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100w Marshall PT, how do I hook it up in a humble clone

8/28/2000 12:34 AM
Tom 100w Marshall PT, how do I hook it up in a humble clone
I building an amp using an original 100w MarshallPT,  
my questions are ,do I need to connect the yellow tap that is supposed to go to between the two 100uf caps? and should I or can I use say 220k bleeder resistors on the caps like on fender.  
Can I use a "Blackface Fender type" rectifier instead of the bridge ? If so how should i connect the yellow centertap that is supposed to go between the 100uf caps  
I also believe my PT has common centertap ground wire for bias and heaters,but the humble schems that supposed to have these 100w Ma PTīs have 100ohm resistors to ground, and also fender style recifiers.  
my PT has the standard 100w colorcode but it has also a green tap on the primary side that is not shown on the 100w schems I have seen (in the G.T book)  
My goal is how to use it in a Humble type 100w  
anyone ?
8/28/2000 3:42 PM
Gil Ayan
Does not Depend On Amp Desing
There is pnly one way to hook up a power transformer correctly, Tom. So, if your tranny needs a full bridge in the Marshall, it will need the full bridge rectifier in the D-clone or whatever other amp you want to build.  
Regarding the wire that goes between the two halves of the first capacitor, you can connect it or leave it unused. The former is supposedly better, but I have used a Marshall PT where I just decided to not use that extra wire and the amp was no noiser than with the thing connected.  
Regarding the center tap for the heaters, once again... use the iron the way it is supposed to be used. You can use the center tap, or use a hum balance arrangement (100 ohm resistors), or BOTH if you'd like. (Yes, I have checked that too, and all three options seem to sound the same to me in terms of hum).  
The only thing I'd say is: whether you are building a Dumble cloner or a Fender, the transformer will be fine. HOWEVER, the Dumble amps usually have "conservative" voltages on the plates of the power tubes (say 450VDC or so), whereas your Plexi iron will generate 500+VDC on the plates. So, that's going to change the sound and there is not much you can do about that. I would encourage NOT using 6L6s with that transformer too, since that voltage is just too high for 6L6s.  
Good luck,  
8/28/2000 11:07 PM
Thanks for your reply Gil !  
The Pt I am using is not a plexi, it is from a late 70s 100w amp and it has more like 465v, so I think Nos 6l6 tubes will work,  
the bias-supply and heaters share center tap on this one so I guess I have to stick with the original Marshall design then, but still why is the "later"Humble schem showing a fender type rectifier and grounded center tap when people saying these late El34(6ca7) models were using Marshall 100w PTīs ? any thoughts on that Gil? Just curious.  
8/29/2000 9:14 PM
Gil Ayan
Marshall OT, Not PT
Tom, the later ODSs were using TF170 Marshall type OTs from Magic Parts. The PTs were Fender Twin style -- and don't ask me why, since the amps use EL34s and draw quite a bit more from the filament supply than a Twin would.  
8/29/2000 10:52 PM
Ohh well I guess I have been misinformed ! Thanks for the correction!  
I have hooked up the Marshall PT as the original Marshall schem, and now it gives around 457v on the &l6 plates .  
Thanks for your help !  
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