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Are my ears lying? Marshall content.....


 
5/31/2000 1:13 AM
Kevin Hamilton
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Are my ears lying? Marshall content.....
Hi all,  
 
I picked your brains about four months ago concerning a Marshall 73 superlead.  
 
I put a Hoffman board in and replaced the bad output transformer with one from Mercury Magnetics. In addition, I used the unused polarity switch location on the front of the amp for selecting between different negative feedback. Either 27k @ 16 ohm, 47k @ 8 ohm, or 100k @ 4 ohm.  
 
Also, I used one speaker output jack location as a switch for either split or shared cathode on V1 and one other jack location to put in the .68 cap on V2 or take it out. The amp will cover ballpark JTM45 tone to Plexi to 70's Superlead by the use of these 3 switches.  
 
I've been playing it for 3 months now and channel 2 seems just a little too bassy. Anyway, I replaced the 820 ohm resistor/330uf cap with a 1k resistor/220uf cap.  
 
To me, it reduced the bass. Am I right electronically or is it just my imagination?  
 
Also, I want to make sure that doing this is OK electronically and I won't hurt the amp.  
 
I also replaced the V2 .68 cap with a 1uf cap. I heard it will give just a hair more gain. Is this true, and again, will it hurt the amp?  
 
One last thing. I sprung for 2 Mullard 12AX7's. These tubes are incredible.  
 
Thanks again,  
 
Kevin
 
5/31/2000 2:19 AM
Peter S
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Kevin,  
changing the cathode resistor to 1k/220uf on v1 will sound ever so slightly slightly less bassy....most people wouldnt be able to hear the difference though. Raising the resistor value raises the lower-3db point. On my amps I change the cap to 2.2uf with the 820r resistor. None of the changes you made will hurt your amp. Lowering the .68 bypass to 1uf doesnt actually increase the overall gain of the stage, but it will drop the lower -3db point downward slighlty. This has the effect of increasing the lower mids slightly more than the .68uf. Again the 2.2uf will extend that even further into the low mids without muddying up the bass. The .68/820ohm combination gives you a lower-3db point of about 440hz. The 1uf lowers this to about 290hz....the 2.2uflowers this to about 130hz.
 
5/31/2000 6:11 PM
Tonefactor
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quote:
"I've been playing it for 3 months now and channel 2 seems just a little too bassy. Anyway, I replaced the 820 ohm resistor/330uf cap with a 1k resistor/220uf cap."
 
 
Try putting a 220pf or 470pf cap across the 470k mix resistor on the bass channel. This might give you what you're looking for.
 
5/31/2000 6:48 PM
Rebel420
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quote:
"Try putting a 220pf or 470pf cap across the 470k mix resistor on the bass channel. This might give you what you're looking for."
SHHHHHH don't give away my secrets!!! *laugh* seriously, though, since i never 'jumper' channels, doing this gives me the variance between the 'grindy' superlead' and a mellower SuperBass or JTM type sound...  
lowering the Ck from the 330uF WILL brighten it up a bit as well tho ;)
 
5/31/2000 6:13 PM
Tonefactor
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quote:
"I also replaced the V2 .68 cap with a 1uf cap. I heard it will give just a hair more gain. Is this true, and again, will it hurt the amp?"
 
 
I can barely hear the difference between a 1uf and a .68uf cap. The 1uf cap will add a touch more lows.
 
5/31/2000 7:43 PM
Trace
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quote:
"To me, it reduced the bass. Am I right electronically or is it just my imagination?"
 
 
You are right! It did reduce the bass a touch. Peter had some cool ideas!! a 250-500pF cap across the mix resistor will brighten up that channel. I would try a different bypass cap if it were me. Personally speaking a 22uF will usually give you all the low-end you'll need. You can use what ever sounds best to your ear and you'll be fine. It's all personal preferance really!!!  
 
Trace
 
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