Need help to understand a part of a schematic

Pat_rocks92

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Hello,

after building my first amp successfully, i wanted more so i got into learning to read schematics. I'm currently a newbie at doing this. I'm building a layout for the micro superlead of the tube roaster and i almost finished. However there are still some things i don't understand at all :

resistor help.png


Here is my issue : the resistors above the treble and middle pots (220k and 22k). Do they come before lug 3 or do they connect lugs 2 and 3 ?

it's unclear as the wave symbol with the arrow represents the pots and the pots are middle (25k) and treble (250k).

could it be that he measured the pots and that they weren't quite the same values and went to be 22k instead of 25k and 220k instead of 250k ?

I currently took a look at pictures pictures of his build and there seems to be resistors there.

Thanks for your help !
 
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Pat_rocks92

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I take those two resistance values to be the potentiometer values. If not, I do not know and am…..Subscribed to learn more.
I believe you are right but that's hyper confusing. In the component list i got 250k treble pot and 25k mid pot. How would someone expect that the designer then measures them and changes the values on the schem x)...
 

KokoTele

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Those are both potentiometers, and the arrow indicates the wiper lug. So both of those pots have a cap connected to the wiper.

I have never figured out how to tell from a schematic which end is which if it's a log taper pot.
 

Pat_rocks92

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The way the Bass pot is drawn means it is a variable resistor as opposed to a potential divider.
So, the 1Meg pot will vary between 1Meg and zero Ohms.
Use the centre pin and the third pin.
The Middle pot is drawn as a potentiometer as is the Mid range pot.
Thank you for noting this too, i was kinda lost... and thought i had to use pin 1 and pin 3...
 

Swirling Snow

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Pot values, like capacitor values, are traditional. Over the years, there's been a tendency to change them to "rounder" values such as "250" instead of "220". Since most pots are 20% accuracy, there's likely not much difference there.

Schematics are drawn to make the lines (wires) short and straight, not to make the pots aligned with reality. Pots really should have the pins numbered on the schematic. JMHO. Which way the actual pot goes.... ah, that's the skill of electromechanical assembly! ;)

In the course of a project, I spend hours staring at pots again and again, figuring out which way they turn when they're facing that way, 'cuz I never remember!
 

Phrygian77

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It's a Marshall Lead circuit tone stack. Look at any Marshall model 1959 or 1987. I can't make out what that bass cap says, but it should be 22nF (.022uF).

Sometimes in amps and schematics you see pots in E12 values, e.g. 220k and 22k. In what you're building, they should be 250k and 25k anyway.
 

loopfinding

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It's a Marshall Lead circuit tone stack. Look at any Marshall model 1959 or 1987. I can't make out what that bass cap says, but it should be 22nF (.022uF).

Sometimes in amps and schematics you see pots in E12 values, e.g. 220k and 22k. In what you're building, they should be 250k and 25k anyway.

yes, exactly this. they're just the values of the pots. not resistors connected to them.
 

Lowerleftcoast

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The Tube-Roaster Micro-Superlead parts list shows 250k and 25k pots. There are no 22k or 220k resistors on the parts list. Imo, we can assume the values written on the schematic are typos.

 

Pat_rocks92

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It's a Marshall Lead circuit tone stack. Look at any Marshall model 1959 or 1987. I can't make out what that bass cap says, but it should be 22nF (.022uF).

Sometimes in amps and schematics you see pots in E12 values, e.g. 220k and 22k. In what you're building, they should be 250k and 25k anyway.
Yeah i didn't understand why when i compared to the super lead schem... he replaced it with a 20nf orange drop. Perhaps that he needed it to improve the tone as same value components don't act the same way with different voltages (that's my guess but not sure).
 

Phrygian77

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Yeah i didn't understand why when i compared to the super lead schem... he replaced it with a 20nf orange drop. Perhaps that he needed it to improve the tone as same value components don't act the same way with different voltages (that's my guess but not sure).

The frequency response is determined by the tone stack component values, the source impedance, and load impedance. Those things don't change regardless of "voltage". There is zero reason to sub a 20nF for a 22nF, or vice versa, other than maybe having to use what is available.
 

Pat_rocks92

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The frequency response is determined by the tone stack component values, the source impedance, and load impedance. Those things don't change regardless of "voltage". There is zero reason to sub a 20nF for a 22nF, or vice versa, other than maybe having to use what is available.
Thanks then i'll take my layout of the original SL68 and change them for the micro ! this build is getting me more knowledge than i thought x)... I might look like an idiot, but at least i'm trying to get into it. I better buy some books for beginners about how tube amps work and how they are designed.
sl68.png


it seems i have many values to change...
 




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