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Re: No CT on PT, how to lower B+ ????


 
1/17/2004 5:22 AM
Dan McCue
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Re: No CT on PT, how to lower B+ ????
Ian, I know this might not be what your looking for, but it might be time to do a cathode biased hybred. If you wanted to experement with the nfb, with less, you could compensate with the degen efects of an unbypassed cath res. or just do one bypassed if not.  
a thought,hth  
 
DM
 
1/17/2004 9:47 PM
Ian Anderson
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Dan,  
 
Cathode biased in an amp pushing 600v on the plates - even around 550v by the time these 'mods' are done. Is that possible?  
 
... Ian :)
 
 
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1/18/2004 2:03 AM
Dan McCue
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Sorry Ian, I think you misunderstand me, a Hybred incorperates fixed and cathode biasing. If your tubes are getting chewed up by the voltage on the plates, reference them up from ground with an additional cathode resistor. Depending what you are looking for, you could choose to bypass the cath res or not.  
 
DM
 
1/18/2004 2:59 PM
Ian Anderson
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Dan,  
 
I get what you mean now - a small resistor off the cathodes of each EL34 at around 50ohms per tube, right? What value would you use for a single cathode resistor with fixed bias?  
 
I'd imagine you'd not want to use a bypass cap as big as a standard cathode biased amp (i.e. 220uF). Maybe something like 50uF (?)  
 
... Ian :)
 
1/18/2004 6:43 PM
Dan McCue
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There were a few people on here who realy got into that, and said they had found the right value for their amp to give a good mix between the two, This is not my forte'. I would recomend trying a few, same goes for the cap (or none at all) it depends what you want in the end.  
Good luck Ian.  
 
DM
 
1/18/2004 7:47 PM
satamax
Ian, one word of caution, some people were saying that the fenders which were biassed that way, were bad sounding.  
 
I think you could try the zenner trick, may be! You know the one with the fet and a low power zenner!  
 
Best regards.  
 
Max.
 
1/19/2004 3:43 AM
markh
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The only problem with the stud-mount zeners is that they expect to be heat-sinked to get near 5W in practise, and therefore serialising them (i.e., separate non grounded heatsinks) can get a little hairy mechanically.  
 
Make a "super zener" using a small zener and resistor, and a high voltage NPN TO220 or TO203 transistor which you can bolt directly to the chassis. This is cheap and rugged. There used to be a PDF on the Aiken site describing how to do this, but I can't find it there now. I can send it to you offline if you like.  
 
--mark
 
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