Why is this 4.7k resistor in the presence circuit?

[12AX7]

I have a build thats got the same NFB circuit as a 2204 except the pot value. Mine is 5k which i used because 25 did nothing for 20k of it's travel. In the JCM theres that 4.7k resistor parallel to the presence pot.  I looked at a superlead circuit and noticed that the 4.7k isn't there. I decided to lift the 4.7k and see how it sounded with the SL's circuit, aside from the pot difference that is. I figure they put that resistor there to lower the pot value. Maybe they had a lot of 22k pots laying around back then and just used them instead of buying a stock of lower value pots. That brings the question why they wanted a smaller pot value than the SL in the JCM. But thats not really my concern.

What i AM wondering is why mine sounds so much better with the 4.7 lifted. It not only changed the pot's range, but i can find far more usable tones with the presence. Is this because i used a 5k with that 4.7k resistor that was only in the circuit of the JCM to lower the pot value, and that since mine was already low to begin with that resistor lowered it too much?It still seems odd tho because if thats the case it should have rendered the pot useless or at least given it much less range. But instead the range from bright to dark is about the same, but the treble that you hear as you roll the presence clockwise sounds a LOT better. Before i left it all the way down even when i wanted more brightness because the treble it brought was nasty. But now it's much much sweeter brightness and sounds great and i've been using the presence a lot now. The whole amp's top end is just a lot better and more open and usable.

[sluckey]

In the JCM theres that 4.7k resistor parallel to the presence pot.

Show me a schematic please. I've never seen any Marshall with a 4.7K parallel to the presence pot.

The later Marshalls used a .1µF cap in series with the 22K presence pot and a 4.7K resistor paralleled the series cap and pot. This improved presence circuit kept dc voltage off the pot thus preventing the scratchy sound that the earlier presence circuit had when turning the pot. The 4.7K in this improved circuit provides a DC ground path for the PI tube cathodes. Without that resistor, the PI can't work and the sound will be extremely starved, that is, if you even get any sound.

The attached is the typical later presence circuit. Please post your schematic.

[12AX7]

Well, you are correct. I just spaced on the fact the resistor is parallel to the pot AND CAP. But as to the notion the resistor must be there to give the cathode a ground reference, i guess i'd agree but the thing is it not only works great and sounds better than ever, but the superlead appears to be the same. With my serious lack of knowledge and propensity to space easily maybe i'm missing something and hopefully if i am you can point it out. But isn't this also the case with the superlead below? I looked over the wiring and it's dead simple so i'm positive i didn't space when i knew what to look for. and theres no ground part on the cathode with that resistor lifted. I would never second guess you because I've read your posts for years and you are extremely competent. But i can't argue with what i'm seeing and hearing. Anyways, look at this....

[sluckey]

But as to the notion the resistor must be there to give the cathode a ground reference, i guess i'd agree but the thing is it not only works great and sounds better than ever, but the superlead appears to be the same.

If your amp is wired IAW the schematic then that 4.7K is absolutely necessary for the PI to function properly. When you lift that 4.7K in a properly wired amp, the PI cathode is forced to find ground through the 100K feedback resistor and the OT secondary. That 100K will only allow a tiny fraction of the current to flow through the PI tube. I'm surprised it makes any sound without that 4.7K resistor.

The schematic you posted is the old style presence circuit. Notice that the cathode has a dc path THRU the 5K pot. That means the pot has dc current flowing through it and will sound scratchy. That's the reason Marshall changed to the "improved" presence circuit with the later amps.

Tell me more about your amp. Is it actually a Marshall? Or is it an exact clone of some Marshall? If so, which one? Or is this a clone that's close but not exact? Who built it? Do you have an as built schematic and/or layout?

[12AX7]

No, it's a home brew i built years ago as a 18 watt that evolved thru endless incarnations to what it is now. I see what u mean about the ground thru the pot. Yet another thing i missed. And to think i haven't had a single beer ! Well, i plugged n again after several hours with fresh ears and while the presence control still sounds better like this, i think i was in the midst of ear fatigue because now it sounds better to me with the resistor. I'm going to try wiring it like the SL. Scratchy pot doesn't bother me as i tend to set it and forget it. Thanks for your observations Sluckey. Don't know where i left my brain today, as even with very little knowledge i usually don't miss such simple things.

[sluckey]

I've got two plexi style amps. Both have the old scratchy presence pot. I don't mind either. I can't remember when I last turned the presence knob! 

[12AX7]

Well heres an update. what i said about ear fatigue was not that but something that changed in the amp after i said i liked the tone with that resistor lifted. What happened is after i did that i determined a change i had made before was unnecassary and in the process of removing it i unknowingly lifted one leg of the 500pf cap on the treble peaker in front of the gain pot. So tone was muddy and dull. Found that and once i resoldered it i tried lifting the 4.7k again and sure enough like at first it sounded good. However, I DO hear it differently in that the change while good is a bit much. So what i'm thinking of trying is putting a  pot in place of the resistor. My NFB resistor is 56k so i need to determine what size pot to use but i think 50 or 100k will work. This way i can dial in just the right amount of what it sounds like with the resistor lifted. And at stage volume i may find it best one way or the other. but just another option to dial it in more precisely.

[Platefire]

Regarding the schematic that sluckey posted, what does the slanted arrow through the 22K pot mean?
When I see that symbol, it always confuses me? Platefire

[sluckey]

Regarding the schematic that sluckey posted, what does the slanted arrow through the 22K pot mean?
When I see that symbol, it always confuses me? Platefire

That indicates the pot is simply being used as a two terminal variable resistor. IE, connect to one end lug and connect to the middle (wiper) lug. One lug will be not used, however, my suggestion is to connect the unused lug to the center lug.

Open the attached pdf. Look at the BASS pot on the schematic. Then look at the BASS pot on the layout to see how it's really wired. My suggestion is to connect the right lug to the center lug.

     http://el34world.com/charts/Schematics/files/fender/Fender_BASSMAN_5F6A.pdf

[PRR]

> what does the slanted arrow through the 22K pot mean?

What Sluckey said.

It goes back to capacitors. You know the symbol for a cap. Actually a fixed-value cap. In radios we like to tune different stations. We have to vary either a cap or a coil to change frequency. The conventional symbol was to draw the cap or coil then add a "pointer" to it, maybe to suggest a tuning pointer.

This did carry-over to variable resistors. Especially in filament trimmers which were always 2-terminal variable resistors.

But 3-terminal pots were more generally useful. They needed their own 3-leg symbol. Today when we need a 2-terminal variable resistor, we tend to find a 3-terminal pot symbol and connect as needed. However some draftsmen will use the 2-terminal resistor and a "tuning arrow".

[Platefire]

As usual PRR, thanks for the explanation. So as I understand what you and sluckey said, the slanted arrow through a resistor means a two term variable resistor required. So just like a normal fixed resistor, you have one lead as the input and the other as the output(two terms), the slanted arrow makes it variable two term--so if you use a normal 3 term pot, you can forget the ground.

So I assume by the same token, on our normal pot symbols with a resistor symbol with a perpendicular arrow in the middle means a 3 term variable pot? Platefire   

[sluckey]

I assume so.

[Platefire]

Yes and I assume that your assumption is so!

Seriously, ever time I would see the slanted arrow through a resistor---I knew it was a pot but just drew a blank. So I always referred to a layout drawing or an actual picture to figure out the correct wiring.
So after all these years of seeing that, I finally have an understanding of the meaning. They say confession is good for your soul Platefire

[sluckey]

You're just feeling guilty because of all that excessive eating yesterday!