Quick! JVM Bias question

SkyhighRocks

SkyhighRocks

New member
How do I get to the bias pins on this amp? What do I need to take apart? I have 2 gigs this weekend and I want to check the settings asap.
 
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SkyhighRocks":8e887 said:
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the 4 screws on the bottom of the amp? i'm used to the DSL bias being right on the back.
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Yep.

Do it with the amp on a table, overhanging to expose the screws from the bottom, so that the chassis doesn't rest it's weight against the knobs.

Typically I flip them over, but I found out that it will rest on the knobs doing it that way, so I don't with the JVM.
 
I got the chasis out but how the hell can I bias it if I need it upside down AND plugged into a cab and powered on???
 
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SkyhighRocks":75718 said:
I got the chasis out but how the hell can I bias it if I need it upside down AND plugged into a cab and powered on???
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Just turn it upside down. The transformers are usually taller than the tubes, set it on it's tranny's, then plug it in and bias it.
 
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Marshall Freak":8a006 said:
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Just turn it upside down. The transformers are usually taller than the tubes, set it on it's tranny's, then plug it in and bias it.
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Nope!

Set it in the headshell, so that the tubes are "pointing" at the front panel.

Basically lay the headshell with the Marshall Logo facing down, then set the chassis in the headshell with the tubes facing downwards/guts up.

Plug in, turn on, etc..
 
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kannibul":c844d said:
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Nope!

Set it in the headshell, so that the tubes are "pointing" at the front panel.

Basically lay the headshell with the Marshall Logo facing down, then set the chassis in the headshell with the tubes facing downwards/guts up.

Plug in, turn on, etc..
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I wasn't sure if the back of the headshell was still like the old ones so I didn't know if that was possible with the JVM. :thumbsup:

I've got to get one, driving me crazy. :doh: :doh: :lol: :lol: :lol:
 
I'll have to try that next time. I basically had it upside down on my table with 2 books under the shorter transformer. It was biased a little low...like 57 on each side. I put them both to 70.
 
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SkyhighRocks":219cc said:
I'll have to try that next time. I basically had it upside down on my table with 2 books under the shorter transformer. It was biased a little low...like 57 on each side. I put them both to 70.
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Mine was at 60 per side. Not too bad for a stock amp actually. I kept it there because it sounds great as is. However, I have a set of NOS RFT (Siemens) EL34's coming in any day that will be going in as soon as they get here. That is going to friggin' rule. :thumbsup: :D
 
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Marshall Freak":84ad7 said:
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I wasn't sure if the back of the headshell was still like the old ones so I didn't know if that was possible with the JVM. :thumbsup:

I've got to get one, driving me crazy. :doh: :doh: :lol: :lol: :lol:
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The trannies are "Z" mounted so they aren't tall enough for the tubes to clear. :)

Yes, with a handle like "Marshall Freak" - you NEED this amp :)
 
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SgtThump":d8914 said:
I just lean the chassis up against my speaker cab with the transformer part touching the cab. Works like a chart when accessing the inside to set the bias.

I already forgot what I set my bias to. Was id 70 or 75? I can't even remember now. May have even been higher.
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I set mine to around 60% idle - I think your set yours higher - maybe 75%-80% idle?
 
Thanks for posting this guys, I wanna check mine too. I'm pretty sure I read it's supposed to be 60 per side stock, but I can't recall were I read that. Damn CRS. :doh:
 
From JVM410.com:
http://jvm410.com/forum/viewtopic.php?p=355&highlight=#355

The biasing a JVM is similar to a DSL/TSL. There are independent bias controls for each side of the push pull and shunt 1 ohm resistors to measure the current.

Again, I cannot stress enough that biasing an amplifier is not a user adjustable feature and that working inside an amplifier can be very dangerous. An incorrectly biased amplifier can lead to valve and/or output transformer failure. Leave it to qualified people.

Procedure is as follows:

With the amp open and upside down, place both master volumes fully counterclockwise (OFF), highly recommended a speaker or resistive load into its relative socket and let the amplifier warm up for few minutes. Switch the standby to ON (playing position). Underneath the power valves, there are two small trimmers and a 3 pin connector (I think they are VR2, VR3 and CON1 but just talking off of my memory). VR2 controls the bias of V1 and V2, and VR3 does the same for V3 and V4.

To measure the current of each side place a multimeter set to millivolts, mV, between the center pin of CON1 and each side. Pin on the left indicates the current flowing through the left pair of valves, pin on the right indicates the same for the other pair.

the conversion is as follows: Voltage in mV=current in mA per push/pull side (if valves are matched current per valve will be half of that). So, if the reading is 60mV that means 30mA per valve (factory setting). I wouldn't go any higher than 45mA per valve (90mV), 30-35mA is a good compromise tone/valve life.

S.
 
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