Amps and Bright caps.

[Mike McLane]

In reading thru your threads on tone stacks I was reminded of a "non-pedal" question that's always bugged me, but no amp tech I've ever talked to can seem to comment on it with any authority. My Deluxe Reverb has a 120pf bright cap. When amp volume is low a lot of the top end gets lost, hence, you flip the bright switch and allow the high freqs (determined by cap value) to bypass the resistance imposed by the VOL pot and go straight into the signal path. Voila, no more muddy. Turn up the VOL to 10 and it becomes a non-issue as the VOL pot resistance "goes away". This is kind of "all or nothing" proposition. We have the same issue with volume pots on guitars, but overcome that with treble bleed circuits. . . a cap and resistor in parallel (1n & 100K is a very popular combination). It does a good job in maintaining highs when the VOL is backed off and provides for a consistent "tone" throughout the sweep of the VOL pot. Can this approach be applied to amps in place of the bright cap?

 

[Chuck D. Bones]

The bright cap on an amp is not an all-or-nothing affair, it rolls in gradually as the Volume pot is turned down. Some amps have a Bright switch to disable the bright cap. There are many ways to implement a treble bleed. Personally, I find a cap and resistor in series are more effective over a wider range of the Volume control. The purpose of a treble bleed on a guitar is to counter the effect of cable capacitance on the guitar's tone. With an amp, two things are going on:
1) the ear is disproportionately less sensitive to high frequencies at lower volume
2) the Miller capacitance of the tube following the Volume control interacts with the Volume control's resistance to roll off the treble
The bright cap counters both of these effects. The value of the bright cap is a compromise to get the right amount of treble boost over the most useful range of the volume control. There are more complicated configurations that use a dual-gang Volume pot to obtain an improved treble bleed. Or why not just turn the Treble & Presence controls to balance the tone?

 

[Mike McLane]

Ah ha!! So there's method in this madness!! Thx for the explanation. My bright cap is switched so I can fiddle, diddle and piddle my way to sonic satisfaction. What cap-resistor series combination would you suggest?

 

[Chuck D. Bones]

Depends on the application. Last time I put treble bleed in a guitar, the VOL pots were 500K so I used 330K in series with 330pF.

 

[Mike McLane]

I'm a Fender guy

 

[Chuck D. Bones]

Deluxe Reverb is a sweet amp.
For amp applications, I would not add a resistor in series or parallel with the bright cap. You can experiment with different cap values. Larger caps will increase more of the midrange. YMMV.

 

[JamieJ]

I’ve been gassing hard for a deluxe reverb recently!

 

[fig]

I am glad I bought mine.

 

[Mike McLane]

Mine's actually an Allen Accomplice Jr. I bought as a kit for about what a good silverface would cost. Top quality components, no conversion costs or filter caps, etc to replace. Has MID control, RAW feature that gradually "lifts" the tone stack out of the circuit for a fatter, grittier tone and a PPI Master Volume. Also, will take 6L6's for 35W output. Would recommend highly.