Marshall JTM30 bias mod problem & IC3 (LM348) problem

[fastfolkert]

Will you explain why?

The wiper is "floating" therefore the resistance does not change. Connect the wiper to one "end" of the pot.

[J M Fahey]

This one is wired the same way I recall doing it in the past.

That means nothing to us, just redraw the actual circuit you wired, looking at it.
And a closeup picture showing the bias pot/trimmer would be fine.

[Jazz P Bass]

Goofy way to draw a potentiometer.

No wonder the confusion.

[52 Bill]

Goofy way to draw a potentiometer.

No wonder the confusion.

Almost like the old Fender schematics where the wiper was drawn diagonally across the resistor.

Check the values of the two "56K" they look like 5K6 to me.

[Tone Meister]

That means nothing to us, just redraw the actual circuit you wired, looking at it.
And a closeup picture showing the bias pot/trimmer would be fine.

See post #11 Juan. The drawing of this circuit is identical to the way I have wired it in the past, with the exception of the pot.

Goofy way to draw a potentiometer.

No wonder the confusion.

This is a 3 terminal potentiometer and I thought sure I had represented the correct schematic symbol. You do know I'm a hack, don't you? Symbols fixed in all drawings and schematics





The pot is a three terminal Bourns 3299Y-1-503LF (datasheet attached) and one end of the pot was left unused. I have since connected the wiper to the unused end and and still unable to adjust bias with the pot. Here is how I did it.

[Tone Meister]

Almost like the old Fender schematics where the wiper was drawn diagonally across the resistor.

Check the values of the two "56K" they look like 5K6 to me.

Alright den. Point taken on the erroneous schematic symbol.

Bill, the resistors measure 56K. Here's a closeup.

[Enzo]

Hey, resistor with an arrow drawn across has always meant variable resistor to me.

[J M Fahey]

Hey, resistor with an arrow drawn across has always meant variable resistor to me.

Same here, but on this schematic the way the arrow was drawn means "wiper contact".

As of the pot wiring, as the British say : "the proof of the pudding lies in the eating" : measure resistance across those white wires and turn the setting screw end to end : the resistance either varies from 0 to 50k or not. (I suspect the latter) .
If not, maybe the pot was damaged with rough handling, replace it with a good one.

And next time, avoid using this screwdriver to adjust it:

[Tone Meister]

Same here, but on this schematic the way the arrow was drawn means "wiper contact".

As of the pot wiring, as the British say : "the proof of the pudding lies in the eating" : measure resistance across those white wires and turn the setting screw end to end : the resistance either varies from 0 to 50k or not. (I suspect the latter). "Don't suspect without investigating." - JMF http://music-electronics-forum.com/t40431/

If not, maybe the pot was damaged with rough handling, replace it with a good one.

Schematic symbols fixed on all documents in this thread. Because this pot has three terminals, I mistakenly thought I was using the correct symbol, based on this information.

Schematic Symbol

With the pot out of the circuit, the resistance DOES vary from 0 to 50K when I measure resistance across those white wires and turn the setting screw end to end. I could replace the pot since it could be damaged, but the only thing I knew to check was the resistance the way you described and it checks out in isolation.

Side Note: There is not one of you I can't strike out with a 90+ mph fastball

[g1]

Side Note: There is not one of you I can't strike out with a 90+ mph fastball

I remember a kid in school who used to say he could kick my ass on skates .

Now that it's back wired up, measure resistance across R132 with pot set to either end.

[Tone Meister]

I remember a kid in school who used to say he could kick my ass on skates .
Now that it's back wired up, measure resistance across R132 with pot set to either end.

I thought everyone here knew I'm a novice when it comes to circuits. A simple notation mistake turned into cauliflower ear.

R132 = 61.5K

[TimmyP1955]

As drawn, the pot just presents a variable load to the bias supply. The pickoff point for the junction of R105 and R106 needs to be inside the resistor/pot string.

[g1]

R132 = 61.5K

Measure with pot set to min. and again with pot set to max.

[J M Fahey]

As drawn, the pot just presents a variable load to the bias supply. The pickoff point for the junction of R105 and R106 needs to be inside the resistor/pot string.

Weird as it seems, for bias supplies the tried and true method is using the pot as a variable load.
Probably because it's fail safe: if pote opens or wiper contact is dirty, bias becomes maximum voltage and tube cools (in every meaning of the word ) while the conventional way it leaves tubes unbiased , so red hot or worse.

That said, *some* amps wire bias the conventional way as you suggest.
But then they often add a fixed resistor in parallel.

[Tone Meister]

As drawn, the pot just presents a variable load to the bias supply. The pickoff point for the junction of R105 and R106 needs to be inside the resistor/pot string.

I've tried to go back and find the exact pot I used before, but I am sure that when I did this mod myself that all I did was remove those 3 zeners and replace them with the 56K>50Kp>50K resistor string using the holes left by the zener diodes, nothing more. I need a solution. Is this Bourns pot the wrong part for the job at hand, or is it just wired into the circuit incorrectly in your estimation?

Measure with pot set to min. and again with pot set to max.

Will do this afternoon when I can get back to that amp.

Weird as it seems, for bias supplies the tried and true method is using the pot as a variable load.
Probably because it's fail safe: if pote opens or wiper contact is dirty, bias becomes maximum voltage and tube cools (in every meaning of the word ) while the conventional way it leaves tubes unbiased , so red hot or worse.

That said, *some* amps wire bias the conventional way as you suggest.
But then they often add a fixed resistor in parallel.

Same question as I asked Timmy: I need a solution. Is this Bourns pot the wrong part for the job at hand, or is it just wired into the circuit incorrectly in your estimation?