Official original Marshall JVM appreciation thread

One thing I experienced today: setting the bias of this amp is kinda tricky. It has two test points and trim pots inside, one per pair of tubes. However, these pots interact with each other. If you turn up one side, the other side will come down again a little bit. So it takes a couple of re-adjustments to get both sides set to similar values.

I have to repeat the procedure tomorrow because the values I measured from the internal test points are quite different from the ones I got directly from the tube sockets using a bias probe. Strange…

Oh, and the amp came from the factory biased very, very cold. I measured 21 mA per tube. With a plate voltage of approximately 450, a much higher current (38 mA) should be optimal. :idk
 
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What albums do we know that JVM’s were used?

Off the top of my head, Children of Bodom and Megadeth used them. I’ll try and dig out some examples. Anyone got any others?







 
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TheTrueZoltan! said:
One thing I experienced today: setting the bias of this amp is kinda tricky. It has two test points and trim pots inside, one per pair of tubes. However, these pots interact with each other. If you turn up one side, the other side will come down again a little bit. So it takes a couple of re-adjustments to get both sides set to similar values.
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That's pretty standard with amps that have 4 power valves in my experience. They need some time to settle, much like guitar strings.
 
TheTrueZoltan! said:
One thing I experienced today: setting the bias of this amp is kinda tricky. It has two test points and trim pots inside, one per pair of tubes. However, these pots interact with each other. If you turn up one side, the other side will come down again a little bit. So it takes a couple of re-adjustments to get both sides set to similar values.

I have to repeat the procedure tomorrow because the values I measured from the internal test points are quite different from the ones I got directly from the tube sockets using a bias probe. Strange…

Oh, and the amp came from the factory biased very, very cold. I measured 21 mA per tube. With a plate voltage of approximately 450, a much higher current (38 mA) should be optimal. :idk
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that happens in all amps: the hotter the bias, the more current it draws, the lower the high voltage (HT is) due the internal resistance losses so you need to bias again to compensate. In amplifiers with 2 or 4 bias pots it is trickier to balance everything but the effect is the same
 
MirrorProfiles said:
What albums do we know that JVM’s were used?

Off the top of my head, Children of Bodom and Megadeth used them. I’ll try and dig out some examples. Anyone got any others?







Click to expand...

pretty much all Joe Satriani albums since 2011 or so have either a standard JVM or a modified one before the HJS was launched, then his HJS once it was launched.
Then Saxon, Iron Maiden, John 5, Ritchie Sambora for a while, Nita Strauss, Arch Enemy, Neal Schon, Frank Hannon, Akira Takasaki... I just don't know in which albums
 
The amp came biased very cold. I raised the bias to approximately 35 mA per tube, which improved the sound of OD2 quite a bit. The amp sounds excellent!

Unfortunately... since I made the bias adjustment the power transformer is humming quite audibly. :cautious:
 
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TheTrueZoltan! said:
The amp came biased very cold. I raised the bias to approximately 35 mA per tube, which improved the sound of OD2 quite a bit. The amp sounds excellent!

Unfortunately... since I made the bias adjustment the power transformer is humming quite audibly. :cautious:
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a bit of hum is normal but if hums too much to the point of being clearly audible at some distance from the amp then something may be going on
 
santiall said:
a bit of hum is normal but if hums too much to the point of being clearly audible at some distance from the amp then something may be going on
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Yep, it‘s strange. Lowering the bias setting to a significantly lower value (less than 25 mA per tube) makes the hum go away.
 
santiall said:
a bit of hum is normal but if hums too much to the point of being clearly audible at some distance from the amp then something may be going on
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Hum is concerning when:
“she’s out back counting stars”…
That powerhouse drummer rules!!

For me “hum” is taxed out power tubes or..
-too many chefs in not a big enough kitchen.
I like all them chefs!!,
-just give them ‘elbow room’ to work their magic.
The transformers.

Honestly Santy, you got shafted man.
That footswitch you created is super killer man.
For a gigging guitarist whom switches guitars with different pickup combinations, -it’s insanely useful
how you can program all that & pull it up.
I get it, it ain’t about us being gear herbs
on music equipment chat forums that moves the units being sold..,
-it’s about units sold & being cost sensible.
Individuals can pull up so much info so quickly
nowadays with gear..
human word of mouth is weak..
-but posting up on Google can be forever.
And in large scale bid’ness,
-f’n heads fly real quick!!!

Numbers don’t have feelings..,
-who said profit was a bad word?
Gentlemen…Welcome to the fold.
Your hands were tied Santiago.
(like Axl said in “Civil War”).
I think you did the best that any designer out there
at the time could have done with the resources on your bench.💯
 
It was so bad that they even let me use the transformers of the JCM800 2203 instead of the smaller ones in the DSL and they are used in the AFD and YJM too. In fact, the power transformers are larger than the ones in the plexi or 2203 to cater for a bit of extra power needed in the circuit.
But you are right, I'm out of the tyranny, my hands are free, no more shafting! (well, by them at least 😄)
 
I’m a little heartbroken. Dude was selling a 410HJS on Reverb this weekend. Mint. Like still had the original box, all the promo materials, footswitch in the original plastic. The guy had a line of vintage amps in the background in immaculate shape, (and a seller record of great amps) so everything checked out. He listed for $2,700 + shipping and I offered $2,100 + shipping. He countered with $2,400, and rather than counter back I figured I’d give it a couple days and see if could squeeze a bill or two out of him. It sold the next day. :cry:

I’m kicking myself for going full tard and not buying the amp.

I never wanted it anyway!
dule hill crying GIF
 
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Whizzinby said:
I’m a little heartbroken. Dude was selling a 410HJS on Reverb this weekend. Mint. Like still had the original box, all the promo materials, footswitch in the original plastic. The guy had a line of vintage amps in the background in immaculate shape, (and a seller record of great amps) so everything checked out. He listed for $2,700 + shipping and I offered $2,100 + shipping. He countered with $2,400, and rather than counter back I figured I’d give it a couple days and see if could squeeze a bill or two out of him. It sold the next day. :cry:

I’m kicking myself for going full tard and not buying the amp.

I never wanted it anyway!
dule hill crying GIF
Click to expand...
7t6lqp.gif
 
So the JVM410H is good or what?

I'm looking at the schematic and it's basically a Plexi and a JCM800 with an extra gain stage and a few tweaks.
A couple of minutes of work and I can make Crunch Red a stock 2203, then tweak OD1 and OD2 around that.

@texhex How do you like your so far?
Anyone else appreciates this amp?
 
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