Bias

[Mats A]

You seem to have looked inside the new DSL. Is it the same as the old?

[Cipher]

They are not the same. The new version uses 1 board for most of the components and the power tubes are chassis mounted, while the original versions had pcb mounted power tubes. (of course there are other differences but I'm just focusing on the boards here.)


JCM 2000 DSL100


DSL 100H (new)
http://marshallroadhouse.com/viewtopic.php?f=73&t=13741

[Mats A]

Ok. Curcuit wise is there big differencies?

[Mats A]

According to a well known amp modder here in Sweden Marshall must have made a misstake in there manual and on the JCM 2000 DSL 100W you should set it to 60mv and that 90 is way to high. Anybody checked that?

[Anitoli]

That's why you SHOULD measure the plate voltage. You cannot get an accurate dissipation number by just looking at the current draw through the cathode. These arbitrary mv numbers will set the bias quite cold so the tubes outlive the warranty period.

The formula is simple:

Voltage X Current = Power

You also need to look at the tubes maximum plate dissipation rating and when setting bias do not exceed 70% of that maximum. Example: 6L6GC has a 30watt plate dissipation. The maximum plate dissipation you would want is 21watts, 70%. Of course you can go lower if you like the sound. Going higher will wear out the tubes faster and if too high cause redplating and a very short tube life.

[Mats A]

But if you have no bias probe gear that is hard. If the bias is not absolutely perfect and maybe a bit cold or warm will it hurt just the valves or the amp itself also.

[Micky]

But if you have no bias probe gear that is hard. If the bias is not absolutely perfect and maybe a bit cold or warm will it hurt just the valves or the amp itself also.

No it's not hard. What are you? Lazy?

Put your meter on millivolts DC.
Read voltage.
Adjust.

Pretty simple.

Unless you are a 4-year old and can't do the math to figure where to adjust them to...

[Mats A]

Well people tell different things and i want to get it right. 90mv or 60mv? I'm not 4 years old are you?

[Micky]

Here you go.
http://www.webervst.com/tubes1/calcbias.htm

You can't go by some 'set' voltage. You have to read YOUR plate voltage and figure it out according to YOUR tubes.

[Mats A]

Ok thanks
But first you say put your meter on mv and read voltage and that it's simpel. Than i must meassure plate voltages and calculate? Can't you go by the service manual from Marshall according to the JCM 2000 DSL and use the prongs?

[Micky]

Of course use the test points.
Forget about the service manual and what Marshall says. It is only a general guideline.
YOUR amp may be different.

Between 30-40 mv per side average on a 40-50 watt amp, 60-80 mv per side average on a 100W amp.

[Mats A]

Then i wont have to calculate i use a meter and mv and the test points. But i'll let my brother do it when he got the time. I'm just a player and no tech.

[Anitoli]

Just try it. You cant hurt anything by using the ports on the amp. Set the meter to MV range or auto if applicable and take a reading. The ports measure ACROSS the resistor, it doesn't break any connection.

[Mats A]

Thanks. I'll probably take it to a tech later for a propper bias adjust.

[Mats A]

My brother did it. We meassured the valves that were in before and they were at 58 and 62 mv/side. Now the amp is more quiet and with no sparkles and pops whatsoever. It sounds just great.