Mpjotone JTM45 kit grounding scheme mod - please review

[ThreeChordWonder]

Thanks for that.

I was aware that it was DDC causing the scratching, and was wondering if the cap would block the DC if put before the pot.

The WD is shown above, repeated below. The cap is a 0.1 uF, attached between the pot middle lug and ground. The input goes to one outer lug and the third is grounded. I suppose I could put the cap between the middle lug and the grounded lug, then ground that lug in to the ground bus.

Other points:

1. I'm getting low voltages across the board, maybe all 10% down on what's shown on the wiring diagram. I don't think that'll be a problem, generally, but I'm wondering if the voltages quoted are optimistic or require a higher power PT than the one provided (Mojotone 7020300). Is there anything I should check?

2. Another problem I'm having is the the power tube, EL34, bias currents are running away, up to 100mA, maybe more, before the master fuse blows.

I suspect a dodgy tune socket, and I'm going to go ahead and replace both, but is there anything else I should check?

Instead of checking tubes in, I can check the tubes out (and later tubes in) voltages at pins #3, right? What range should I be seeing?

3. One final point. The resistors to the EL34 pins #4 (screw grids) are shown as 470 ohm 1W ceramics or cement resistors (bigvgray ones anyway). They're both fed in parallel off a 1 kohm 5W ceramic / cement one on the turret board. I'm going to up them to 2 or even 5-watts anyway. The question is that I've seen some diagrams with 1 kohms instead of the 470s. Is there a reason and / or advantage in this?

[Roe]

1. I'm getting low voltages across the board, maybe all 10% down on what's shown on the wiring diagram. ...

2. Another problem I'm having is the the power tube, EL34, bias currents are running away, up to 100mA, maybe more, before the master fuse blows.

I suspect a dodgy tune socket, and I'm going to go ahead and replace both, but is there anything else I should check?

Instead of checking tubes in, I can check the tubes out (and later tubes in) voltages at pins #3, right? What range should I be seeing?

3. One final point. The resistors to the EL34 pins #4 (screw grids) are shown as 470 ohm 1W ceramics or cement resistors (bigvgray ones anyway). They're both fed in parallel off a 1 kohm 5W ceramic / cement one on the turret board. I'm going to up them to 2 or even 5-watts anyway. The question is that I've seen some diagrams with 1 kohms instead of the 470s.

2 pulls the voltages (in point 1) down. you need to increase the negative bias voltage at pin 5
3 1k 5w is the standard with el34s. it is needed, unless you run very low voltages. but the one 1k flying resistor in series with the 470R resistors should work if they have sufficient current capability. A 10w 1k in series with 3w 470R resistors would be safer with el34s and tube that draw alot of screen current

[ThreeChordWonder]

Thank you sir!

[ThreeChordWonder]

One question.

When you say "increase the negative bias at pin 5", do you mean make it more negative or more positve?

[Roe]

One question.

When you say "increase the negative bias at pin 5", do you mean make it more negative or more positve?

I mean more negative: adjust the bias so that the tube draws less current and does not blow fuses or damage anything.

Adjust to max negative voltage. if that does not do, increase the resistor to ground connected to the bias pot (e.g. increase a 47k or 56k to a 68k)

[ThreeChordWonder]

Last question (fingers and toes crossed).

The bias pot.

I'm using a 0-50k pot mounted in a DIY bracket in favor of the tiny, mechanically feeble, one in the kit that would go on the turret board. More robust and keeps fingers and screwdrivers at a distance.

The pictorial wiring diagram shows it wired middle plus one lug, as a simple variable resistor. The schematic wiring diagram shows it wired as a potential divider. See cellphone screen captures below.

Which is correct?

[martin manning]

The pictorial wiring diagram shows it wired middle plus one lug, as a simple variable resistor. The schematic wiring diagram shows it wired as a potential divider...
Which is correct?

Both are wired as variable resistors. Electrically they are the same, unless wiper contact is lost. In that case, the schematic version is much better since the negative bias voltage will still be present.

[angelodp]

So simply add jumper