1M ~ 250K bass pots… I don't know what MORE Ω accomplishes

[deci belle]

I am building a 5F8 and am curious why those bass pots are 1M. btw it seems that other parts of the 5F8 circuit are bass-frequency biased (especially the first cathode-bypass caps)

The 5F6 circuit I made a while back only has a 250K bass pot (It is supposed to have a 1M pot).

The 6G8 Twin has a 500K volume pot. Most other Fender volume pots are 1M.

I'm just trying to understand what the resistance values are doing and wondering why the circuits usually call for such big ones, but other times, not.

[daz]

This free app is good. It'll gives you the ability to see what different values do to the freq curve.
TSC

[Chuck H]

Daz (or Duncan rather) offered a good tool there. It's great for a sense of perspective. In the end the listening will still be the acid test. And...

There is no "right" or "wrong" in guitar amp voicing. Only in the operational parameters. As long as the tubes are operating within safe parameters there are no other rules. So... A 1M bass pot is going to offer a higher load impedance than a 250k pot. Simple as that. The 1M pot is capable of more bass at the high end of the adjustment. It's entirely possible that A10 pots (10% value at 50% rotation) were used in this design. I don't know. But it's something I do regularly WRT design. Select a pot value and taper that will give me the desired "nominal" setting at the halfway point of the knob, OR, a pot that gets as close as possible to the voicing I want at nominal and still offers enough adjustment for non nominal settings. The pot value and taper are part of an overall design and it's expected capabilities.

[deci belle]

Thank you for the link, daz! Dangs~ I've tried that one, but my mac won't work with it!!

Oh! Thank you, Chuck— at least in bass pots, a higher value equals more bass.

Do volume pots work similarly— or just get to maximum volume sooner in the dial's sweep (than a lower-value volume pot)? Maybe a higher value volume pot would distort more/sooner?

I had a gain pot between two gain stages that went to ground. That was a variable grid-load resistor, so I learned that more resistance in that position results in more gain.

[Chuck H]

Used as a divider there would be a small decrease in HF with the lower value pot. Depending on circuit impedances there may not be much difference in gain. Maybe 1 or 2 dB for a typical high impedance gain stage between a 250k and a 1M pot. It's important to note that the actual gain at a given adjustment will typically be the same for the different values because the voltage division would be the same. It's at the highest setting that any difference in gain between the different values is useful. Using a linear pot for an example, a 250k pot set to 5 would present a 125k resistance to a 125k load for a 50% voltage division. A 1M pot would present a 500k resistance to a 500k load for a 50% voltage division. Because of the high impedance of most guitar amp circuits this is apples to apples. The difference in the actual resistance values CAN be used to tune other circuits like "bright" switches and such since the greater resistance would allow for more effect with the larger value pot, and vice versa. It's important to note that this also changes that circuits capacitor knee with the divider resistance (impedance).

Used as a variable load (using the bass pot again for an example) the larger pot will do everything the smaller pot will do but obviously there's more range of adjustment, so, more bass. The taper is the real trick here. Different tapers won't change the tone at any given performance level, only the actual dial setting. So if you use a typical audio pot (lets call it a 25% taper) in a 250k value for a bass pot you would have a load of 62.5k with the control set half way. With a 500k pot you would have 125k. With a 1M pot you would have 250k. Now, if you choose a different taper, like a 1M "A10" pot (10% audio taper) you would have 100k when the knob is adjusted half way. So, in this case the 1M A10 pot has less bass at the halfway setting than the 500k (typical audio taper) pot does. This is really handy because if you, for example, have an amp uses a 250k audio pot and seems to need more bass for clean tones, but you don't like the idea of setting the knob at 1.5 for aesthetic reasons when running dirty, you can change to a 1M A10 pot and get the extra range at the top of the control adjustment. so the bass still feels like it adjusts normally up to 5 and then has even more range on top of that. I find different tapers useful for volume controls too. Especially in amps like the 5f6a where the volume seems to jump up all at once. Using A10 pots can really help with making the controls feel more natural. If a standard audio pot had the amp already hitting max volume (amp is just starting to clip) at only 2.5 on the knob then the A10 pot will give you the same ratio at 4.5 or 5 on the knob and all the same adjustment range is still above that. Of course it goes the other way too. If, for example, your treble pot always seems to be at a high setting for a nominal amount of HF, but you don't want to change the overall range of adjustment, you can move to a faster taper. If you have an A10 pot in there you can move to the more standard audio pot (about 25% taper) or if you have a standard audio taper you can try a linear taper pot. I hope I made this as confusing as possible

[deci belle]

Oh yes, slightly confused (or out of control) is always a good thing, Chuck! I really want to hear as much as possible as I am thinking about this amp build now, so the thoughts can start to gel before the soldering iron gets warm.

What you said is very helpful— thanks, man!!

[J M Fahey]

Or in a nutshell: sound goes through a resistor , usually from 100K (Blackface Fender) to 56K (Tweed/early Marshall) to 33k (modern Marshall) into a capacitor which kills all frequencies above a certain value, essentially leaving only "Bass" there .

The capacitor usually goes from .047uF (Fender Blackface) to .022uF (Tweed and most Marshalls).

The crude but simple way to control bass amount is to short across the capacitor with a pot (Log/Audio for smoother perceived range).

250 K Log is fine and very smooth; 1M log provides *a little* extra Bass (1 or 2 dB at best) for the price of a less smooth range, because, really, *most* of available bass was reached by 250K.

I mean audible bass change, the *curve* looks more important than what we actually hear.

Edit: contrary to what calculatins show, Fender controls have more/deeper bass than Marshall ones.

The perceived idea of the contrary comes from the real fact that popular Fenders were open backed combos , while popular Marshalls were closed back cabs.

Also Marshall/Tweed controls "cheat" by providing more midbass frequencies, because the turnover frequency is higher.

As in: 100K + .047uF is roughly 32 Hz, while 33K + .022uF is 6X higher, some 180 Hz, a HUGE difference by any standard.

[Chuck H]

See? That's just confusing Juan

I actually got all that. It's just that most builders and tinkers have a tough time getting that a load can act as a low pass filter and the same .022uf cap isn't created equal in all circuits. The principals are impedance dependent and involve formulas "I" never troubled to learn. But it's more important to know that these are design considerations in the first place. Once you know THAT it happens you can make adjustments and it is your ears that do any final decisions anyway!

You never mentioned the differences due to the TS driver impedance being much lower in the Marshall/5f6a type amps as compared to the BF Fender types.

[deci belle]

Please do!

[J M Fahey]

Chuck H already did
I didn't want to write a book on Tone Controls, but yes, the cathode follower lowers impedance nd modifies curves.
To be more precise, it's the contrary, it approaches more the theoretical curves.
Easy to check, Duncan's (genius) TSC software admits setting the generator impedance which is driving the tone control.
Use any stack, let's start with Blackface Fender, simulate it twice, first with some 40K , the net impedance of a 12AX7 (68K internal in parallel with 100K plate resistor) , and then repeat with, say, 1k (cathode follower).
You'll be amazed.

[Chuck H]

Yupper. The load effect from the lower driving impedance becomes apparent with the impedance increasing with frequency. This is exaggerated by the low impedance driving a higher impedance circuit. Not to mention the lower losses.

[deci belle]

I have noticed that because I have a cathode-follower circuit along with a blackface one on a bf-style head.

Because of the facts I learned I decided to forgo the 1M base pots and use 250k. I did up the value of the 6G8's volume pot from 500k to 1M so the bright switch I'm installing will have more effect (if I understood enough of what y'all told me).

The fun part was changing out the guts of the CTS dual concentric pot (front AND rear) for one of the channels (not enough room on the tweed-still twin chassis for all the holes)! I ganged-up the volume and bass and kept the 2nd hole for the 350k/70k treble pot.