DesertRose
New member
re-biasing when replacing it with new tubes ?
Do Marshall amps need ....
- Thread starter DesertRose
- Start date
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Lewguitar
Guest
Re: Do Marshall amps need ....
Yes...if you're replacing the output tubes. No, if you're just replacing preamp tubes. Lew
Yes...if you're replacing the output tubes. No, if you're just replacing preamp tubes. Lew
DesertRose
New member
Re: Do Marshall amps need ....
how about if its the same kind of output tubes ?
thanks
how about if its the same kind of output tubes ?
thanks
kmcguitars
GuitarArtologist
Re: Do Marshall amps need ....
At any rate... it's aways wise to rebias after any power tube change. Even same style/brand. There could be variances?
Do you mean the same style (i.e.) EL34's or the same brand (i.e.) JJ EL34's?DesertRose said:
At any rate... it's aways wise to rebias after any power tube change. Even same style/brand. There could be variances?
stevie_bees
Cat In The Hatministrator
Re: Do Marshall amps need ....
It never hurts to check the bias when replacing valves, you just never know.
It never hurts to check the bias when replacing valves, you just never know.
Kamanda~SD
New member
Re: Do Marshall amps need ....
(I don't own a tube amp) whats this biasing your talking about?
(I don't own a tube amp) whats this biasing your talking about?
stevie_bees
Cat In The Hatministrator
Re: Do Marshall amps need ....
In simple terms, it's an adjustment of the voltages around a valve to ensure it operates properly. The bias is a "switching" voltage, usually just holding the valve "off" until your guitar signal goes through it.
In simple terms, it's an adjustment of the voltages around a valve to ensure it operates properly. The bias is a "switching" voltage, usually just holding the valve "off" until your guitar signal goes through it.
DesertRose
New member
Re: Do Marshall amps need ....
what is the voltage reading best suited for Marshall amps on the DMM ? eg. 68mV ?
thanks
starting trial and error
what is the voltage reading best suited for Marshall amps on the DMM ? eg. 68mV ?
thanks
starting trial and error
stevie_bees
Cat In The Hatministrator
Re: Do Marshall amps need ....
I'm not sure. There is a Marshall amps forum. They might have some info there
I'm not sure. There is a Marshall amps forum. They might have some info there
stevie_bees
Cat In The Hatministrator
DesertRose
New member
Re: Do Marshall amps need ....
Thanks man ..
All the help is relaly appreciated
Hopefully i don't commit suicide
Thanks man ..
All the help is relaly appreciated
Hopefully i don't commit suicide
Gearjoneser
Gear Ho
Re: Do Marshall amps need ....
Marshall DSL/TSL's have the bias points on the back panel, so you only need a voltmeter.....you don't even have to remove the chassis from the headbox.
On all other older Marshalls, there's a bias pot inside, somewhere near the power tubes. Then, you'll need a bias tool and multimeter. You can find bias tools on Ebay for around $20. It's just a socket that fits between the amp and 1 tube, with two leads that attach to your meter.
Marshall DSL/TSL's have the bias points on the back panel, so you only need a voltmeter.....you don't even have to remove the chassis from the headbox.
On all other older Marshalls, there's a bias pot inside, somewhere near the power tubes. Then, you'll need a bias tool and multimeter. You can find bias tools on Ebay for around $20. It's just a socket that fits between the amp and 1 tube, with two leads that attach to your meter.
DesertRose
New member
Re: Do Marshall amps need ....
YES !
Mine is a JCM900 and 800. old models, I read on the article of removing the back chasis.
The bias tool is a screwdriver with a plastic holder. The voltmeter with 2xleads, think one black and one red. Heard only use the black crocodile clip to clip on the chasis to ground or something like that.
Its quite interesting especially you do it yourself. Well, most to most if the thing cannot be bias, then I just take it to the nearest Mr. Bias
Gearjoneser said:
YES !
Mine is a JCM900 and 800. old models, I read on the article of removing the back chasis.
The bias tool is a screwdriver with a plastic holder. The voltmeter with 2xleads, think one black and one red. Heard only use the black crocodile clip to clip on the chasis to ground or something like that.
Its quite interesting especially you do it yourself. Well, most to most if the thing cannot be bias, then I just take it to the nearest Mr. Bias
Lake Placid Blues
New member
Re: Do Marshall amps need ....
There are several methods of biasing used. One is using scope. In this method the bias is set cold and the sine wave observed on the scope. It will show a kink. The bias is then gradually adjusted hotter until the kink dissapears. In this method the bias is generally colder than other methods but the amp usually sounds more agressive.
Another method is to measure the plate dissapation in mv, or ma. If you measure between the cathode and ground, then it must go through a 1 ohm resistor to correct the readings mathmatically. You measure across the resistor. Tools like the bias probe do this, and some amps have the one watt resistor soldered there all the time. Another way is to measure how much current is flowing from the output transformer to the plates. This is more involved and more dangerous, but delivers good results.
When figuring the amount of MV, you take the amp's nominal plate voltage and divide this into the tube type plate dissapation in watts. For example, if your Marshall has 450 volts at the plates (pin 3) you divide this into 25 if your running EL34's. 30 or 35 if your running 6550's or KT88's. In the case of EL34's and 450 volts, it would be .05555~, or 55mv. This is 100% plate dissapation and the tubes won't last long. You would normally operate at 70% if you like your bias hot, or 60% if you bias cold. 70% of 55 is 38mv, and 60% is 33mv. The range from 60% to about 73% allows you to fine tune the tone of your amp. After setting the bias mathmatically, play the amp both clean and distorted at gig volume. Note how it sounds. Try playing it with the bias set a few mv colder and hotter too. Leave it were you liked the tone best. Of course be careful if you do all this your self.
Bias should always be adjusted with a re-tube for best tone, and best tube life. Bias should also be periodically adjusted as the tubes wear.
On edit: I changed the terms "1 watt" to "1 ohm."
There are several methods of biasing used. One is using scope. In this method the bias is set cold and the sine wave observed on the scope. It will show a kink. The bias is then gradually adjusted hotter until the kink dissapears. In this method the bias is generally colder than other methods but the amp usually sounds more agressive.
Another method is to measure the plate dissapation in mv, or ma. If you measure between the cathode and ground, then it must go through a 1 ohm resistor to correct the readings mathmatically. You measure across the resistor. Tools like the bias probe do this, and some amps have the one watt resistor soldered there all the time. Another way is to measure how much current is flowing from the output transformer to the plates. This is more involved and more dangerous, but delivers good results.
When figuring the amount of MV, you take the amp's nominal plate voltage and divide this into the tube type plate dissapation in watts. For example, if your Marshall has 450 volts at the plates (pin 3) you divide this into 25 if your running EL34's. 30 or 35 if your running 6550's or KT88's. In the case of EL34's and 450 volts, it would be .05555~, or 55mv. This is 100% plate dissapation and the tubes won't last long. You would normally operate at 70% if you like your bias hot, or 60% if you bias cold. 70% of 55 is 38mv, and 60% is 33mv. The range from 60% to about 73% allows you to fine tune the tone of your amp. After setting the bias mathmatically, play the amp both clean and distorted at gig volume. Note how it sounds. Try playing it with the bias set a few mv colder and hotter too. Leave it were you liked the tone best. Of course be careful if you do all this your self.
Bias should always be adjusted with a re-tube for best tone, and best tube life. Bias should also be periodically adjusted as the tubes wear.
On edit: I changed the terms "1 watt" to "1 ohm."
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DesertRose
New member
Re: Do Marshall amps need ....
hey man .. thanks for the info.
My JCM900 runs on 6L6EH .. i'm intending to replace them with the original 5881s. So i would need info
As for the JCM800, its EL34s .. so no issue with that. Your point are really relevent. thanks
hey man .. thanks for the info.
My JCM900 runs on 6L6EH .. i'm intending to replace them with the original 5881s. So i would need info
As for the JCM800, its EL34s .. so no issue with that. Your point are really relevent. thanks
ErikH
Well-known member
Re: Do Marshall amps need ....
When my tech biased my amp last year with new tubes, he used a scope. Signal comes out the scope to the amp's input then out the speaker out and back in to the scope (I think that's right). I got to watch him adjust it. It was pretty neat. The sine wave was actually pretty good from the start so there was minimal adjustment. He adjusted at low volume and then gradually increased it and readjusted until things were steady. He can bias my amp that way all the time. It sounds incredible.
When my tech biased my amp last year with new tubes, he used a scope. Signal comes out the scope to the amp's input then out the speaker out and back in to the scope (I think that's right). I got to watch him adjust it. It was pretty neat. The sine wave was actually pretty good from the start so there was minimal adjustment. He adjusted at low volume and then gradually increased it and readjusted until things were steady. He can bias my amp that way all the time. It sounds incredible.
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Kommerzbassist
Thunderbirdologist
Re: Do Marshall amps need ....
Uhm... I'm not so shure if that's correct... there IS a little whatever it is, but it's acover and doesn't seem to be meant to be taken off... dunno... you can pull the cover out a bit, but then it stops.... where is that spot supposed to be excactly?Gearjoneser said:
Lake Placid Blues
New member
Re: Do Marshall amps need ....
I don't know why I wrote "1 watt" resistor between cathode and bias. It should read a "1-ohm" resistor. This resistor needs to be a 3-5 watt presision resistor. What this resistor does is allow you to equate voltage to current, when using your multimeter. This way, you don't have to have current running (The circut) through your multimeter, and a more expensive multimeter that reads millamps. You can just read millivolts across the 1-ohm resistor instead.
If your not experianced with these things, then by all means find some one whom is, and have them show you how, and set it all up, and all the needed safty precautions. It's not really difficult, but these are high voltage machines.
Both 6L6 and 5881 have a plate dissapation of 20-22 watts. Even though they have the same rating, no two matched pairs behave, or bias, exactly the same way.
Some plate dissapation numbers for popular output tubes:
6L6 early glass case and metal case models from the 50's-20 watts
6L6-22 watts
5881-23 watts
KT66-25 watts (28 absolute max)
EL34-25 watts
6CA7-25 watts
KT77-25 watts (31 absolute max)
E34L-25 watts
6550-28 watts
KT88-35 watts (42 absolute max)
6V6-12 watts
DesertRose said:
I don't know why I wrote "1 watt" resistor between cathode and bias. It should read a "1-ohm" resistor. This resistor needs to be a 3-5 watt presision resistor. What this resistor does is allow you to equate voltage to current, when using your multimeter. This way, you don't have to have current running (The circut) through your multimeter, and a more expensive multimeter that reads millamps. You can just read millivolts across the 1-ohm resistor instead.
If your not experianced with these things, then by all means find some one whom is, and have them show you how, and set it all up, and all the needed safty precautions. It's not really difficult, but these are high voltage machines.
Both 6L6 and 5881 have a plate dissapation of 20-22 watts. Even though they have the same rating, no two matched pairs behave, or bias, exactly the same way.
Some plate dissapation numbers for popular output tubes:
6L6 early glass case and metal case models from the 50's-20 watts
6L6-22 watts
5881-23 watts
KT66-25 watts (28 absolute max)
EL34-25 watts
6CA7-25 watts
KT77-25 watts (31 absolute max)
E34L-25 watts
6550-28 watts
KT88-35 watts (42 absolute max)
6V6-12 watts
Lake Placid Blues
New member
Re: Do Marshall amps need ....
It is essentially correct, and I have confirmed this in the past. However, on the JCM2000 amps, the ma number that should read on the meter, is actually about twice that of normal, because of the way the leads are wired up. For example, instead of about 40ma, it will be 80-90 ma on your meter, IIRC.
Kommerzbassist said:
It is essentially correct, and I have confirmed this in the past. However, on the JCM2000 amps, the ma number that should read on the meter, is actually about twice that of normal, because of the way the leads are wired up. For example, instead of about 40ma, it will be 80-90 ma on your meter, IIRC.
