Blackface Vibro Champ mystery cap?

[blueznet]

I have looked at several layouts and shematics for the Fender AA764 Vibro-Champ amp. Almost half of them are showing a 330 PF cap from Cathode to Control Grid. Most are showing this amp to NOT have a 330 PF cap there.

What is the reason for Fender putting a 330 PF cap there? Do I need to put one there?

[John]

I think Tubenit has done something like that; he called it a smoothing cap.

[jojokeo]

One man's high freq oscillation reducer enhancing stability (CBS-Fender) is another man's smoothing cap (tubenit) is another man's tone robber (G Weber).

[blueznet]

One man's high freq oscillation reducer enhancing stability (CBS-Fender) is another man's smoothing cap (tubenit) is another man's tone robber (G Weber).

Well, which one is it?   

[firemedic]

Depends on what you are trying to do.
Try your build without the cap first, then if it's unstable try small value caps from grid to cathode or plate to cathode 'til it's stabilized. It's just local NFB.
 
To me a SE amp's appeal lies in the rawness of the sound, I guess it CAN be too raw. If you are conscientious about preventing oscillation & noise in your layout (shielded wire, etc.) you shouldn't need the cap.

[sluckey]

It's all of them and more. It's also an anti-brite cap. It was probably put there to suppress some high frequency parasitic oscillation. It attenuates high frequencies, including guitar signal high frequencies. It will remove some of the ice pick treble. IOW, make the tone more mellow, smoother. It's the opposite of the bright cap on the bright switch of some Fender amps.
 

[HotBluePlates]

Same reason the bigger Fenders have a 2000pF cap to ground from the grid. Running the cap from pin 5 to pin 8 is "same as" to ground.

[ThermionicEngine]

I only find the cap on AA764 and later Vibro Champs.  The Champs got them later.  Maybe the vibro champ was subject to oscillation issues not present in the champ amp.  I usually cut these caps out on the CBS amps without any problems at all.  Customers with "dog's ears" say they hear a difference and it is a positive one.

[HotBluePlates]

Come to think of it, I have had several blackface VibroChamps. Can't say I ever remember seeing that cap. Maybe it was there and I overlooked it, or...

[echuta13]

If you look at the AB764 schematic (Bronco) they're in there.   They weren't in the blackface amps (AA764) I think these caps were pretty common in the CBS era amps primarily due to sloppy wire dressing.  It was easier to slap a few ceramics in then do clean wiring apparently. 

[stratele52]

Poor wiring dress need have sometimes parasitics oscillations and need a cap to remove. Your amp could have a good wiring an don't need this cap. You have too look at the wiring and do proper led dress if needed and remove the capacitor

[jjasilli]

Ditto to Firemedic & Sluckey  On my rebuild I removed the cap & the NFB loop.  But there was parasitic oscillation.  I wound up replacing that cap and a stronger NFB loop.  (N.B.: I hotrodded my amp with a 6L6 > larger OT > 10" Spkr.)