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Gain with Plexi style amps

guitarcal

Inspired
Quick question for you guys!

I own a few real plexi amps including a '71 50 watt, Prs HxDa (similar to a late 60s plexi), plus a few others. I pair with a 4x12 loaded with greenbacks.

What I have noticed is in the Axe Fx, if the gain knob is set similar to my real amps, the Axe Fx amp model has way more gain.
For example, for my real amp, I might have the gain set at a 8/10 whereas to get a similar tone in the Axe Fx I have to place on a 4/10. I should mention the Master Volume on the axe fx model and real amp are both set to 10.

Any thoughts to why this is? If I disable the bright cap in the axe fx, the gain is drastically reduced and is closer to my real amps, but in my real amps the bright cap isn't clipped. I believe my '71 has a 5000pF bright cap installed.

Also, I can adjust the input trim of the axe fx model and it more closely matches the real amp, but thinking that shouldn't be necessary.
 

muudrock

Experienced
I've had a few older plexis and they were all different from each other in terms of sound, feel and gain on tap. The input trim and gain controls can get you what you need.
 

guitarcal

Inspired
Plexis vary greatly. Some have cathode bypass caps, some don't. The models match our reference amps which apparently are different than your amp.
Thanks for the quick responses. Are there any other advanced settings you recommend I tweak to more closely match my reference amps? I know the bright cap value can be adjusted, wasn't sure if any of the other caps can be tweaked?

I'll pull my amp apart this weekend and take some measurements, I'm curious if that cathode bypass cap is installed and if so, what the value is.

Thanks!
 

simeon

Axe-Master
check what preamp tubes are in there and select similar in the axe. beyond that, it's difficult to know. you may have more idea once you've done some measurements.
 

muudrock

Experienced
Additional settings that work for me.....I typically set Variac setting to 97%, set output comp up a little, increase Negative feedback a bit. I like using default speaker comp settings too. I pretty much always run the mids high and set the Bass, Treble and Presence to the IR I'm using-typically a greenback or the like.
 

guitarcal

Inspired
What’s wrong with just setting the gain on 4 if that’s the tone you like?
Great question! Its depends on the model, but usually with gain on 4 and bright cap enabled, the amp is too bright and harsh. If I remove some of the highs and mids, the tone gets dull and no longer has the high end plexi sizzle.

check what preamp tubes are in there and select similar in the axe. beyond that, it's difficult to know. you may have more idea once you've done some measurements.
12ax7s, so should be adding the correct amount of gain there
 

FractalAudio

Administrator
Fractal Audio Systems
Moderator
Great question! Its depends on the model, but usually with gain on 4 and bright cap enabled, the amp is too bright and harsh. If I remove some of the highs and mids, the tone gets dull and no longer has the high end plexi sizzle.
Turn the Drive up to where you like the tone and turn down the Input Trim. That's what it's there for.
 

Hotplate

Power User
Turn the Drive up to where you like the tone and turn down the Input Trim. That's what it's there for.
This is what I do, along with adjusting the bright cap to what I need. I find the plexi/vintage Marshalls modeled tend to have more gain than I'm used to as well, but the Axe allows you to adjust for this no problem.
 

Tahoebrian5

Fractal Fanatic
Stupid question regarding the bright cap. If the bright switch is off does it defeat the bright cap altogether?
 

Semih Yanyali

Power User
Turn the Drive up to where you like the tone and turn down the Input Trim. That's what it's there for.
I dont wanna hijack this post but i think the issue is solved and with my appologies i would like to ask about smth else, is there a reason why we dont have modifers available for "fat" and "overdrive" ?
 

guitarcal

Inspired
Turn the Drive up to where you like the tone and turn down the Input Trim. That's what it's there for.
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!

I dont wanna hijack this post but i think the issue is solved and with my appologies i would like to ask about smth else, is there a reason why we dont have modifers available for "fat" and "overdrive" ?
+1, I second this question!
 
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