JVM410H Issue

[gezza]

Hey Guys,

So I have a JVM410H that was loaned to me by my band member.
I took it home and tried it out, WTF??
I have owned 2 JVM410H and 1 JVM410HJS before.
It just sound weak and dull and lacked gain.
So I checked the tubes.
Amp manufactured date - M2007 - 34 -1150.
EL34's are also WK32/07.
So I am guessing they are 14 years old.

So I put my Bias Probe on it.
Plate voltage 480.
EL34 #1 - 35mv.
EL34 # 2 0mv.
EL34 # 3 0mv.
EL34 # 4 0mv.
I also swapped the Bias Probes just to be sure.
Same results.

So would putting in a new set of EL34's fix the problem or is there something else going on?
I hoping others have been through this before.
I also plan on replacing all the preamp tubes as they are also 14 years old.

Looking forward to any help.

Cheers.

 

[MonstersOfTheMidway]

Yeah, if those power tubes are from 2007 (YIKES!!!) then it's very likely the tubes are shot. I can't say for certain because I don't know the particular history of the amp (how often it was used, how it was used, hours in use, etc), but power tubes that old probably dead. And yes, if those preamp tubes are also 14 years old, then consider changing those out, too. Again, the particular history of this amp would need to be considered, but to be safe I'd just change them all out.

I'm the original owner of stock 2007 410H. My original preamp tubes malfunctioned almost a year after I bought it, and the power amp tubes malfunctioned a few years later. Replacing the power tubes really brought back a lot of life to the amp and biasing was easy. I suspect once you put new tubes in the amp and bias, it will be back to its old self.

Good luck the rig.

 

[gezza]

Hey Thanks,
I just had a quick look inside. Looks very clean and original.
I can't see any burntout resistors or capacitors.
I also checked my multimeter on the 3 pin connector, just to be sure.
0Mv on one side and 35Mv on the other.
Can an amp run safely on 1 power tube?
I am hoping to replace all the tubes this week.

Cheers.

 

[gezza]

Just had another look.
I don't have the original schematic for the JVM410H.
But FS3 and FS4 don't have fuses but now have wire in it's place.
Does that sound right?

 

[South Park]

Don’t run the amp with one power tube . Pull all power tubes and check for bias voltage . Trace it back from the socket to the source to find the break . The tubes might be good

 

[gezza]

Hello Southpark,

So remove all the power tubes and I'm not sure how to check the bias voltage.
They are 14 years old.

Cheers.

 

[gezza]

I should have mentioned all power tubes are glowing

 

[gezza]

see image attached.

 

[South Park]

The heater circuit is working . But if the bias has no voltage no power from the plates goes to ground . It could be several things bad tube socket open resisters bad tubes . Take it to a tech and have it looked at this stuff can be dangerous

 

[gezza]

What if I remove the working valve and place in the opposite socket V1 to V4.
if that then shows a reading, then all 3 EL34's are dead.
If that doesn't change and I still get zero, then it's something else.
let me know,

Cheers.

 

[Spanngitter]

But FS3 and FS4 don't have fuses but now have wire in it's place.
Does that sound right?

Yes, that's right as Marshall issued a Service Bulletin to have these fuses removed as they tend to burn the PCB due to contact resistance building up over time which leads to heat and final PCB charring rendering the whole amp being a quite massive door stopper.
For the Power Tube Issue I need to state that as per your postings I do not recommend to work on this on your own, bring it to a Tech who can checkout what's going on.
SInce this is a 2007 it normally should have still Svets in it, they are robust as hell and normally only fail when awfully mishandled.

 

[Ken Underwood]

Just had another look.
I don't have the original schematic for the JVM410H.
But FS3 and FS4 don't have fuses but now have wire in it's place.
Does that sound right?

 

[gezza]

I also forgot to mention.
I placed the bias probes on all 4 sockets, as I have the one x 2.
So I tried every EL34 and only one give a current the #1 EL34.
So I swapped #1 with # 4 - EL34.
and I now have the 38Mv on the opposite side.Confirming it's the tubes.
Which I think proves that 3 x EL34's are dead, but still light up.
I'm gonna to put in some spare EL34's and see what happens.
If it is a series issue I'll take it to a tech, but if they are start giving the correct reading.
I'll bias it and rock on.

 

[Spanngitter]

I would not put in a new set of tubes before you have verified that there is sufficient negative Bias Voltage on Pin 5 ( around ~-42Vdc) of any of the Power Tubes.
If there is none then there is an issue on the Amp and a new set would have core meltdown again...

 

[gezza]

OK,
So I was correct, only one EL34 was working.
Put in some near new EL34's and biased it to 70mv per side and what do you know.
It has roared back to life.
Just need to replace the preamp tubes as they are also 14 years old.
So a good result.

Cheers

 

[Malima]

Check if R26 and R27 are Ok.
I had one blown (still looked ok but it was in open circuit). They are 1R, 1W but if one fails the bias goes completely off chart.

 

[David Rivers]

Hey Thanks,
I just had a quick look inside. Looks very clean and original.
I can't see any burntout resistors or capacitors.
I also checked my multimeter on the 3 pin connector, just to be sure.
0Mv on one side and 35Mv on the other.
Can an amp run safely on 1 power tube?
I am hoping to replace all the tubes this week.

Cheers.

no...never on 1 power tube