Nice mess in a 78 JMP Superlead...need advice!

[donzoid]

OK I am doing this and want to clarify Martin's instruction for:
"...and D1 goes from the junction of C1 and the existing bias diode to ground (ground it with the existing bias supply caps.)"

1. The existing Bias supply caps are the two 10uf caps that are side-by-side on the PCB.
2. "ground" represents actual "-" end of those caps (the non-ridged end since we are dealing with a negative voltage feed in the end) and point out in that case that there are two separate traces for each of the caps "-" (does that matter)? Or, am I tying this to every other ground (such as the bus wire on the back of the pots for example)
3. Stripe end of the new diode is heading toward this ground point.

I'll set this aside until someone chimes in...don't want to let the smoke out.

[martin manning]

OK I am doing this and want to clarify Martin's instruction for:
"...and D1 goes from the junction of C1 and the existing bias diode to ground (ground it with the existing bias supply caps.)"

1. The existing Bias supply caps are the two 10uf caps that are side-by-side on the PCB.

Yes.

2. "ground" represents actual "-" end of those caps (the non-ridged end since we are dealing with a negative voltage feed in the end) and point out in that case that there are two separate traces for each of the caps "-" (does that matter)? Or, am I tying this to every other ground (such as the bus wire on the back of the pots for example)

Ground is circuit ground which is the "+" end of the bias supply caps, same as I show in the schematic above.

3. Stripe end of the new diode is heading toward this ground point.

Yep.

[martin manning]

By all means test the mod with the power tubes removed, and sweep the bias trimmer through its range to see what range of negative voltage you have at pin 5, and check it at both sockets. If it looks good, -45 to -50V say, set it to the most negative voltage before powering down and installing the tubes.

[donzoid]

OK...thanks. Glad I asked since I'm not used to working with sub-circuits that are "backward" (reverse polarity). That will actually make it easier due to the component location.

Yes once finished, I will pop out the tubes before power up (especially since I don't have a variac) and take measurements.

[donzoid]

Do I need to attach a speaker load when taking a measurement (even with the tubes popped out)? Almost ready... diode is on the back side of the pcb in case you were wondering...

[JazzGuitarGimp]

WIthout the output tubes, you do not need a speaker connected.

[donzoid]

Thanks Lou!

OK, powered on, and no smoke. Voltage now ranges from -52.4 to -59.9 with only a 0.1v difference from side to side.

Note this is using the 3k9 resistor where the 4k2 was recommended...'cause that's the closest one they had.

[donzoid]

OK...installed tubes, connected speaker load, and switched it on. I am not sure at this moment if its just my crappy meter auto-ranging or if it is now quite low, at 3.9 on the highest setting of the trim pot. But hey, no smoke is emanating from the beast and thats the most important thing. I suspect what I am seeing is actually 39 and not 3.9 which would land me in a perfect spot. Maybe even a little more than I want...but easily in adjustable range. Meter is set at the 200mA range. One tube being read.

Is this the actual, or am I now in range?

[Cameron]

Thanks Lou!

OK, powered on, and no smoke. Voltage now ranges from -52.4 to -59.9 with only a 0.1v difference from side to side.

Note this is using the 3k9 resistor where the 4k2 was recommended...'cause that's the closest one they had.

Adjust the 3.9k or the 47k to get it into the range you need

[martin manning]

I think your bias voltage is too low now. You have a 4k3, right? I'd try that in place of the 3k9.

[Cameron]

Wait...... do you have the cap just over the 3.9k resistor ....in parallel ??
...If so ........then you probably have enough voltage with the stock circuit..by adjusting the 3.9k resistor.

[Cameron]

If you have been trying to adjust the bias with this transformer and the stock 27k resistor ...it wont work you need to just adjust the ...now 3.9k and it should get into range with the stock Marshall circuit.

[donzoid]

Yes I have a 4k3 on hand.

Yes the cap is in parallel with the 3k9 resistor (only).
The diode is on the back side, it seemed cleaner to put it there.

[Cameron]

Yes I have a 4k3 on hand.

Yes the cap is in parallel with the 3k9 resistor (only).
The diode is on the back side, it seemed cleaner to put it there.

Its wrong .....you then have enough voltage with the stock circuit. Take out the cap and tell us what voltage you have??

[Cameron]

Yes I have a 4k3 on hand.

Yes the cap is in parallel with the 3k9 resistor (only).
The diode is on the back side, it seemed cleaner to put it there.

You want the cap ..after the resistor.