tube if you ever have the need for diagnostics.

The current shunt method is prone to disaster - if you do it use crocodile clips and turn off between tests - this takes time. People do move the probes on live amps but it's a bad idea. One slip and ...disaster! Worse as the meter is in current mode has a low resistance meaning you can short something else out.

Pete's suggestion of measuring the transformer resistance is better. It's still a little slow and critically dependant on measuring the resistance and voltage accurately and then taking the time to do the calculation.


PS: One bias probe buys LOTS of 1 ohm resistors at $0.02 each

[The Dude]

I can't disagree, for the most part. The only thing I would add is that, if you're a full time tech, a bias probe is still a good thing to have. They can be found on the web for less than 30 bucks. You don't need anything real fancy. Or, you can build your own. The reason I say this is because a lot of modern amps have the tube sockets on a PC board. It wouldn't be worth the time to cut traces and modify boards simply so you could bias the amp. IMO, A bias probe is a much quicker option and cheaper for the customer in many instances.

[drewl]

Even easier to use the Aiken method.

God forbid if I had to install cathode resistors on all my amps or ones I work on.

[Justin Thomas]

Maybe it's best to be aware of all the "better" methods and use whichever one the combination of time, parts, and precision allowed. Personally, I install the 1R resistors, but I make a point to NOT buy amps with board-mounted tubes. But as Dude said, I'm not gonna cut traces, etc. We should all be ABLE to use different ways, even if we don't use them all the time.

Personally, I <WOULD> like to one day overcome my fear of the shunt method, because I know it's not just not having the right tools to do it safely, but that I'm genuinely afraid of slipping. Goals...

Justin

[drewl]

Buy some " mini grabber" tm. clips for your meter.
They're everywhere, I have at least a dozen.

[jalexquijano]

ill just keep on using the Weber Bias Rite and bias the power tubes 65 to 70%. The only thing ive seen with this device is that it does not show any minus(-) on its measurements. Is this normal?

[Jazz P Bass]

ill just keep on using the Weber Bias Rite and bias the power tubes 65 to 70%. The only thing ive seen with this device is that it does not show any minus(-) on its measurements. Is this normal?

The Weber device is measuring the Current through the tube.

Not too sure what a minus current is.")

[Tom Phillips]

ill just keep on using the Weber Bias Rite and bias the power tubes 65 to 70%...

That's a good approach. Especially if you are just maintaining your own amp. You can also continue learning tube amp and general electronic theory so you can understand the concept behind biasing and all the other things going on inside your guitar amp.

Well, i had one tech come to my house yesterday and taught me a weird way of doing bias. I explain as follows...

As already mentioned we don't intend to belittle the person that tried to help you. Something could have gotten lost in the translation. Another thing that could be at play is that anyone can call themselves a tech. The label doesn't really tell one anything about the expertise level of the person who uses the label. I'm a guitar player. So is Eric Clapton. I think that each of us can play better than Leo Fender. What does that all mean?

To be fair. Some people are good at doing certain things but they are not good at teaching others to do the thing. There could also have been a misunderstanding about your question.

Cheers,
Tom