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extra preamp stage for Marshall


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9/23/2000 6:37 AM
mills extra preamp stage for Marshall
I was thinking of putting a gain stage in a superlead on channel I. There is an extra hole for a preamp (super trem chassis). I'm looking for information on what values of resistors to use for the plate, cathode etc... I'm thinking of using the same values in the orignal first stage then tapping of a 1 meg pot into the next stage, using this pot as the gain control. Any ideas?  
 
mills
 
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9/23/2000 6:50 AM
Don Symes

Assuming you have the heater current to spare...  
 
it might be interesting to slip a Dumbleoid OD stage (and post OD EQ) in there.
 
9/23/2000 3:33 PM
Carl Z

Don;  
 
Been there, done that, sounds like shit! :(  
 
It'd probably improve a lot if you were commited to the topology and were willing to fiddle with component values till you hit the right combination  
 
Carl Z  
http://www.summitamps.com
 
9/24/2000 4:04 AM
Don Symes

I figured somebody had and was curious.  
 
Any advice for our original querant?
 
9/24/2000 2:08 PM
Trace
Extra gain stage, plexi's, Master volumes..long winded
I know where Carl's coming from and I agree with him. It's hard to suggest an extra gain stage for a plexi for a few reasons. What tone are you looking to get really close to? Is there a Master Volume on the plexi now? If not try a post PI Master Volume and you might be surprised (most people are) at just how much gain is in a plexi's preamp.  
 
Now with all that having been said...are you a fan of the PI breaking up? If not and you add another gain stage you will be dancing on some pretty funky ground. "Yes" you'll have more gain with an extra stage but in my opinion if you don't have a master (ala-JCM800 style) the PI will start breaking up the more you turn the amp up (non-master) and you'll end up with mush/mud and an over all hard to control amp.  
 
People who want to add another gain stage typically want to hear more preamp gain and hence the need for a master volume that's connected off from the treble pot because you really don't want the PI to break up when you have an extra gain stage in a plexi. It's a pretty ugly sound in my opinion.  
 
 
It's pretty early in the AM and I've been working about 70 hours a week so I hope this all makes sense (no coffee yet either--ha, ha)  
 
MASTER VOLUMES: There's always a lot of talk about what master volume is the best and to be honest I was also a "master volume hater" for a very long time. I'm only going to talk about the plexi for this post (just we're all on the same page--ha, ha).  
 
Most Hard-core-tone-nuts hate master volumes on vintage amps. I can totally relate to where you are coming from! Most techs install the JCM800 style (master connected off from the treble pot) when it comes to vintage amps.  
 
I should preface all this by saying "there is no right or wrong" when it comes to tone. Now having said that (ha, ha) vintage-tone-nuts are NOT hip to that sort of master at all (this would include vintage Fenders as well) for two main reasons  
 
1.) The PI doesn't breakup & when you are used to hearing that you WANT to hear that and the JCM800 is not going to work for that and that's where people complain about the "buzzy" preamp sound of a master volume.  
 
The Post PI MV that uses the dual ganged pot (100K or 250K) is in my opinion...the best sounding master for guys who want to control the volume on stage (or at home) but want the same tone. This includes the tone of the PI which is critical to a plexi. It's a trade off as far as power tube break up but you'll be surprised at how good this master volume sounds at lower volumes compared to oehter masters. Low-end still comes through which is a good thing! (ha, ha)  
 
2.) When it comes to EL34's tone-nuts want to hear the EL34's breaking up. This is where the sacrifice or comprise comes in.  
 
Plexis were not designed for small clubs, they were designed at a time when you needed an amp to throw some serious volume into an arena!! People didn't mic guitar amps back then so it was pretty important to crank the holy bejesus outta a 100 watt amp if you wanted to be heard in a live situation.  
 
Well times changed (as they always do--ha, ha) and now we WANT to hear a plexi on at least "5" or "8" and that's a little bit hard to do when most clubs only hold about 150 to 200 people. Heck even a 500 seater is too small to be cranking a plexi! (ha, ha)  
 
So seeing a need for power tube breakup while still having the volume low...enter the power attenuators. Vintage-tone-nuts hate them because "it still doesn't sound the same as when the amp is cranked."  
 
"Yes," and "No." The Power Brake and Hotplate do sound pretty much the same and they reproduce the tone pretty accurately, HOWEVER, what most people hear as "different" is the fact the speaker is not getting pounded as hard and that of course means the speakers are not breaking up (assuming they are vintage low wattage speakers).  
 
So the saga goes on... (ha, ha) Anyway, for people building plexis I would just like to say that a great deal of the plexi is the output transformer (as far as comparing the originals to the 70's or the reissues).  
 
Have fun building!!!!  
Trace
 
9/24/2000 5:26 PM
Peter S

Trace,  
Not bad for not having had your coffee yet. As one of those vintage tone whackos, I gotta say, you hit it on the head big time. That's why for a Plexi Marshall type thing, I have to reccommend power attenuators. It does change the sound, but with a little eq and some finesse, a power attentuator can yeild excellent results. My favorite is the Hotplate, but there are others that work well too.  
 
Peter
 
9/24/2000 8:26 PM
Richie{~}==:::

Trace: So many miss that point with the attenuators, you don't get the speaker breakup and that is a big part of the whole picture. You can get the sound of the amp,but loose the sound that is added when the speaker breaks up. Very good point.  
Richie{~}==:::
 

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