Help: I've never worked on an amp, but I'm going to attempt to bias my amp safely.

SonexReducer

Member
Messages
371
Hi,

I'm a complete newbie when it comes to the guts of an amplifier. I've watched several youtube videos on biasing an amp and it all seems very simple but I have a few questions about tools and safety.

People are suggesting I put one hand in my pocket while adjusting the bias pot / near the open chassis. What are the potential risks if I follow this advise?

I would like a Bias Master type tool. I would like to check 6L6's, 6V6's, KT66's. Does it check these tube types?

Also, The Bias Master is out of my budget. Is there something similar available that is reliable and priced a bit lower?

I live in a remote part of the country now and before it was very easy to have somebody just take a peek at my amps, but that is no longer an option.

I appreciate you guys being here, for I don't want to just poke around a mine field blindly.

Thanks:)

EDIT:

So I went ahead and ordered a bias probe called the pro one from eurotubes. It measures both the plate voltage and tube output. You can play and bias at the same time. Its a cakewalk to use and I ended up biasing it "cold" at 31. Just above 50%.

Here's me in action biasing for the first time:

SAM_0416_zpsr80frmfm.jpg
 
Last edited:
M

Member 1963

The best and easiest way that safe too is to remove each power tube's cathode from ground then solder a a 1 ohm (yes, ONE ohm) resistor from each cathode to ground. The you can read the voltage between ground and the cathode, and because they are only separated by 1 ohm you will get a reading in milliamps which due to the 1 ohm resistor, when you do the math translates exactly to the current drawn. EG: 30 millivolts means the tube is drawing 30 milliamps.

I usually put a bias pot on the back panel and a probe jack (pic below) for each cathode on the back panel too. You can put one for ground too or just use the chassis, but it's nice to have a ground jack for the other probe too rather than having to hold it against the chassis. What i like to do is use 2 probe jacks on a 2 tube amp, one for ground and one that goes to a switch that selects which tube's cathode the jack goes to. Saves a 3rd jack and also quicker to select which tube you are checking. This is simple, safe, and with a jack to each tube you can see which is drawing most and use that one to bias the amp so you don't bias too hot. This way you also only need a multimeter and don't have to buy some expensive bias tool.

Oh, and it should go w/o saying the pot should be a screwdriver type so it never gets turned by accident. Even safer, put some sort of cover over it secured in place by screws.

s-l300.jpg
 
Last edited by a moderator:

Blue Strat

Member
Messages
30,722
The best and easiest way that safe too is to remove each power tube's cathode from ground then solder a a 1 ohm (yes, ONE ohm) resistor from each cathode to ground. The you can read the voltage between ground and the cathode, and because they are only separated by 1 ohm you will get a reading in milliamps which due to the 1 ohm resistor, when you do the math translates exactly to the current drawn. EG: 30 millivolts means the tube is drawing 30 milliamps.

I usually put a bias pot on the back panel and a probe jack (pic below) for each cathode on the back panel too. You can put one for ground too or just use the chassis, but it's nice to have a ground jack for the other probe too rather than having to hold it against the chassis. What i like to do is use 2 probe jacks on a 2 tube amp, one for ground and one that goes to a switch that selects which tube's cathode the jack goes to. Saves a 3rd jack and also quicker to select which tube you are checking. This is simple, safe, and with a jack to each tube you can see which is drawing most and use that one to bias the amp so you don't bias too hot. This way you also only need a multimeter and don't have to buy some expensive bias tool.

Oh, and it should go w/o saying the pot should be a screwdriver type so it never gets turned by accident. Even safer, put some sort of cover over it secured in place by screws.

s-l300.jpg

Great stuff but might not be considered a beginner's project.

You can get inexpensive bias probes that require the use of a seperate Digital Multimeter. If you have one of these, and you should if you wish to expand your amp work skills, then it won't cost you much.
 

NotWesYet

Silver Supporting Member
Messages
5,358
Great stuff but might not be considered a beginner's project.

You can get inexpensive bias probes that require the use of a seperate Digital Multimeter. If you have one of these, and you should if you wish to expand your amp work skills, then it won't cost you much.


I just purchased two of these and a multimeter as the compu-bias I ordered had been delayed for sometime.

https://www.tubedepot.com/products/tubedepot-bias-scout-kit

Sonex, if you are interested I'll be be glad to sell two for the price of one as I now have the compu-bias.

I would also suggest buying some books and dvd's. I have a Gerald Weber DVD and book and have another book on the way. Worked on amps in high school with my physics professor who worked for NASA. Showed me the hard way capacitors can hold a charge! Best to study up and always take precautions!!!!!!
 
M

Member 1963

Great stuff but might not be considered a beginner's project.

You can get inexpensive bias probes that require the use of a seperate Digital Multimeter. If you have one of these, and you should if you wish to expand your amp work skills, then it won't cost you much.

Maybe, but i wrote the post with the thought in mind that we never know who we are talking to. Some would find this quite simple even if they are a beginner with a little guidance, others may find it about as simple as brain surgury. So i wrote it in case he might be capable. It really is very simple, but it may not seem so in a text message like this.
 

Blue Strat

Member
Messages
30,722
Maybe, but i wrote the post with the thought in mind that we never know who we are talking to. Some would find this quite simple even if they are a beginner with a little guidance, others may find it about as simple as brain surgury. So i wrote it in case he might be capable. It really is very simple, but it may not seem so in a text message like this.

Agreed. If this would be his first soldering job (my hunch it is) it might not be a good idea.
 

zenas

Member
Messages
8,871
A bias probe of some sort is the way to go. However there's the possibility the amp in question doesn't have an easily accessible bias pot. Or even has a pot at all?
At any rate, none of use was born.knowing this stuff so it can be learned. :)
 
Messages
6,837
+1 on 'What amp is this, and is there an adjustment pot?'
IF the amp is a fixed biased amp with a bias voltage adjustment pot, then I suggest that the OP save up enough money to buy a bias metering tool that reads both current draw and plate voltage.
IN the meantime, reading and studying would be beneficial, imho.... sonex reducer, if you have never been to the Aiken Amp site, do a search for 'Aiken Amps'...go to the TEch info section and read the White Papers....all of them. There is a very good section...maybe tow...on biasing.
 

SonexReducer

Member
Messages
371
Thanks for all the replies. Yes, bias pot in both amps in question. I've watched videos using the suggested method. I've never soldered in my life. I would feel comfortable biasing this way if I had someone do this small mod for me and bias it with me there (so to learn), but honestly, I'm scared to do this. I would like to use a bias metering tool like the above member suggested. The amps in question are a Metroamp JTM45 and a JCM800 inspired build using 2 6V6's.

On a side note, I just came across a couple of threads with comments concerning a (Metroamp) KT66 JTM45 pushing up to 75w dimed? (Even in my ignorance I have my doubts). I have a 2x10 Ported Pine Cab on the way running 2 Celestion G10 Greenbacks which will rate it at 60w. Dime it anyway and relax?

Currently looking at this: https://www.eurotubes.com/store/pc/viewPrd.asp?idproduct=144&idcategory=6

Thanks again.

The best and easiest way that safe too is to remove each power tube's cathode from ground then solder a a 1 ohm (yes, ONE ohm) resistor from each cathode to ground. The you can read the voltage between ground and the cathode, and because they are only separated by 1 ohm you will get a reading in milliamps which due to the 1 ohm resistor, when you do the math translates exactly to the current drawn. EG: 30 millivolts means the tube is drawing 30 milliamps.

I usually put a bias pot on the back panel and a probe jack (pic below) for each cathode on the back panel too. You can put one for ground too or just use the chassis, but it's nice to have a ground jack for the other probe too rather than having to hold it against the chassis. What i like to do is use 2 probe jacks on a 2 tube amp, one for ground and one that goes to a switch that selects which tube's cathode the jack goes to. Saves a 3rd jack and also quicker to select which tube you are checking. This is simple, safe, and with a jack to each tube you can see which is drawing most and use that one to bias the amp so you don't bias too hot. This way you also only need a multimeter and don't have to buy some expensive bias tool.

Oh, and it should go w/o saying the pot should be a screwdriver type so it never gets turned by accident. Even safer, put some sort of cover over it secured in place by screws.

s-l300.jpg
 
Last edited:
Messages
6,837
Sonex, that Eurotubes things looks good for your purposes. I like being able to play and make small adjustments to set things where my ear likes it....all the while knowing that the amp is running safely.
Note: the ear can lead one to very hot bias settings that while maybe sounding great offer the tubes shorter life. Past 65% of max plate dissipation or so, the sonic returns for hotter biasing are decreasingly smaller as one increases that current and plate dissipation factor.
 




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