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Re: thanks Bruce for response


 :
7/28/2003 10:09 PM
Newt
Re: thanks Bruce for response
I understand why the plate voltage is lower than the screen; I just don't understand why it's designed that way.
 
7/29/2003 5:35 PM
Bruce /Mission Amps

Because the DCR of the OT, used with only one tube, is not high enough to cause the screen and plate voltages to be way way off, it's probably not a big deal, not good but not really destructive Newt.  
Looking later in years, you'll see that amp designers finally decided that the screens really did need to be at or below the plate voltage, possibly in order to keep the tube life up.  
In any case, using a bigger and better OT with lower DCR and at least a 510 to 5000 ohm 2w resistor before or after the choke... but before the screen tap in his amp so screen and preamp current has to flow through it, would probably solve many of the voltages and current problems.  
 
Bruce
 
7/30/2003 3:45 PM
Newt
DCR of trannys & chokes
Bruce, once again I thank you for your patience.  
 
A couple of questions:  
 
When calculating current flows through power supplies and tubes, for some reason I've never felt comfortable using voltage drops across chokes and OT primaries to derive current. I've somehow had it in my head that the DC resistance of inductors isn't a constant. Have I got this wrong, ie. it will be constant with a constant(DC only) current?  
 
Is there anything wrong with putting a small(say 10 ohms) resistor between the rectifier and first filter component(choke and/or cap) to make it easy to get a reading on total B+ current draw?
 
7/30/2003 8:39 PM
Bruce /Mission Amps

No, a 2 watt 10 ohm resistor is probably OK.  
If you mesured .5vdc (500ma of current) across your 10 ohm resistor it will need to be upped to a 3 to 5 watter.  
The DCR of a good choke or OT is pretty constant even at higher current levels so it's not a bad thing to use for measuring. Only thing is, they are connected to high voltage so you have to be carefull.  
Keep in mind though, every current using device downstream, will be drawing current through any DCR that is closer to the source, the rectifier.  
So the voltage drop across your 10 ohm resistor will be caused by everything, including the little 1ma to 2ma of the very first triode in the preamp.  
 
Bruce
 
7/30/2003 9:36 PM
Newt

Thanks, Bruce. I'm not normally measuring current directly, but dividing the voltage drop by the resistance. In the simple cathode-biased amps I've worked with so far, I've found I can trace the current through the circuit from the rectifier to the cathode grounds. If I have enough resistors and measure them accurately(not using stated values), the currents will add up. Relying on the choke & OT resistances will just give me more to go on.
 
7/31/2003 4:08 PM
Bruce /Mission Amps

quote:
"If I have enough resistors and measure them accurately(not using stated values), the currents will add up. Relying on the choke & OT resistances will just give me more to go on."
EXACTLY! Ohm's Law actually works! :)  
 
Bruce
 
7/31/2003 5:06 PM
Gary Gustin
Re: added extra 5y3 and 6v6 to 5f2 ...problem?
uhhhhhh...........huh huh huh uhhhhhhhh.......huh huh huh (Beavis and Butthead style) Like, whats a good electronics/ amp book to buy, or something?  
I'm totally a rookie when it comes to this stuff. Is a Kevin O'conner tut a good book for a beginner?  
I pretty closely followed the Angela Instruments directions from their website, and they even give the impression that a Hammond 125e will work - they recommended the 125E in the single 6V6 project  
and inform that it can be used in the super 6V6 amp as well( the parallel 6V6's). Their test readings for the super 6V6 with a 125ESE was:  
plate to cathode= 339VDC  
screen to cathode= 348VDC  
Their screen measurement is higher as well. ????  
Also, is a Hammond 272DX a good choice for a tweed deluxe? Might try making one of those.  
Thanks everybody for helping me out.  
 
 
Gary
 

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