Marshall JVM410H Bias

[ca7922303]

Got this with the tolex falling off and no corners or feet on it. Done replacing all that. Would like to check the bias on it. Have a eurotubes bias adapter. Any advice is appreciated.

[The Dude]

No schematic. No tube configuration. The best we can do is guess. I'll submit this:

https://robrobinette.com/Tube_Bias_Calculator.htm

You should be able to find whatever you need, in regards to bias, on that page.

[g1]

From what I've read, idle current should be 30mA per power tube.(plate voltage runs around 480V)

[Enzo]

There is a "bias adaptor" already in it. Look on the schematic, each side of the power tubes has a 1 ohm resistor in the cathode circuit, and convenient test points for each.

[ca7922303]

There is a "bias adaptor" already in it. Look on the schematic, each side of the power tubes has a 1 ohm resistor in the cathode circuit, and convenient test points for each.

I'm seeing a vr2 & vr3 in between v2 &v3.

[ca7922303]

I'm seeing a vr2 & vr3 in between v2 &v3.

V1-V4(492vdc)

[ca7922303]

V1-V4(492vdc)

Getting (17mAdc) on first el34

[ca7922303]

Getting (17mAdc) on first el34

V1-V4(17-18mA) 495vdc

[g1]

VR2 will adjust V1 & V2.
VR3 will adjust V3 & V4.

[ca7922303]

VR2 will adjust V1 & V2.
VR3 will adjust V3 & V4.

With all power tubes at 17-18 mA, should I adjust up to 30mA when voltage is already @ 495vdc? Thanks.

[Enzo]

I hope you realize that the hotter you run the tubes the LOWER the high voltage supply will sag.

Did you know that stock Peavey 5150 heads - huge gain monster heads - typically idle at like 15ma or so?

You are talking about adjusting the amp, but how does it sound now? Does it sound good? If so, leave it as it is. Yes, 18ma is a cool setting, but please keep this one thing in mind as you work on this stuff:

IT IS JUST A GUITAR AMP.

It isn't precision NASA space lab gear.

18ma @495v is less than 9 watts idle dissipation. No stress from that. You want to run them at 30ma? Sure, why not. That is not quite 15 watts, but I suspect at 30ma your 495 will be a few volts lower, so call it 14 watts. And that is OK too.

[g1]

Agree with Enzo above. If there is no complaint and it sounds good, you don't need to touch the bias.
If you just put in new power tubes, and you want to set to factory spec., then adjust so they are about 30mA each. That will bring the plate voltage down a little bit.
If they sound fine as they are, the lower setting where it is now (18mA), will give longer tube life.

[The Dude]

Maybe already understood, but it should be pointed out that each pair of tubes shares a cathode resistor, so the voltage drop(s) at the test points is for 2 tubes.

[ca7922303]

Maybe already understood, but it should be pointed out that each pair of tubes shares a cathode resistor, so the voltage drop(s) at the test points is for 2 tubes.

Can you explain what that means with all power tubes @ 18mA/495vdc and 2-bias pots. Thanks.

[The Dude]

If you're using a bias probe on separate tubes, it doesn't matter. If you are using the test points to measure voltage drop on the shared cathode resistor, you are measuring two tubes at the same time, so divide by 2 to figure approximate bias for each tube.