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Re: Cathode Bias on Matchless Amps


 
8/23/1999 3:21 PM
pgambon
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Re: Cathode Bias on Matchless Amps
It's going to take me awhile to digest the info in Doc's reply! As for some practicle application, let me tell you about my "champ". I had an extra 12ax7 in the chassis, so I wired both tubes in parrallel using stock values (100k/1.5k). The sound is as close as I've gotten to Cream era Clapton or ZZTop (think Rythemean). This amp won't do clean. Happy accidents can be good!
 
8/24/1999 5:44 PM
Steve A.
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... so I wired both tubes in parallel using stock values (100k/1.5k)  
 
 
 
    So did you run both halves of the two tubes in parallel, or just one of the halves? Did you double up the plate and cathode resistors, or just use one 100k and one 1.5k resistor, and jumpered the respective tube pins together?  
 
 
 
Thanks!  
 
 
 
Steve Ahola
 
 
  Wednesday
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8/25/1999 2:15 AM
pgambon
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I paralleled both tubes with one 100k and one 1.5k per tube (25uf/25v on first stage). Now for the really sick admission, I have a switch on the first cathode to give me a "marshall" tone, using a 2.7k. Some my say that is over doing it, but it works good! If you try this let me know. thanks pg
 
8/29/1999 4:45 AM
SpeedRacer
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in the old Mullard book, their 20w EL34 circuit uses seperate cathode bias resistors explaining that this particular setup does the best job of balancing DC currents in the transformer primary.. why, I dunno. Being that they are Mullard (were) and they designed the tubes, I would take their word for it. :-) It is an interesting statement though and I am looking for some additional info on it.
 
8/30/1999 3:27 AM
Bruce
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Is that the "grey book"?  
 
Bruce
 
8/30/1999 3:36 AM
SpeedRacer
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I guess it is kinda grey.. :-)  
 
"Mullard Tube Circuits For Audio Amplifiers" reprinted by the Audio Amateur Press. That the one you're thinking of?  
 
P#32 is the mention of the bias arrangement on the 20W EL34 amp.  
 
 
 
PS: For all the newbies out there, this book is not only PACKED with killer tech info, but also *complete* plans to build pre-amps, power amps and the chassis.. a real "how to" book. It may be the best of its kind in this respect.
 
8/30/1999 4:00 AM
Bruce
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Yup, that's the one.  
 
Very good book with lots of read between the lines stuff for us old tube hackers.  
 
Bruce
 
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