Negative feedback

skydog

skydog

Experienced
(Made you look, lol.)

Is the negative feedback damping parameter a relatively linear adjustment? Example: my 50W plexis are a bit older than the common ones of 1970 using a 47K resistor in the negative feedback circuit. I assume the same for the AF3’s plexi model. I want to simulate a 27K resistor that is in my circuit. (An early evolutionary step between the 45W and the 50W heads.) Is there a good guess as to how much to decrease damping?

Btw,
Yes, I know that I can use my ears. But my ears have been attached to my head and standing in front of two 50W plexis the better part of my life.

Yes, I know that I’m being pedantic. Big surprise, I own an Axe FX III.

Yes, I know about tone matching. My tone matches always really suck. They make my amp sound like it’s buried under a giant termite nest outback.
 
These are my notes for the various Plexis:

// Plexi 50W: 100K from 4-ohm tap
// 1987x: 100K from 8-ohm tap
// Plexi 100W: 47K from 8-ohm tap (off speaker jack with two 16-ohm cabs)
// 1959SLP: 47K from 8-ohm tap
// Plexi 6550: 100K off 4-ohm tap

// 50W Normal - Normal channel. 100K feedback from 4-ohm tap. Cathode cap on second triode.
// 50W High 1 - Treble channel. 100K feedback from 4-ohm tap. No cathode cap on second triode.
// 50W High 2 - Treble channel. 100K feedback from 4-ohm tap. Cathode cap on second triode.
// 1987x Normal - Normal channel. 100K feedback from 8-ohm tap. No cathode cap on second triode.
// 1987x Treble - Treble channel. 100K feedback from 8-ohm tap. No cathode cap on second triode.

You need to know what tap the feedback is from. Going down a tap decreases the feedback by 0.707. Conversely going up a tap increases the feedback by1.414. Up two taps doubles the feedback (4-ohm to 16-ohm).

FWIW, our reference Plexis are 1967 and 1968. I've never seen one with a 27K feedback resistor. That's pretty low so I have to assume it's from the 4-ohm tap. If that is the case you would want to multiply the feedback by about 3.3. So if it's 1.5 you would set it to 4.95.

I've seen JTM45s with 27K off the 16-ohm tap. That's a lot of feedback. In this case you would then double the feedback on top of the 3.3 so 9.9 or so.
 
FractalAudio said:
These are my notes for the various Plexis:

// Plexi 50W: 100K from 4-ohm tap
// 1987x: 100K from 8-ohm tap
// Plexi 100W: 47K from 8-ohm tap (off speaker jack with two 16-ohm cabs)
// 1959SLP: 47K from 8-ohm tap
// Plexi 6550: 100K off 4-ohm tap

// 50W Normal - Normal channel. 100K feedback from 4-ohm tap. Cathode cap on second triode.
// 50W High 1 - Treble channel. 100K feedback from 4-ohm tap. No cathode cap on second triode.
// 50W High 2 - Treble channel. 100K feedback from 4-ohm tap. Cathode cap on second triode.
// 1987x Normal - Normal channel. 100K feedback from 8-ohm tap. No cathode cap on second triode.
// 1987x Treble - Treble channel. 100K feedback from 8-ohm tap. No cathode cap on second triode.

You need to know what tap the feedback is from. Going down a tap decreases the feedback by 0.707. Conversely going up a tap increases the feedback by1.414. Up two taps doubles the feedback (4-ohm to 16-ohm).

FWIW, our reference Plexis are 1967 and 1968. I've never seen one with a 27K feedback resistor. That's pretty low so I have to assume it's from the 4-ohm tap. If that is the case you would want to multiply the feedback by about 3.3. So if it's 1.5 you would set it to 4.95.

I've seen JTM45s with 27K off the 16-ohm tap. That's a lot of feedback. In this case you would then double the feedback on top of the 3.3 so 9.9 or so.
Click to expand...
There are exactly zero modelers on the market besides Fractal Audio Systems that come with this kind of service. After over a decade of using FAS gear, you’d think I’d get used to it, but it still blows me away when the main man gets on and shares his knowledge like this. Kudos, Cliff.
 
FractalAudio said:
These are my notes for the various Plexis:

// Plexi 50W: 100K from 4-ohm tap
// 1987x: 100K from 8-ohm tap
// Plexi 100W: 47K from 8-ohm tap (off speaker jack with two 16-ohm cabs)
// 1959SLP: 47K from 8-ohm tap
// Plexi 6550: 100K off 4-ohm tap

// 50W Normal - Normal channel. 100K feedback from 4-ohm tap. Cathode cap on second triode.
// 50W High 1 - Treble channel. 100K feedback from 4-ohm tap. No cathode cap on second triode.
// 50W High 2 - Treble channel. 100K feedback from 4-ohm tap. Cathode cap on second triode.
// 1987x Normal - Normal channel. 100K feedback from 8-ohm tap. No cathode cap on second triode.
// 1987x Treble - Treble channel. 100K feedback from 8-ohm tap. No cathode cap on second triode.

You need to know what tap the feedback is from. Going down a tap decreases the feedback by 0.707. Conversely going up a tap increases the feedback by1.414. Up two taps doubles the feedback (4-ohm to 16-ohm).

FWIW, our reference Plexis are 1967 and 1968. I've never seen one with a 27K feedback resistor. That's pretty low so I have to assume it's from the 4-ohm tap. If that is the case you would want to multiply the feedback by about 3.3. So if it's 1.5 you would set it to 4.95.

I've seen JTM45s with 27K off the 16-ohm tap. That's a lot of feedback. In this case you would then double the feedback on top of the 3.3 so 9.9 or so.
Click to expand...
Can't possibly offer enough thanks for your detailed response. Sorry it took me a few to respond - in Austria, working on a problem all night, and temporarily forgot to check in.

I have several old Marshall heads, and apologies for my amp-biguous question:
// Plexi 50W: 27K off 16-ohm tap (two amps)
// Plexi 1986: 47K off 16-ohm tap (one amp)
// Plexi 1987: 47K off 8-ohm tap (one amp)

and an almost Marshall
//Traynor YBA-1 (very early tube-rectified): 100K off 8-ohm

You've helped now with both the input and output questions for matching my AF3 to my Marshalls. Support and attention to detail are astonishing. Thanks again.
 
FractalAudio said:
These are my notes for the various Plexis:

// Plexi 50W: 100K from 4-ohm tap
// 1987x: 100K from 8-ohm tap
// Plexi 100W: 47K from 8-ohm tap (off speaker jack with two 16-ohm cabs)
// 1959SLP: 47K from 8-ohm tap
// Plexi 6550: 100K off 4-ohm tap

// 50W Normal - Normal channel. 100K feedback from 4-ohm tap. Cathode cap on second triode.
// 50W High 1 - Treble channel. 100K feedback from 4-ohm tap. No cathode cap on second triode.
// 50W High 2 - Treble channel. 100K feedback from 4-ohm tap. Cathode cap on second triode.
// 1987x Normal - Normal channel. 100K feedback from 8-ohm tap. No cathode cap on second triode.
// 1987x Treble - Treble channel. 100K feedback from 8-ohm tap. No cathode cap on second triode.

You need to know what tap the feedback is from. Going down a tap decreases the feedback by 0.707. Conversely going up a tap increases the feedback by1.414. Up two taps doubles the feedback (4-ohm to 16-ohm).

FWIW, our reference Plexis are 1967 and 1968. I've never seen one with a 27K feedback resistor. That's pretty low so I have to assume it's from the 4-ohm tap. If that is the case you would want to multiply the feedback by about 3.3. So if it's 1.5 you would set it to 4.95.

I've seen JTM45s with 27K off the 16-ohm tap. That's a lot of feedback. In this case you would then double the feedback on top of the 3.3 so 9.9 or so.
Click to expand...
@yek, any chance of adding this to the Wiki page, if/when you have time?
 
Darrin66 said:
@yek, any chance of adding this to the Wiki page, if/when you have time?
Click to expand...
More notes from Cliff from another post;
“The following models have the 0.68uF cap on the second triode:
Plexi 50W Normal
Plexi 50W High 2
Plexi 50W Jumped
Brit Brown
Plexi 100W Normal
Plexi 100W High
Plexi 100W Jumped
Plexi 100W 1970
1959SLP Normal
1959SLP Treble
1959SLP Jumped”
 
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