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Gain with Plexi style amps

SeeD

Fractal Fanatic
Confirmed, input trim gets me most of the way there! I am now playing with a combination of tone match and the sine generator to better amp match the the frequency of my HXDA. I’m getting extremely convinving results! Thanks all for the help!
Would be cool if you could share the presets,when you are done :) ?
 

marshall2553

Experienced
I've never owned a Plexi, but for some reason I always had the impression that the real thing didn't have nearly as much gain on tap as the Plexi 100 and Plexi 50 models do. Well I just bought a fairly nice 50W Plexi clone and I was shocked at how much gain it has. It pretty much sounds identical to the Plexi 50W Hi model with the same input drive/volume settings.
 

FractalAudio

Administrator
Fractal Audio Systems
Moderator
I've never owned a Plexi, but for some reason I always had the impression that the real thing didn't have nearly as much gain on tap as the Plexi 100 and Plexi 50 models do. Well I just bought a fairly nice 50W Plexi clone and I was shocked at how much gain it has. It pretty much sounds identical to the Plexi 50W Hi model with the same input drive/volume settings.
They're all over the map. There are several factors:

1. Existence of bypass cap on V2 (in parallel with the 820 ohm cathode resistor). If the cap is there there's about 6 dB more gain.

2. Feedback resistor and tap. Our 50W reference amp has 100K from the 4-ohm tap IIRC which is very little feedback. Some had 100K from the 8-ohm tap which is a bit more feedback. Some had 47K from the 8-ohm tap which is even more feedback. Finally some had 47K from the 16-ohm tap which is a lot more feedback. Some even had the feedback off the speaker jack so the feedback was dependent on the speaker impedance (Friedmans do this). The more feedback the lower the gain. The Negative Feedback parameter allows you to change the model's feedback. Turn it up and you'll notice the gain decrease and it will get more "open" but the distortion will get "harder".

3. Power supply voltage. The lower the supply voltage the more "gain" the amp has. Actually the gain is mostly independent of the voltage but the tubes clip sooner so effectively you get more distortion as the supply voltage is decreased. Some Plexis had voltages well over 500V. Our reference 50W Plexi is about 400V IIRC which is quite low. It might even be less than 400V, I'd have to check the notes.

4. Output Transformer. These varied over the years as well. The higher the turns ratio the sooner the tubes clip. Some 50 watters had primary impedances close to 4K while others were around 3.2K. Our reference 50W is around 3.5K. The Transformer Match parameter allows you to adjust the relative turns ratio.

5. Power Tubes. Some had KT66s, some had 6550s, some had EL34s. The transconductance varies quite a bit between these tube types.
 

marshall2553

Experienced
They're all over the map. There are several factors:

1. Existence of bypass cap on V2 (in parallel with the 820 ohm cathode resistor). If the cap is there there's about 6 dB more gain.

2. Feedback resistor and tap. Our 50W reference amp has 100K from the 4-ohm tap IIRC which is very little feedback. Some had 100K from the 8-ohm tap which is a bit more feedback. Some had 47K from the 8-ohm tap which is even more feedback. Finally some had 47K from the 16-ohm tap which is a lot more feedback. Some even had the feedback off the speaker jack so the feedback was dependent on the speaker impedance (Friedmans do this). The more feedback the lower the gain. The Negative Feedback parameter allows you to change the model's feedback. Turn it up and you'll notice the gain decrease and it will get more "open" but the distortion will get "harder".

3. Power supply voltage. The lower the supply voltage the more "gain" the amp has. Actually the gain is mostly independent of the voltage but the tubes clip sooner so effectively you get more distortion as the supply voltage is decreased. Some Plexis had voltages well over 500V. Our reference 50W Plexi is about 400V IIRC which is quite low. It might even be less than 400V, I'd have to check the notes.

4. Output Transformer. These varied over the years as well. The higher the turns ratio the sooner the tubes clip. Some 50 watters had primary impedances close to 4K while others were around 3.2K. Our reference 50W is around 3.5K. The Transformer Match parameter allows you to adjust the relative turns ratio.

5. Power Tubes. Some had KT66s, some had 6550s, some had EL34s. The transconductance varies quite a bit between these tube types.
Out of curiosity, what year were the 50W Plexis you based the Hi 1 & 2 models on made? I don't know the details, but the clone I bought is supposed to be based on a '68 (12000 serial number range). But knowing Marshall the components they used varied based on what they had on hand from day to day.
 

wildolympic

Inspired
Try wiki axe fx there is a page that shows axe settings VS real amp settings, I think it's under Yeks guid to the fractal audio amp models.
 
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