JCM 800 2203 Bias

[sixstrings]

Can someone provide a quick step-by-stop of how to bias a JCM 800 2203 reissue? Do I need a bias kit or just the multimeter?

 

[MrDan666]

There's a few simple ways of doing it.

Most simple IMO is to buy a Weber Bias Rite and use that. You just put the BiasRite sockets in the amp's power tube sockets. Then you load the tubes into the BiasRite sockets. Switch amp and BiasRite on. Once amp is warmed up switch the amp's Standby off and then read the bias reading on the BiasRite.

If it needs adjusting, simply adjust the bias trimpot inside the amp slowly until you get the correct bias reading. Be VERY careful if you adjust inside while the amp is switched on. You can always power down and adjust with the amp switched off, if you're concerned for your safety.

The other method of biasing is a little more dangerous because it involves probing inside a live amp, so you need to be confident with working with dangerous voltages. If you aren't 110% confident working on a live amp, do not use this method. If you do want to know more about that though, then let me know and i'll explain further.

I personally use the BiasRite to bias my own amps or customers amps. Its just so much quicker, safer and easier IMO No math or amp skills required, anyone can use it. Highly recommended!

 

[sixstrings]

There's a few simple ways of doing it.

Most simple IMO is to buy a Weber Bias Rite and use that. You just put the BiasRite sockets in the amp's power tube sockets. Then you load the tubes into the BiasRite sockets. Switch amp and BiasRite on. Once amp is warmed up switch the amp's Standby off and then read the bias reading on the BiasRite.

If it needs adjusting, simply adjust the bias trimpot inside the amp slowly until you get the correct bias reading. Be VERY careful if you adjust inside while the amp is switched on. You can always power down and adjust with the amp switched off, if you're concerned for your safety.

The other method of biasing is a little more dangerous because it involves probing inside a live amp, so you need to be confident with working with dangerous voltages. If you aren't 110% confident working on a live amp, do not use this method. If you do want to know more about that though, then let me know and i'll explain further.

I personally use the BiasRite to bias my own amps or customers amps. Its just so much quicker, safer and easier IMO No math or amp skills required, anyone can use it. Highly recommended!

Thanks. When I open the chassis, will the trimpots be obvious or is there something else I need to know to make the right adjustments? Also, can I just use a digital multimeter if I don't have a bias kit?

 

[MrDan666]

Thanks. When I open the chassis, will the trimpots be obvious or is there something else I need to know to make the right adjustments? Also, can I just use a digital multimeter if I don't have a bias kit?

If you plan to use the method using a Multimeter, again as i say you need to be confident on working on a live amp, plus you will also have to slightly modify the tube sockets before you can bias the amp. You have to de-solder the jumper wire that jumpers pins 1+8 and ground on each powertube socket. Then you install a 1-Ohm resistor in place on every Powertube socket. One leg goes through pins 1+8 and the other leg goes to ground. The resistor must be atleast 1% tolerance or the reading will not be accurate.

Then when the amp is hooked up to a load, powered up and off standby... you set the DMM to read DC Milivolts. Place the black DMM probe in one of the chassis mounting screw holes so its touching ground, and then with the red probe you measure the leg of the 1-Ohm resistor that's going to pins 1+8, one at a time. That will then read the bias. Even though it will be in DC MiliVolts, just think of it as MiliAmps.

Yes the trim pot should be obvious inside, it should be the only one in there. But if you aren't sure just post a pic and i'll highlight it.

This method works just fine but i do MUCH prefer the method using a Weber Bias Rite, as it doesn't require the soldering of the resistors on each tube socket. Its much safer if you don't already have the experience working on amps.

 

[sixstrings]

Thanks. When I open the chassis, will the trimpots be obvious or is there something else I need to know to make the right adjustments? Also, can I just use a digital multimeter if I don't have a bias kit?

If you plan to use the method using a Multimeter, again as i say you need to be confident on working on a live amp, plus you will also have to slightly modify the tube sockets before you can bias the amp. You have to de-solder the jumper wire that jumpers pins 1+8 and ground on each powertube socket. Then you install a 1-Ohm resistor in place on every Powertube socket. One leg goes through pins 1+8 and the other leg goes to ground. The resistor must be atleast 1% tolerance or the reading will not be accurate.

Then when the amp is hooked up to a load, powered up and off standby... you set the DMM to read DC Milivolts. Place the black DMM probe in one of the chassis mounting screw holes so its touching ground, and then with the red probe you measure the leg of the 1-Ohm resistor that's going to pins 1+8, one at a time. That will then read the bias. Even though it will be in DC MiliVolts, just think of it as MiliAmps.

Yes the trim pot should be obvious inside, it should be the only one in there. But if you aren't sure just post a pic and i'll highlight it.

This method works just fine but i do MUCH prefer the method using a Weber Bias Rite, as it doesn't require the soldering of the resistors on each tube socket. Its much safer if you don't already have the experience working on amps.

Ok fuck that. The Weber tester is a little pricey, but I found this one

https://cgi.ebay.com/Bias-Tool-probe-tes ... 53de03fe96

What do you think? Should I buy it?

 

[MrDan666]

I don't know for sure as i haven't got any experience with that Bias tool.. but from what i can see in the pics it only has a single socket. Which is gonna be a pain in the ass if you run a 100watt amp with 4 tubes.

The Weber has individual sockets for each powertube, mine is the BR-4 model with the 4 sockets. This way you can monitor the Bias of each tube all in one go, without having to power down. MUCH faster than if you had only 1 socket.

I bought my BiasRite used off a forum member over at HC a while back. You could probably score one for cheap if you put up a 'Wanted' post in the Classifieds here at RigTalk or any other amp forums that you visit I think i only paid the equivalent of like $70 bucks for mine!

 

[sixstrings]

Ok inside the chassis I only saw two blue dials one said V1 and the other V2. I'm assuming those are the bias pots since I couldn't find anything else like it inside the chassis. So what is V1 and V2 for? It's a 100w amp so is it for inside/outside tubes or something?

 

[MrDan666]

Ok inside the chassis I only saw two blue dials one said V1 and the other V2. I'm assuming those are the bias pots since I couldn't find anything else like it inside the chassis. So what is V1 and V2 for? It's a 100w amp so is it for inside/outside tubes or something?

Is it a Reissue model?

If so, VR-1 is the Bias trimpot which you adjust to change the bias

 

[sixstrings]

Ok inside the chassis I only saw two blue dials one said V1 and the other V2. I'm assuming those are the bias pots since I couldn't find anything else like it inside the chassis. So what is V1 and V2 for? It's a 100w amp so is it for inside/outside tubes or something?

Is it a Reissue model?

If so, VR-1 is the Bias trimpot which you adjust to change the bias

Yes it's a reissue. What is the V2 trimpot for?

 

[MrDan666]

Yes it's a reissue. What is the V2 trimpot for?

I'm pretty sure VR-2 is a hum balance circuit for the heaters, so don't touch VR-2. Just adjust the VR-1 trimpot for the bias

 

[sixstrings]

Yes it's a reissue. What is the V2 trimpot for?

I'm pretty sure VR-2 is a hum balance circuit for the heaters, so don't touch VR-2. Just adjust the VR-1 trimpot for the bias

Thanks so much for the help. I'm going to buy the bias tool and give it a shot. Any idea what bias settings Marshall recommends for the reissue?

 

[MrDan666]

Thanks so much for the help. I'm going to buy the bias tool and give it a shot. Any idea what bias settings Marshall recommends for the reissue?

No problem man, glad to help!

Ideally you need to know the plate voltage first before you determine the ideal Bias setting. On my BiasRite it has a switch with 2 settings, you switch to plate voltage and it reads that on the display (usually anywhere between 450-500vdc).

When you know your plate voltage, you can use this Bias calculator to work out your ideal bias range
http://www.webervst.com/tubes1/calcbias.htm