cathode resitor in AB763 second stage

[bluescaster]

Hi,

I read here http://fenderguru.com/amps/super-reverb :

"If you are like most players and only use the Vibrato channel (reverb, tremolo, the bright cap and the extra gain stage), you should pull out the V1 tube. This is the preamp tube for the normal channel which you are not using when playing the Vibrato channel. All AB763-similar circuits are designed so that they share the cathode cap and resistor (25 uF/1500 ohm) and pulling one of the tubes will changes the effective value of the resistor they both share. If you pull one tube the other channel’s tube will be hotter biased and offers more gain.
This mod is one of Cesar Diaz’ tricks in the Fender Custom Shop Vibroverb 64 which he always did to Stevie’s amps. "


If i understand this, when you pull out V1, the second stage of vibrato channel has a 820 cathode resistor instead of a virtual 1K5 one when V1 is here. So decrease cathode resistor offers more gain.

But , on Custom Shop Vibroverb 64 schematic, we can see that fender change cathode resistor from 1K5 (3K3 // 2K7) to 2K7 when switch is on MOD. they increase cathode resistor !

http://www.prowessamplifiers.com/schema ... _mods.html

What is wrong ? please.

Regards

[jelle]

Neither is *wrong*, just different.

[pdf64]

The fenderguru site doesn't seem to have been written by people with a good level of technical competence; give the ideas a go but take any explanations with a pinch of salt.

[Smitty]

For cathode biased stages the current across the cathode resistor develops a positive DC voltage. Because the grid is at zero volts DC it ends up being negative with respect to the cathode's positive DC voltage. The difference between the two is the negative bias voltage or how much below clipping (when the top of the AC signal approaches the zero DC volts of the grid) the center of the AC signal resides. For a given gain stage a smaller cathode resistor will develop less positive voltage and the bias will referred to as 'hotter'. We call it hotter for two reasons: 1) the stage is drawing more current at idle. 2) the DC bias point of the cathode is closer to the zero volts of the grid which allows a smaller AC signal to achieve clipping.

Guitarists often confuse clipping with gain as well as headroom. I don't think there is a practical difference in gain (how many times the AC signal is multiplied by the gain stage) for triode cathode bias points, but there is a marked difference in the amount of headroom or how much AC signal amplitude you can present to the grid before clipping. That is the relevant measure. Think of the DC bias point as the center of the AC signal.

Some designers will set bias to induce clipping before cut off (when the negative side of the AC signal reduces current draw so much that the stage stops drawing current all together) or cut off before clipping like the third gain stage of a Trainwreck. The two types of distortion sound very different and induce differing types of harmonic distortion components.

Hope this helps.