marshall owners. what's your bias set at?

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1big1

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i'm sort of conducting a random unscientific sampling of what folks are setting thier bias at on different models of marshall amps.
the only info i'm looking for is the model.. (jcm 800.. jmp.. etc.) tube type (el34.. kt 66.. etc) and how many ma or mv your bias is set at.
just curious.. that's all.
 
My Marshalls are all biased a little differently. I bias the amps while playing them to find the swet spot with whatever particular set of tubes is in the amp. Since even tubes of the same make and model can sound different, I find that is the best way to go to get the tone I am after. My '87 JCM 800 2205 with KT88's is biased to around 45mA per tube. My Jubilee 2555 with EL34's biased to 37mA per tube. My '77 JMP 2203 is biased to 40mA per tube. :rock:
 
i'm trying to figure something out. i was reading a ritchie fliegler book last night and noticed he included a chart from a bias probe. it listed some suggested ranges for various amps like fender.. etc. and various power tube compliments.. and output tranny voltages.
i'm trying to cure my sick dsl and need to do a bias mod on it. maillet's site suggested running dsl and tsl's quite a bit cooler than the factory spec. so i tried it this morning. i was shooting for 32 mv. after i did a bunch of re-soldering on the boards etc.
i may wind up ordering the bias mod parts soon so i wondered if i was going to get cross over distortion at the lower bias settings. the short answer is no. i suppose the only real way to tell what the amp is actually doing is to scope it. i've got a scope.. but i need a load resistor. i've got a sine wave generator i can use to boot. i'm trying to put a parts list together so i can order what i need all at once. so.. thanks for your reply. it helps.
 
my 600 with kt77s is at 47
my 73 jmp with 6550's at 52 1/2
jvm at 45 with el 34s
the dsl 401 and the VM I cant remember
 
1big1":3d0bsx4u said:
i'm trying to figure something out. i was reading a ritchie fliegler book last night and noticed he included a chart from a bias probe. it listed some suggested ranges for various amps like fender.. etc. and various power tube compliments.. and output tranny voltages.
i'm trying to cure my sick dsl and need to do a bias mod on it. maillet's site suggested running dsl and tsl's quite a bit cooler than the factory spec. so i tried it this morning. i was shooting for 32 mv. after i did a bunch of re-soldering on the boards etc.
i may wind up ordering the bias mod parts soon so i wondered if i was going to get cross over distortion at the lower bias settings. the short answer is no. i suppose the only real way to tell what the amp is actually doing is to scope it. i've got a scope.. but i need a load resistor. i've got a sine wave generator i can use to boot. i'm trying to put a parts list together so i can order what i need all at once. so.. thanks for your reply. it helps.
Click to expand...

I have re-tubed lots DSL and TSLs (in fact I am working on a TSL today) and the sweet spot seems to be around 32mA. Higher values seem to loosen up the bass on those amps. 32mA is still well within spec so it's all good. :thumbsup:
 
DSL 50 --> 41.6 per side

But am going to try the 32ma suggested and see what happens :rock:
 
My 2553 is about 34mA for the EL34's. Plate voltage is 471VDC.
 
killertone":34351spr said:
1big1":34351spr said:
i'm trying to figure something out. i was reading a ritchie fliegler book last night and noticed he included a chart from a bias probe. it listed some suggested ranges for various amps like fender.. etc. and various power tube compliments.. and output tranny voltages.
i'm trying to cure my sick dsl and need to do a bias mod on it. maillet's site suggested running dsl and tsl's quite a bit cooler than the factory spec. so i tried it this morning. i was shooting for 32 mv. after i did a bunch of re-soldering on the boards etc.
i may wind up ordering the bias mod parts soon so i wondered if i was going to get cross over distortion at the lower bias settings. the short answer is no. i suppose the only real way to tell what the amp is actually doing is to scope it. i've got a scope.. but i need a load resistor. i've got a sine wave generator i can use to boot. i'm trying to put a parts list together so i can order what i need all at once. so.. thanks for your reply. it helps.
Click to expand...

I have re-tubed lots DSL and TSLs (in fact I am working on a TSL today) and the sweet spot seems to be around 32mA. Higher values seem to loosen up the bass on those amps. 32mA is still well within spec so it's all good. :thumbsup:
Click to expand...

i consulted the table and came up with a range of approximately 57 to 68 ma per pair. what really made me question the marshall service bulletin that says to bias them at 90 per pair was my laney aor. it's a 50 watter and the bias figures out to 33 ma. so why the hell would a dsl bias so much hotter? i was sort of expecting distortion on the low end when i backed the bias down. didn't happen. not that i'm unhappy about that.
i wonder if the boys in whatever department were out late when they wrote that bulletin? 45 ma is a number you'd try to hit if the tranny was 350v.
 
I keep mine about 60-65%.....but I hammer my amps volume wise all the time....if you never turn the amp up the bias can be hotter of course....
 
Ok, I'll be the guy to ask the stupid question. How do you calculate diss. % ? I just set mine at the level I read online, which was around 40-45ma for a DSL 50, so I set it to 41.6 and left it.... But it would be nice to do the math and figure out the correct way to do this.
 
What George said, my 100 watt non masters get biased at about 25 mil since I run it wide open
 
ted weber now sells a build it yourself bias probe for 16 usd. i gots to get me one. i could post how to read the plate voltage on pin three.. but i don't want to be responsible for what happens if someone screws up.
 
psychodave":1s3w0wns said:
70% diss.
Click to expand...
exactly, it has to do with your tubes and amp voltage there is no magic stock number they all vary.
 
tweed":10f39whq said:
Ok, I'll be the guy to ask the stupid question. How do you calculate diss. % ? I just set mine at the level I read online, which was around 40-45ma for a DSL 50, so I set it to 41.6 and left it.... But it would be nice to do the math and figure out the correct way to do this.
Click to expand...

Use this formula to find out what the bias range is:

Max Dissipation of tube type / Plate Voltage * (.6 to .7) = bias in mA

So, for an EL34 (25w) with Plate Voltage of 450VDC running at 65% Max Dissipation (you can use anything from 60%-70%):

25/450 * .65 = ?

.0555 * .65 = .36

So, 36mA is the bias per tube.
 
Marshall Law":1e2rr1zx said:
psychodave":1e2rr1zx said:
70% diss.
Click to expand...
exactly, it has to do with your tubes and amp voltage there is no magic stock number they all vary.
Click to expand...

70% dissipation isn't a magic number, either.
 
also.. just because your amp is rated at 100 watts doesn't mean it's putting out 100 watts either.
a simple method you can use that will get you in the ballpark as far as your wattage is concerned is this.. if you don't have a scope.. dummy load.. etc.
you can take your dmm.. set it to ac volts.. and hook it up to your speaker leads and start playing at the volume you'd normally play at. try to determine an average reading while looking at the meter. once you think you have a good idea what the middle ground is voltage wise.. you square it.
so if your amp is putting out an average of 17.5 volts x itself = 306.25. divide that number by the ohms your speakers are set at.. say 8.. and you get 38.28 watts.
it's just another method you can use to determine wattage.
 
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