JCM800 DC voltage on input jack!

[Randy486]

Hi,

I've done #36 mod on my 2013 JCM800 2203X and it sounds really good, but I have a problem (I think I had this problem before modding the amp too, I'm not sure), when I connect my pedals in front of the amp they make the amp sound really thin, they make my humbuckers sound like single coils.
I used my MXR EQ, BOSS RC-3 and TC Electronic HoF 2 and they all make the amp sound thin and I don't know if this is normal or not.

Also when I try to measure the 1Meg ohm input grid leak resistor through my guitar cable it shows 1Meg ohm, but when I turn the amp on (just the power switch, the amp is on standby) it drops to 27K ohm and I can measure 0.5v DC between sleeve and tip of guitar cable but when I pull V1 tube the voltage goes away, I also replaced V1 with another tube but nothing changed and it measured 0.5v again. Is this normal on JCM800s? because when I measure this resistor on my Laney CUB12R (which is basically a clone of JCM800) it measures 1Meg ohm even when the amp is on.
I should mention V1a is hot shielded, but since the amp is on standby (therefore, no plate voltage) I don't think this should cause any problems.

Thanks.

[92Volts]

That is not supposed to happen.

Lower resistance reading is because the multimeter cannot properly read resistance with an external voltage present. That doesn't mean the value of that resistor is actually changing.

It's strange that the tube's presence changes this considering the only power there should be the heaters. Maybe it is a short in the socket or PCB, which connects heaters to (something wrong), but only when the tube is inserted because it bends/pressures something.

Can you confirm high voltage isn't present at this time, or maybe the standby is faulty?

If the tube were actually passing cathode current there would be a much wider range of possible causes, like a wrong connection or bad ground for the entire section of the circuit.

[Randy486]

That is not supposed to happen.

Lower resistance reading is because the multimeter cannot properly read resistance with an external voltage present. That doesn't mean the value of that resistor is actually changing.

It's strange that the tube's presence changes this considering the only power there should be the heaters. Maybe it is a short in the socket or PCB, which connects heaters to (something wrong), but only when the tube is inserted because it bends/pressures something.

Can you confirm high voltage isn't present at this time, or maybe the standby is faulty?

If the tube were actually passing cathode current there would be a much wider range of possible causes, like a wrong connection or bad ground for the entire section of the circuit.

I will check that, I should also mention that it has a hum balance pot. Could the hum balance circuit cause the problem?
Here's my amp schematic: http://www.thetubestore.com/lib/thetubestore/schematics/Marshall/Marshall-2203-Reissue-Schematic.pdf

[Randy486]

I measured filter caps voltage when the amp was on standby and it was only 200-300mv. but I found something else, when it's not on standby, the voltage on input jack drops to 100mv and resistor measures 650k ohm.

[pdf64]

I measured filter caps voltage when the amp was on standby and it was only 200-300mv. but I found something else, when it's not on standby, the voltage on input jack drops to 100mv and resistor measures 650k ohm.

Standby, by definition, is not a valid operating condition, so any measurements made in that mode are nonsense.
In normal operating mode, the test meter voltage may be biasing the tube into grid current mode, so again, trying to measure a resistance that is in parallel with the tube grid is pointless.

Check it with the amp off and de-energised, or take the tube out.

[Randy486]

I measured filter caps voltage when the amp was on standby and it was only 200-300mv. but I found something else, when it's not on standby, the voltage on input jack drops to 100mv and resistor measures 650k ohm.

Standby, by definition, is not a valid operating condition, so any measurements made in that mode are nonsense.
In normal operating mode, the test meter voltage may be biasing the tube into grid current mode, so again, trying to measure a resistance that is in parallel with the tube grid is pointless.

Check it with the amp off and de-energised, or take the tube out.

I know any voltage presence will interfere with resistance measurements, but why is DC voltage present on the input jack (therefore, on V1a grid)?

[92Volts]

Can you provide a schematic or at least description of what changed compared to the original schematic you provided, when you performed this "#36 mod"?

[Lauri]

I know any voltage presence will interfere with resistance measurements, but why is DC voltage present on the input jack (therefore, on V1a grid)?

When there's no plate voltage and the cathode is heated up, some electrons will hit the control grid.

[Randy486]

I know any voltage presence will interfere with resistance measurements, but why is DC voltage present on the input jack (therefore, on V1a grid)?

When there's no plate voltage and the cathode is heated up, some electrons will hit the control grid.

With the amp out of standby position and a guitar connected to it, I measured V1a grid and it was almost zero volts, this shouldn't be a problem, right?

[Randy486]

I've done #36 mod on my 2013 JCM800 2203X and it sounds really good, but I have a problem (I think I had this problem before modding the amp too, I'm not sure), when I connect my pedals in front of the amp they make the amp sound really thin, they make my humbuckers sound like single coils.
I used my MXR EQ, BOSS RC-3 and TC Electronic HoF 2 and they all make the amp sound thin and I don't know if this is normal or not.

Could the 2200pf treble peaker cap cause this issue? I mean even Boss RC-3 makes my les paul sounds like a strat,

[92Volts]

With the amp out of standby position and a guitar connected to it, I measured V1a grid and it was almost zero volts, this shouldn't be a problem, right?

It should be fine in that case.

Could the 2200pf treble peaker cap cause this issue? I mean even Boss RC-3 makes my les paul sounds like a strat,

No, it couldn't cause this issue, though it sounds like you might not have an issue. Lauri shared information in this thread (and a similar thread) I never knew before-- some voltage at the input may be expected in standby mode.