JCM900 4100 Correctly Biasing

[Snake_j]

In my reading on the forums I have seen people mention adding an additional 5ma to the idle current calculation for the screen current (65%+5ma). Does this vary by model and only relevant on some or is this common to most models...Some people bring it up and some don't, which is correct for my JCM900 4100DR

Thanks!

 

[ampmadscientist]

In my reading on the forums I have seen people mention adding an additional 5ma to the idle current calculation for the screen current (65%+5ma). Does this vary by model and only relevant on some or is this common to most models...Some people bring it up and some don't, which is correct for my JCM900 4100DR

Thanks!

The hotter it's biased, the better it sounds.
The shorter the tube life will be...

The colder it's biased, the rougher it sounds, gritty.
The longer the tube life will be.

So you see, its a compromise between sound quality and tube life.

Go ahead and bias it 5% hotter...does it sound better?
The tube life is now 5% shorter.

Most people who understand will bias it hotter, and buy tubes more often.
Because they prefer the sound quality of hot bias.

If there is no noticeable improvement, leave it colder.

 

[iron broadsword]

I've got 6L6's in mine and I run it at a little over 70%. Just sounds so friggin good that way. I change tubes about once a year, haha, cause at about that point I hear it change quickly and re-biasing doesn't fix it. Iirc the math worked out to 43ma for 70%, and it's running at 45.

 

[tschrama]

Love my tubes between 15 and 25mA. Hot biasing is a myth

 

[iron broadsword]

Myth? Idk, I think there are definitely some nuances. Actually this would be a fun test... next time I have my amp on the bench I'll record some different settings and see if it is like I think it is.. I do know that whenever I set it less than 65% I always end up coming back to change it because it seems to be missing something.

 

[Ghostman]

There's been a lot of posts of bias on the cold side sounding much better. Especially in the JCM2000's. I know it sounds much better in mine to be on the cold side of life.

 

[mickeydg5]

There's been a lot of posts of bias on the cold side sounding much better. Especially in the JCM2000's. I know it sounds much better in mine to be on the cold side of life.

Which DSL and what is your idle bias current and plate voltage?
I bet it is not cold at all.

 

[jakub421]

In my 77' 1987, I run the tubes at almost 70%. At a plate voltage of 365, thats around 48 mA.
Would I be able to safely run my tubes hotter since the plate voltage is lower?

I believe I read somewhere that with that low of a plate voltage, running the bias a bit over 70% isn't that much of a problem. Opinions?

 

[Ken]

Hotter vs. colder seems to vary by amp model with Marshalls. On the JCM 2000 DSL, we tend to use the pins and not measure plate voltages, so we're really biasing by ear. For whatever reason, few people like the factory 90 mV bias. Most people prefer 10% lower, myself included (halve that 90 mV for the 50 watt models). The factory number tends to be too fizzy.

I've read here that the first JVM's were shipped with a cold bias and people tended to punch them up. For those of us who are just measuring the mV and not measuring plate voltages, I suggest you do what I do: start at the factory number and then adjust by ear up or down until you get the best tone.

Ken

 

[JWRE]

I have 30 watt 6L6GC tubes, a plate voltage or 486 and found that 42 ma sounds best to me. Less sounds weak and more wasn't any better than 42, so that's where I've kept mine .

 

[Ghostman]

Which DSL and what is your idle bias current and plate voltage?
I bet it is not cold at all.

it's a JCM2000 DSL MLB edition. I don't know the plate voltage but it's biased at 72ma a side.

 

[mickeydg5]

it's a JCM2000 DSL MLB edition. I don't know the plate voltage but it's biased at 72ma a side.

DSL are usually average 460 to 470 VDC on plates I believe.
So you would have a midway bias, nice and average but not cold.

 

[Ken]

DSL are usually average 460 to 470 VDC on plates I believe.
So you would have a midway bias, nice and average but not cold.

72 would also be where I'd bias the 1st generation DSL's. Besides the stripe color, I don't know if the MLB is any different. Why did Marshall change the name anyway?

Ken

 

[mickeydg5]

The whole 45mA situation with those amplifiers makes one wonder if it were planned to have a hotter bias or if the B+ was expected to be lower.

 

[Ghostman]

72 would also be where I'd bias the 1st generation DSL's. Besides the stripe color, I don't know if the MLB is any different. Why did Marshall change the name anyway?

Ken

They changed the color to go with a budget package promotion from what I understand. The Amp was paired with a cheap MC412 cabinet, one with a 1960 dimensions but built like an MG cab, particle board and Rocket 50 speakers.

 

[Ken]

They changed the color to go with a budget package promotion from what I understand. The Amp was paired with a cheap MC412 cabinet, one with a 1960 dimensions but built like an MG cab, particle board and Rocket 50 speakers.

I remember the package...I'm just not sure if the amp was the same except for cosmetics.

Ken

 

[Snake_j]

Thanks for the replies, my jcm900(90' el34s) was never right from day one. The old tubes were at 18ma... I swapped out my tubes and could not get anywhere near where they should be even with the pot maxed out (25ma). I ended up replacing r30 (56k) with a 47k and got them dialed in at 36ma with the pot about in the middle with plenty to go in each direction. Turned out sweet...sounds so great compared to before.

 

[IOSEPHVS]

In my reading on the forums I have seen people mention adding an additional 5ma to the idle current calculation for the screen current (65%+5ma). Does this vary by model and only relevant on some or is this common to most models...Some people bring it up and some don't, which is correct for my JCM900 4100DR

Thanks!

This thread is dated, but for the JCM900 4100, reading from the junction of R26 and R27 to chassis ground, set bias with VR2 to approximately -50V. It's as simple as that.

 

[gibson17]

Hello, I'm resurrecting this thread because I am having the same issue with the same amp. I have a JCM900 DR 4100. The bias adjustment at full is about 23ma. I clipped r30 which in my amp was 22.5k and put a decade resistor in place to find a suitable replacement. I then moved the bias pot to the middle position which reads at about 8ma. No amount of changing the value of r30 changes the bias range. What am I doing wrong?

 

[anitoli]

Hello, I'm resurrecting this thread because I am having the same issue with the same amp. I have a JCM900 DR 4100. The bias adjustment at full is about 23ma. I clipped r30 which in my amp was 22.5k and put a decade resistor in place to find a suitable replacement. I then moved the bias pot to the middle position which reads at about 8ma. No amount of changing the value of r30 changes the bias range. What am I doing wrong?

According to the schematic R30 would be a 1 watt 10K ( brown, black, orange) and is not in the bias circuit, that provides voltage drop to the PI tube. The resistor in question would be R28. Take out the power tubes, turn on amp standby off. Measure the negative bias voltage at pin 5 with the adjustment pot fully CCW and then fully CW. What is the voltage range? And what/how are you using to measuring the bias current?

https://www.drtube.nl/schematics/marshall/4100-60-02-2-iss11.gif