Bias Meter Favorites?

[Livingstoni813]

How's it goin guys. I'm starting to gear up to be able to perform retubes for myself and possibly a few friends. First piece of gear I'm starting with is a solid dmm. From all the info I've gathered, seems like Fluke is the best name for reliability/longevity/accuracy, their meters range from $200-800, for the handheld units. Then they also have the kits with different connects/probes. My main questions are: Do I need any of those kits containing all the special connecters and probes? Or is most of that stuff gonna be useless to me? How deep down the rabbit hole do I need to go as far as price is concerned? I want to be as accurate as possible... What should my concerns be, if any, about the calibration of the unit? Definitely gonna be more questions to come, but figured this was a good place to start.

Also, if it helps to know what I'll be working on... Got a Metro JTM45 Combo(VA KT66's)at the beginning of this year, and have another on the way, friend has a Fender Blues Jr and is lookin into a Bassman.

[danman]

One day I'm gonna spring for a Fluke myself as they are very nice meters. You can pick up a decent used one on Ebay at a fair price also if you are lucky. I have been using a fifty dollar meter from RadioShack for the past seven years and it hasn't let me down yet. You want to make sure that you get one that has a pretty high voltage scale though as some tube amps can approach 500-600vdc. Some meters only read up to 500v and may not be enough for your needs. A capacitance function is also nice for occasionally checking the value of caps but it isn't necessary for simple retube and bias jobs. I would recommend getting yourself a set of alligator clips and the spring loaded grabbers for attaching to your meters leads. Makes it much safer when you can clip your test leads into the amp and not have to worry about a probe slipping off while taking a reading and causing damage to the amp or yourself.

[Livingstoni813]

Alright, so, alligator clips, spring loaded grabbers, and a voltage scale over 600v. The capacitance func for possible future use. Any other bells and whistles? Other specific functions or function reading tolerances I should be concerning myself with? I don't want to get something with features I won't use, but, I want it to be versitile enough to handle most practical situations when working on amps or guitars.

Edit: I take it this would cover most if not all situations?

http://www.fluke-direct.com/shop/itemDe ... urer=Fluke

[neikeel]

Edit: I take it this would cover most if not all situations?

http://www.fluke-direct.com/shop/itemDe ... urer=Fluke

That looks fine, I went cheaper myself (I use a Fluke 117 - or could be 115 - pretty similar) which is 600v, 10A with most of teh other functions.
Yes you need a set of croc clips and insulated probes.
Also make up a double croc lead about a foot long in a very bright colour and 10A rated wire, this is for cap bleeding before you work on an amp - absolutely essential - make one before you buy your meter!

[Livingstoni813]

Ok, for the double croc lead to drain the filter caps, is that something I'm going to need or need to do for a retube/bias? I'm guessing that draining the caps before you work on the internals of an amp, prevents you from frying the amp, the meter, or yourself. But strictly for a retube/bias, am I going to have to concern myself with draining the caps? If so, why? And then finally how? LOL

[danman]

It's important when working inside an amplifier to always make sure that the B+ voltage has been drained from the capacitors before starting. Some amplifier circuits will naturally drain the caps and others will not, so you want to get into the habit of always checking with your meter first. The lead that Neikeel suggested will allow you to clip onto a plate resistor and ground to safely bleed any stored voltages. Now in your case, you say that you only want to retube and bias the amps. Some amplifiers have bias test ports and adjustment pots on the outside of the chassis but others will require you to make the adjustments directly on the circuit board itself. You will also need to take a plate voltage reading to calculate the proper dissipation. In both cases you will need to work inside a live amp with voltages approaching 500v or more. Having the clips on your test lead will allow you to clip onto a tube pin and take your readings without your hands in the amp. If you invest in one of the nicer bias probes with both the plate voltage and current draw options you will be able to plug into a tube socket and safely take your readings from outside the amp. You may still need to make the bias adjustment inside the chassis with the power on though. It's a good idea to use a non-conductive screwdriver for this just in case it slips or touches something else inadvertently. If you have not done so already, check over at "geofex.com" as they have some good articles on safety and biasing.

[neikeel]

Ok, for the double croc lead to drain the filter caps, is that something I'm going to need or need to do for a retube/bias? I'm guessing that draining the caps before you work on the internals of an amp, prevents you from frying the amp, the meter, or yourself. But strictly for a retube/bias, am I going to have to concern myself with draining the caps? If so, why? And then finally how? LOL

I leave amp plugged into wall but with wall power switch off - amp is grounded, flip it over on steady surface/cradle whatever to clear the tubes.
take croc and clip to chassis then clip to the junction of V1 100k plate resistors, wait a few mins, switch the stand by on to drain mains caps.
Use your DVM between the positive lug of each filter cap and ground to ensure all discharged, you are now ready to go.

Are you setting bias with a probe, with 1ohm output tube ground resistors, shunt method?

[Livingstoni813]

Damn, nothin like finding out how much you don't know lol. Alright, so just to let you know where I'm at as far as understanding both of you. Not too sure what a plate resistor is, I do know what the chassis is, and also, V1 is the first stage preamp tube, farthest from the power tubes. My JTM45combo was made by George, remember him sayin something about trimpots to adjust the bias, would just need a bias probe. I was looking into the dmm first just to add some redundancy to the equation. Glad this came up though. If I get a bias probe that measures plate voltage and bias, does that mean I don't have drop $ on a dmm? Btw, I appreciate both your responses guys. I should probably let you know, I've never retubed/biased an amp before. I did have a bunch of research I wanted to do before I asked questions about "how to bias", or even thought about opening the amp.

[Livingstoni813]

Ok, visited geofex Dan, read the precautions as well as precautions from a few other sites. Also read about methods for draining the caps, they used the exact wording you used Neil, only no pics to go withe the explanation, so I'm still not sure wich pin is which, or which resistor is the plate resistor. Anyway, the more I read, the more practical it started to seem that I should just get a bias meter that reads plate voltage and bias on the same unit. Any thoughts on that route for tube changes? Bias meter brands?

[danman]

Search Youtube for some videos on biasing and discharging filter caps. Quite a few there that you can watch and get some ideas. If you bought an amp from George then he likely added the 1ohm bias resistors on the tube sockets for biasing. You will need a standard dmm to take your current draw readings at these bias resistors and a plate voltage reading at the power tube socket. The safe way would be to turn the amp off and check to make sure the caps are discharged. Then you can safely hook your meter leads with the clips onto the resistors or tube socket pin with the amp off. Then fire up the amp and take your readings. This way you will not need to be reaching in a live amp to take readings. It takes a little longer but it is safer until you get comfortable working around high voltage. You can also spend the money for a bias probe that either plugs into your dmm or comes with it's own. That decision will be up to you but if you already have the 1ohm bias resistors installed then you really do not need the probe. Now if you want to bias other types of amps then the probe might come in handy down the road.

If you would like to see some instructions for biasing your style amp, do a google search for "50 watt plexi instructions" and you will find the Metroamp pdf for the 50 watt kit. In these instructions George does a very good job of explaining how to bias the amps using the 1ohm resistors and a dmm.

[Livingstoni813]

Thanks Dan, already been watching some of those cap discharging vids this evening. Some helpful, some with people doin some goofy stuff. Been looking also at the different cap discharge tools people are using. Some are just a regular lead wire, double croc clips, described/directed to connect one end to chassis, and the other end to V1 pin 1. Then leave it in place while you're doing work to the amp? Now, I took the "leave it conncted" part to pertain to doing soldering or part replacement work on the amp. For the bias job, I just use the cap draining tool as indicated and then remove it directly after? Or leave it in place?
Also, I ran into a vid and some seperate diagrams showing/describing the same type of double croc lead for cap discharging with certain spec resistors in between the 2 leads. Any need for that? The vid said it allowed the voltage to bespread out over the resistor before it ran to ground/chassis. Also, like that idea of hooking everything up after the caps are discharged, but before I turn the amp on, seems logical, and adds just another layer of saftey precautions, the more the better imo on that, quite a ways away from those steps yet, noting everything along the way though.
Got a good lead on a fluke 117, possibly discounted by a relative. Now, if I don't need a bias meter, and can learn to bias my amp without it, cool, however, like you said, if the situation is different, biasing a friends amp etc. What bias meters could you or would you suggest? I've looked at the compu-bias, the bias king pro and the Alessandro so far.
Thanks again

[danman]

I can't give any advice on those because I have always used the dmm. They all have their pros and cons and different ways of measuring current draw. Take your time and find one with the features that best suit your needs. For the cap draining lead you mentioned, if you are attaching to the plate resistor at v1 you do not need an inline resistor in your lead because the plate resistor itself will limit the current flow and give you a nice steady voltage drain. Now the other option would be to clip directly to the capacitor itself to drain it. Having a resistor in your lead will prevent any sparks or pops and will allow the voltage to drain a little slower than directly shorting the cap to ground with just a plain wire. I always just clip to the plate resistor and leave it on until I am ready to power the amp up again. Always make sure to remove the lead before powering up though! A lot of amps will come with bleeder resistors installed at the caps which will automatically drain them when the power is off but it is safe working practice to always check with your meter first before starting work inside the amp.

[Livingstoni813]

Cool cool, I'm gonna gather at least the dmm at this point, make one of those cap discharge tools, and buy the new tubes, check out that 50w pdf, and stop back in on this for clarifying questions. I think I now know at least the physical tools I need to do this properly and safely. Probably gonna post some noob questions on CP 12ax7's and KT66's once I give that section a read.

[Joshabr1]

Got a fluke 87-5 absolutely great meter!! Recommend these big time. Also have an old 27. And a 117. All great. But the the 87 takes the cake.

[Livingstoni813]

Got a fluke 87-5 absolutely great meter!! Recommend these big time. Also have an old 27. And a 117. All great. But the the 87 takes the cake.

Thanks Josh, actually popped for a Fluke 117 with some Christmas $ the old man sent me. Comes with a calibration cert, idk if I needed it, but figured it couldn't hurt. My mom works for Stanley Vidmar, so she was able to get me a $25 discount and free shipping from Stanleys web store, should be here first full week of January. Just learned my cousin is a licensed electrician, so he's gonna show me how to make the cap discharge tool Neil was talking about, and also stand over me to give me a swift kick in the ass if I'm about to do something wrong. Oh, and I guess I should mention due to the thread title, I've decided to skip the bias meter for right now, and just learn how to use the dmm to bias. Kinda figured, if I could do it all with one tool, no need to buy two.