Marshall JVM410H Tube Bias
I started some tubes rolls in the preamp section some time ago.
And settled in with a mix of JJ's, Prefered Series and stock Marshall white 12AX7's.
It had a great sound after I found the right mixture of where to place what in the V1, V2, V3, V4 and V5 spots.
But I never touched the power tubes they were always kind of a mystery in the bias area.
But after playing my amp for several years the tubes were well broke in and developed a great character.
This being said there were a few issues that made me think I needed to at least put a meter on the bias.



I began on researching technical aspects, the specs, what tools are needed and the process.
It didn't seem all that hard although a bit dangerous with the high voltages inside the amplifier.
The JVM 410 was designed by Santiago Alvarez and the power section had a spec of 30ma per tube.
This is perhaps a little cold in the big scheme of things but surely the designer knew what this amp should be.
30ma per tube works out tp 60ma per side there are 4 tubes with two to a side.
The amp came with JJ EL34L power tubes, they were changed from the stock Marshall tubes by the previous owner.
For the record I took the amplifier guts out of the unit and placed it upside down onto the head casing.
This gave a stable work space to do the biasing from. There were four phillips screws on the bottom of the head unit.
Once these were taken out I turned the amp up on it's end, grabbed the tranformer and pulled the chassis out.
I placed it on the head casing and located the bias pins and pots.
There are three pins and two trim pots located on the circuit board.
The center pin is ground and the outer pins are the hot test points.
I made connectors for the pins on the end of an old computer power cord.
The opposite end had a stable place to stick the test leads while I adjusted the pots.
I used a small cheap mutli meter set to 200mv and began work.
I set a speaker load on the amp and turned the master volumes to 0.
Turned on the amp and let sit on standby for 10-15 minutes and let the tubes warm.
I turned on my multi meter and plugged in my leads and switched the amp off standby.
As I swapped side of the test leads I saw the amp was out of balance one side 61ma and the other 65ma.
So I began to set the trim pots and discovered that a small move makes a huge difference in the meter.
I had to recover from too much too soon as it were.
Eventually got both sides in the lower 60's and began on working out a balance.
I eventully got the amp at 61.3 per side which is 30.8 per tube.
It is very close to the spec set by Santiago (who by the way as of this post has left Marshall)
So was this going to be an Earth shattering difference in tone of my amp?
Well in a word yes, the character of my amp had a considerable change.
I had to reset my parameters in relation to channel volume and master volume.
It took a bit to get the volumes at the normal playing level and I did a set.
This is what I noted: the fizzy sounds were gone the bottom end was very tight and articulate.
There was no boominess to the bottom as before the attack and response were much smoother.
The harsh shrillness of the top end had been tamed and rounded a bit.
It still had brightness but brightness as in tone not ice pick.
Then the mid range was full of body and notes rang saturated with tone.
Good tone, the kind of tone that while you were playing you would almost stop to enjoy the sounds you were making.
Power chords were thick with that melt your face off energy.
Lead lines were articlulate and notes were saturated with thick tone.
Even though I biased this amp less hot than someone had tried before I think it was the balance that dialed the sweet stuff in.
It's a little hard to pinpoint exactly the difference outside of the tone but the response has changed also.
I think it has to do with the attack and compression but the response has changed somewhat.
But I think it has changed for the better regardless it just takes a little getting used to.
Once I learn the subtle idiosyncrasies of the amps new paramters it will become the norm.
But I have become comfortable with the bias process so changing tubes in the power section won't be that mystery it was.

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