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Marshall JMP SuperLead-Early '70s


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5/24/2000 12:03 AM
Matt
Marshall JMP SuperLead-Early '70s
Cool new section!  
 
I was thinking I should bounce this off ampagers. I have a basket case JMP 100W that had been truly hack butchered. After gently repairing the PCB and totally rewiring and recapping the power supply, it's almost ready to go.  
 
I have a replacement power transformer for it which I just installed onto the chassis, but not into the circuit. It's a 400-0-400 with a bias tap. My PS caps are two 100 F/500V in series with 5 W 220k bleeders.  
 
Questions:  
 
Should I wire standby per regular 1959 with secondaries being open when in standby mode or just lift the center tap? I'm concerned about too high a voltage on the switch.  
 
Will lifting the CT have any adverse effects?  
 
Has anyone toyed with standby alternatives, such as a 10K or 20K resistor in series with CT when in standby mode to let the caps charge in a more controlled fashion? Any real advantages to this?  
 
I plan on running the amp in 50 W mode, so the plates are going to see up to 550V possibly with this transformer. What are some gut opinions on the following options: a) Just go with the power as it is supplied(using good EL34s, of course), b) Do the Zener diode mod to reduce voltage 50 or 100V, c) Get a lower V transformer.  
 
Cheers,  
Matt
 
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5/24/2000 3:06 PM
Peter S

550 volts if pretty high voltage, but I've seen many amps that work fine with 550 volts and up on the plates. I personally feel a little more comfortable at around 450 volts, but EL 34's should be able to take 800 volts on the plates. I wouldn't try to use sovteks, siemens at those voltages however. If you want to keep the transformer you now have, you could either try to find some Mullards or use Svetlanas and see how they hold up. I think I would look for a transformer that would lower the voltage to about 450, if the amp were mine. I've seen qiute a few old Marshalls that sound great at 450 plate volts. My 50 watt Marshall copy runs at 450VDC on the plates and it sounds teriffic. I tried dropping the voltage to 390 with a zener, but the amp sounded lifeless and lost much of it's great dynamic character.  
 
As far as the stanby switch goes, I always wire it in series with the HT.
 

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