Ab763 cap missing (10pf on the 3.3M/10pf)... what effect does it have?

[JubileeMan 2555]

Well, my quest continues to figure out why my vibroverb copy sounds funny... I found this cap is missing. Any thoughts?

http://www.shelleygrund.com/images/capmissing.jpg

[Mike T]

I can't tell you exactly what it does, but it's part of the reverb circuit.

[pula58]

10pF and 3.3Meg ohm makes a 4.8kHz frequency breakpoint. That is within our hearing (be careful of high voltages).

[JubileeMan 2555]

10pF and 3.3Meg ohm makes a 4.8kHz frequency breakpoint. That is within our hearing (be careful of high voltages).

So...what happens when the 10pf is gone, but the 3.3meg is still there?

[TheAmpNerd]

This is part of the the reverb mix stage.

The 10pf cap brightens up the dry signal as
well as the whole amp.


So you have the following:


470K Ohm wet signal
3.3 meg & 10pf dry signal
220K to ground (bleesd off dry signal)
All feeding the reverb mix amp, this then goes
to the PI and power amp.

Hope this helps

[TheAmpNerd]

I performed My first Torres mod (Super Overdrive) on a Bassman 100. Torres left that cap out of the schem and when I got done it sounded really muffled. My Friend did the same mod to another amp and His sounded normal (which was like a very lame excuse for a Boogie). It took two weeks and several fruitless calls to Dan T. to figure out what was wrong. I was looking at an AB763 schem and noticed the cap. I was so turned off by Torres from then on.

The Bassman 100 did not have reverb.
I'm wondering what cap you really left out
and where you put the 10pf cap that should
have been maybe 100pf/110pf or a 250pf or 500pf cap.

You might want to double check you still might
be very fuffled from where you should be.

[BJF]

Hi,

This is an equalizing cap more than a filtercap. The voltage divider that creates the reverb mix circuit is quiet high impedance to improve the isolaton. The cap is meant to be tuned so it just levels off the loss in high frequencies that may occour at the grid of reverbmixer stage.

Like the tuning cap on an oscilliscope probe, to make a fine square wave approximation

Making this capacitor big enough to pass midrange will affect the reverb sound- so it sounds more distant and then it may require changes in the reverb path to restore reverb shimmer
Removing this cap will make the amp a bit muffled and the degree of this depends on the ghost capacitances present at the grid of reverbmixer.

Controlling the bite of the amplifier is better done at other positions while this cap is fine to just level out losses at this point in the amplifier.

If deeper reverb sound - for e.g. surf style is desired the 3M3 might be increased a bit and then capacitor decreased to retain balance of the bridge created.

For more gain in this stage the 3M3 can be lowered and this cap increased but it may then be required to install a shunt cap from grid of mixer stage to ground - again to balance the bridge but then to shift response of the dry path more to the upper midrange


Have fun
BJ

Affiliations
www.bjfelectronics.com
www.mpamp.com

[JJman]

Hi,

This is an equalizing cap more than a filtercap. The voltage divider that creates the reverb mix circuit is quiet high impedance to improve the isolaton. The cap is meant to be tuned so it just levels off the loss in high frequencies that may occour at the grid of reverbmixer stage.

Like the tuning cap on an oscilliscope probe, to make a fine square wave approximation

Making this capacitor big enough to pass midrange will affect the reverb sound- so it sounds more distant and then it may require changes in the reverb path to restore reverb shimmer
Removing this cap will make the amp a bit muffled and the degree of this depends on the ghost capacitances present at the grid of reverbmixer.

Controlling the bite of the amplifier is better done at other positions while this cap is fine to just level out losses at this point in the amplifier.

If deeper reverb sound - for e.g. surf style is desired the 3M3 might be increased a bit and then capacitor decreased to retain balance of the bridge created.

For more gain in this stage the 3M3 can be lowered and this cap increased but it may then be required to install a shunt cap from grid of mixer stage to ground - again to balance the bridge but then to shift response of the dry path more to the upper midrange


Have fun
BJ

Affiliations
www.bjfelectronics.com (http://www.bjfelectronics.com)
www.mpamp.com (http://www.mpamp.com)


Isn't the 500pf the one that feeds the reverb circuit? And the 10pf/3.3Meg feeds the dry signal directly to the grid of the "mixing" triode? No?

[Blue Strat]

Isn't the 500pf the one that feeds the reverb circuit? And the 10pf/3.3Meg feeds the dry signal directly to the grid of the "mixing" triode? No?

That is correct.