Taming a Bright 5F2A

[MarkF786]

I find the tone circuit in a Princeton Tweed 5F2A to be bright so I was wondering if there was a way to tame it.

In the diagram below, it seems that C1 is working like a bright cap. Is this an accurate assessment? Can I lower the value of this cap to reduce the brightness? On amps I've owned with a bright switch, I typically keep it turned off since I find upper-highs unpleasant to my ears.

Any other tips?

Mark

[JHow]

Duncan amps has a tone stack calculator that will let you tweak values of many tone stacks:

TSC

[MarkF786]

Thanks for the reply but the only single-knob tone stack in models is the Big Muff. I wish I could do a classic Fender single-knob stack.

Mark

[mac1amps]

Hey Mark,
I'm with you with respect to [most] bright switches. They sting my ears!
Personally, I would recommend removing the C1 cap all together, not just lowering the value. In most all tweeds, that's the only difference between a "Bright" channel VS a "Normal" channel.

[JHow]

...the only single-knob tone stack in models is the Big Muff...

Oops, sorry, I just assumed it was in there, I didn't actually look.

With the tone control rolled up I think it is just a bright cap, but wouldn't you want to increase the cap value to lower the cut-off freq?

[echuta13]

The 470pf does act as a bright cap (albeit a variable one). By setting your tone control at the halfway mark... you should be pulling it out of the circuit. At wide open on your volume control, it should be out of the circuit as well.

[MarkF786]

Hey Mark,
I'm with you with respect to [most] bright switches. They sting my ears!
Personally, I would recommend removing the C1 cap all together, not just lowering the value. In most all tweeds, that's the only difference between a "Bright" channel VS a "Normal" channel.

Thanks for the feedback. I'm thinking of seeing what happens when I yank it; optimally I could switch it in & out. I just wanted to make sure that C1 is acting as a bright cap.

With the tone control rolled up I think it is just a bright cap, but wouldn't you want to increase the cap value to lower the cut-off freq?

I think you're right.

The 470pf does act as a bright cap (albeit a variable one). By setting your tone control at the halfway mark... you should be pulling it out of the circuit. At wide open on your volume control, it should be out of the circuit as well.

I'm thinking the "halfway" mark is probably relative to the volume control, and also the physical halfway mark probably doesn't match the electrical halfway mark - though idealy at some value the pot is neither bleeding or cuting treble. For example, with the volume knob around 9 o'clock, I have to turn the tone knob down to around 9 o'clock also before the treble sounds balanced instead of bright; to my ears, 12 o'clock on the tone knob definitely doesn't sound flat. But when I bring the volume knob fully up, then I can also bring the tone knob up while keeping a somewhat balanced treble (though still no ideal to my ears).

Ideally I don't want the tone control to ever act as a treble bleed; I only want it to be a treble cut.

Mark

[JHow]

Not to put words in echuta13's mouth, but I think "halfway" means electrically half way, or 500K on a 1M pot.

I think all of this is an easy experiment to see what you like best.

[MarkF786]

Not to put words in echuta13's mouth, but I think "halfway" means electrically half way, or 500K on a 1M pot.

I think all of this is an easy experiment to see what you like best.

Thinking about it, it seems like C1 boosts the treble while C2 cuts the treble, with the tone pot blending between the two - but there is never a point where the effect is null, especially since the value of C1 and C2 are different. So at 500K, there is a treble boost and treble cut occurring simultaneously, but with a different corner frequency.

Or is my understanding of the circuit way off?

I tried the quick mod tonight and it did what I expected. It sounds much better to my ears; certainly less bright (and yet not dark).

What made me hesitant about the small mod was the amp is a fairly new, "boutique" amp from a respected builder. It's not an exact clone of a 5F2A, but close enough - and the tone circuit is an exact match. Now that I found the mod works, I might ask the builder to add a "bright switch" to the amp so that I can switch between the two.

Thanks for all the feedback!

Mark

[JHow]

...there is never a point where the effect is null

Only when volume is wide open, then the tone circuit is bypassed.

Out of curiosity, what value of C1 did you most prefer?

[ThermionicScott]

I tame my 5F2A's tone by leaving it around 2 or 3 most of the time. Sounds great, at least with the old alnico I've got in there.

I like to think of the one-knob tone circuit as a variable bright-cap on the volume control. At higher settings, the corner frequency is lower so more of the treble is being boosted. And as you turn up the volume, you're bypassing that circuit, so it becomes less effective.

A true treble-bleed circuit would do the same thing as the tone knob on your guitar, wouldn't it?

- Scott

[Groover]

When the volume is all the way up the treble boost (bright cap) part of the tone control does not have any effect, but the treble cut part of the tone control will work regardless of volume setting. The amount of cut for a certain setting of the tone pot may change depending on the setting of the volume pot.