Does a 'bright cap' really boost highs?

[Gregg Hermetech]

Quick question: Does a so called "Bright Cap" on a volume control (whether it be in the guitar, or the preamp input volume control on a simple amp like a 5F1) actually boost highs, or does it really act as a HPF and rip out lows, leaving the highs alone? I am guessing the latter, but I keep seeing it referred to as a way to boost highs, which is most confusing.

[R.G.]

It prevents highs over some frequency from being attenuated by the treble control.

Tone stacks are passive, meaning they only cut some frequencies more than others. The filtering is set so that the treble control mostly affects the highs. A bright cap bypasses higher highs around the treble control's variable cut.

[xtian]

On Volume pots, yes, bright cap lets the high frequencies sneak by while the rest are bled to ground by the pot.

[maxkracht]

This might be too technical, but in the dancing pixies theory of electronics, think of small caps like a narrow path and a big cap like a wide path. Bright cap is a shortcut around the volume control, but only the skinny dancing pixies can make it through, the fat ones have to go the long way if they want to get out.

[JD0x0]

No, there's no actual 'Boost'

What you get is a perceived 'boost' because the cap allows highs to bypass the volume control (on settings below 10) so highs are attenuated less than other frequencies. This is also why the bright switch gets less effective as you increase that volume control. The fact you get no high frequency accentuation on '10' demonstrates how there's no actual 'boost.' Now, all frequencies lack attenuation, so they 'catch up' with the highs bypassing the volume control.

For a genuine boost, you'd basically need an active circuit, or something that actually increased gain. You could possibly argue that adding a small cathode bypass cap to an unbypassed cathode would 'boost' the highs, since you're increasing gain in the high frequencies, but then again, if you're comparing it to a fully bypassed cathode, there isn't actually a 'boost' it's more like a cut at lower frequencies. Now, if you had a switchable gain stage with that small bypass cap, (for perceived high freq accentuation) engaging this stage could be viewed as a high frequency 'boost'

IMO, this applies to many 'Top Boost' circuits too. In many cases the 'Bright', 'High Treble' or 'Top Boost' channel in an amp is simply a bass cut vs the normal channel. (Sometimes with a 'bright cap' across the volume too) It's a perceived 'boost' because there's less bass being passed, but in many cases the highs aren't actually 'boosted' it just seems that way to our ears because there's less of the other frequencies.

[pjd3]

Ha, thats great Max.

I have from time to time imagined electron flow in strange analogous ways to give some visual to the actions.

"But, to truly know the electron, you must "Be" the electron" - Bruce B+

Semi-sincerely yours,

Phil D.

[pdf64]

… Does a so called "Bright Cap" on a volume control … actually boost highs, or does it really act as a HPF and rip out lows …

The distinction between those 2 options might be seen as something of a matter of perspective really.
What matters is that unless the volume is set really high, the amp sounds brighter.

[martin manning]

Does a so called "Bright Cap" on a volume control ... actually boost highs, or does it really act as a HPF and rip out lows, leaving the highs alone?

It's the latter. Sort-of. A volume control is a resistive divider, a series resistance and a shunt resistance, with the junction (wiper) typically feeding the grid of the following stage. The series resistance and the Miller capacitance of the following stage (Cg-a * gain, ~100 pF for a 12AX7) form a low pass filter. The corner point of this filter moves from a very high frequency at high volume settings (where there is a small series resistance), to a low frequency where the series and shunt resistances are equal (50% attenuation), and then increases as the volume control is turned down further. A bright cap bypasses the series resistance, allowing the higher frequencies to get by the filter. The result is the volume control primarily reduces low frequencies, and has little effect on the high frequencies.

[Gregg Hermetech]

This is all brilliant information, thank you so much. I'm coming from an audio engineering perspective (been a full time mastering engineer since 2011), so defining the correct terms is important to me. Electrical engineering, well I'm just a beginner, so thank you guys for helping me grok this stuff!

I'm finding my 5F1 clone a little dark sounding, I usually only have the volume control on 3. I've been reading one way to counter that would be to add a 100pF capacitor between lugs 1 and 2 on the amp's volume pot. I can order the cap, and try it out before soldering it in place, so shall do just that. Was contemplating adding an actual EQ stage (Framus style Mid Cut control for more of a scooped BF sound), but will try this first.

Had forgotten passive circuits can't boost, doh... I used to have a Thermionic Culture Pullet passive EQ with 38dB insertion loss, in my mastering rig. I had to use a mic pre to get it back up to line level.

So, the pF value of the capacitor determines the high pass filter's cutoff frequency, but that also depends on the surrounding circuit? Does 100pF sound like a good place to start with the volume control on my 5F1, or any other recommendations? I'm using a 12AY7 instead of a 12AX7 for lower gain, if that might make a difference.

Thanks once again!

[sluckey]

I'm finding my 5F1 clone a little dark sounding, I usually only have the volume control on 3. I've been reading one way to counter that would be to add a 100pF capacitor between lugs 1 and 2 on the amp's volume pot. I can order the cap, and try it out before soldering it in place, so shall do just that. Was contemplating adding an actual EQ stage (Framus style Mid Cut control for more of a scooped BF sound), but will try this first.

Not much can be done with the tone of the 5F1 circuit but putting that 100pF on the volume pot will help brighten the tone at lower volume levels but will have decreasing effect as the volume is cranked up and no effect if volume pot is set to max.

For more control over the tone you may consider the simple 5F2A tone circuit. Easy to add... Just a pot and two caps. No board mods required. This tone control acts as a variable bright cap as you turn it up and a variable treble bleed as you turn it down. May be just what you are looking for.

https://el34world.com/charts/Schematics ... n_5f2a.pdf

Or for more tone control consider the Champ AA764 style tone controls. This tone circuit is the same as found in bigger blackface era amps. It's lossy so your champ will not be as loud as a 5F1.

https://el34world.com/charts/Schematics ... _schem.pdf

[JD0x0]

I'm finding my 5F1 clone a little dark sounding

Don't forget, as mentioned earlier in the thread. Cutting lows can also increase perceived brightness. Try Cathode bypass caps in the 1uF-2.2uF range. Not only will it brighten the tone at low volume, but it'll control bass flub and muddiness when the amp is cranked. The 5F1's speaker and cab dimensions mean you're getting a very poor bass response anyway, so much of the low end being passed through the circuit isn't even audible, and you're just making the circuit work harder than it needs to, attempting to reproduce frequencies that that are far below what the speaker/cab are able to reproduce.

[Gregg Hermetech]

Thanks for the suggestions. I'll give the 100pF cap on the amp volume control a go and report back when it's done. I'm basically looking for two things with my amp (an Amp Maker WF-55 kit):

1) Less volume. I've addressed this by changing the 12AX7 for a 12AY7 and it worked well, but I still rarely have the amp above 3. It's supposed to be 4w and is still WAY too loud for home playing. In future I will also install a 3 way PT switch so I can switch the overall amp voltage to 2w and 0.4w.

2) Less lows. I've addressed this in two ways so far. Firstly by changing the first coupling cap for a much lower value, which REALLY helped clean up the flub and I am very happy with. And secondly by installing an NFB pot so I can get both more and less NFB than I had before. With more NFB things clean up nicely, particularly the low end sounds tighter.

However, I'd still like it to sound a bit more chimey/more high end if possible. As I never have it above 3 on the volume, I'm thinking a Bright Cap on that control may be the perfect easy solution. The other thing I'd like to do is add the Framus mid scoop control, which is also pretty simple, a pot, a resistor and two caps I think (I made another post about it in the Fender part of this forum, will add link below). This would allow me to hopefully get more into BF sound territory, be better for my Funk strumming practise (where you mostly want just clean highs), and reduce the bass (at max settings it cuts bass by around 6dB as well as scooping mids).

https://ampgarage.com/forum/viewtopic.php?f=8&t=36632

I'm definitely not looking for more gain, or a bassier tone, am quite happy with the Tone control on the guitar for the rolled off sound, so probably won't do the 5F2A tone control. I have a BF AB763 clone (plus a few mods) built for me a while back from Vyse Amps in the UK and that has the standard BF tone stack which I do really like, but not enough space on my 5F1 chassis for a full tone stack I think, and want to keep things as simple as possible. Thanks for those links though, will definitely check them out (not looked at the BF Champ circuit before), I am all up for learning about this stuff!

Ah, speakers, yes, I should mention I had a custom 12" cab built by Tube Town which I put a Weber 12A125A in, so it's not stock at all as far as original Champ speakers go. Cathode Bypass Caps are another thing I haven't fully researched yet, so thanks for bringing that up. My worry there is I heard they also increase volume, which I am looking to avoid. Also I never crank the amp, max 3 here!

Will try to post a pic of the amp if anyone's interested, only room left on the front panel for one more control!

Enlight435.jpg

[martin manning]

In the basic 5F1 the input network is typical Fender where 64k//64k = 34k is the series resistance, and that plus Miller and any other stray capacitance will roll off some of the extreme high end. With 34k in front of the input he corner point frequency is quite high, maybe 30kHz depending on how much additional capacitance you have to ground, but the roll-off begins a couple of octaves below that. Input grid resistors as low as 8-10k are seen in some amps, so you might give that a try.

[Fischerman]

Ha, thats great Max.

I have from time to time imagined electron flow in strange analogous ways to give some visual to the actions.

"But, to truly know the electron, you must "Be" the electron" - Bruce B+

Semi-sincerely yours,

Phil D.

That works better for me too sometimes. I can’t really get a grip on how tubes work when thinking about conventional current flow. But it makes sense when I think of it in terms of electron flow.

[JD0x0]

1) Less volume. I've addressed this by changing the 12AX7 for a 12AY7 and it worked well, but I still rarely have the amp above 3. It's supposed to be 4w and is still WAY too loud for home playing. In future I will also install a 3 way PT switch so I can switch the overall amp voltage to 2w and 0.4w.

2) Less lows. I've addressed this in two ways so far. Firstly by changing the first coupling cap for a much lower value, which REALLY helped clean up the flub and I am very happy with. And secondly by installing an NFB pot so I can get both more and less NFB than I had before. With more NFB things clean up nicely, particularly the low end sounds tighter.

1. A master volume would get this sorted. Change the 220k grid resistor in front of the power tube to a 250k pot. Very easy. I usually use the 'Volume' control position for the master, and turn one of the input holes into a 'gain' control (your original volume control)

2. The cathode bypass adjustments I mentioned will help with this. You can also have multiple values on a switch. (Use a small value on the tube, then parallel a second cap wired to the switch for a 'full'/'tight' switch) You can also do the same thing with the power tube's cathode capacitor. The VHT special 6 ultra wires theirs to a rotary switch for a few different selectable values. Check their schematic for more details.
There's also the cap between the second gain stage and output tube. Trimming this up I believe will also change the bias excursion of the circuit. A smaller cap will generally have a faster excursion and recovery, all other things equal. Probably not something you need to worry about if you're not turning the amp up loud, though.

Lastly, you can increase perceived highs by cutting highs in the NFB loop. This is exactly how a 'Presence' control works. Cut highs in the NFB loop, which in turn reduces less gain in the high frequencies, when the NFB is applied.