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getting "sag" with resistors


 
8/26/2000 5:44 PM
Josh
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getting "sag" with resistors
I really wanted some "sag" when I built my amp. I think I made my power supply a little to good to get any though.  
this is a 5E3 type amp. the power transformer is the hammond 270fx (275-0-275 @150 mA) after the first filter cap there is a hammond choke(5H)  
anyway, I have heard that you can wire a resistor in the b+ line of SS rectifiers to simulate "sag". I am wondering if I could do the same with a tube rectifier. would this work to give me some sag? what sould the resistor's value be? I have a 220 ohm 5watt; would this work?  
Josh
 
8/26/2000 9:56 PM
Graywater
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Josh,  
 
This has been covered in depth before, but....you can insert a tube rectifier after the SS rectifiers to get sag. Since there are oodlin's of old TV damper diodes out there for a humm (much less than a song) you can use one of these w/o a 5v rectifier supply as long as you've got a couple of amps at 6.3v to play with.  
 
GW
 
8/26/2000 11:17 PM
Josh
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Oops, I guess I didn't mention it, but this amp has a tube rectifier (5Y3). but I am not getting any sag because the PS is too strong.  
Josh
 
8/30/2000 11:17 PM
Randy Jamz
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Start with 100-ohms @ 10 watts. Then run the amplifier into a dummy load and measure the voltage drop across the resistor and the current through it with a signal applied . You may get by with a lower power rating, but I doubt it. These values work for me.  
P.S. Be sure to put the resistor in series with the rectifier and the first filter capacitor. You could also use lower value capacitors in the power supply to soften things up. If you have 40uF or higher, try 20uF.
 
8/31/2000 2:36 PM
Graywater
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Josh,  
 
How much current capacity does your rectifier filament winding have? If you've got at least 3 amps you could try a 5U4 and the 5U4B as these tend to have more voltage drop across them. Your mention of the "strength" of the power supply is confusing. The "sag" is a function of the resistance of the rectifier at the current drawn by the rest of the amp and the capacity of the first filter capacitor. If your filter cap's too large it will limit the "sag" but the current capability of the PT doesn't come into play until you draw lots of current and if you're not getting much drop across the recitifier you're not drawing very much current.  
 
 
GW
 
8/31/2000 4:41 PM
Josh
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what I meant by "strength" is that the PT puts out 150mA- more than plenty for a 5E3 circuit. I do have 3A on the rectifier. but I had always thought that tubes like 5U4 and 5AR4 had less drop and that a 5Y3 would be the "saggiest".  
I did have pretty good luck mounting a 220 ohm resistor on the rectifier tube socket in series with the B+. It gets pretty warm, but its still alive.  
Josh
 
8/31/2000 5:36 PM
Graywater
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Josh,  
 
The B supply current available from the PT still has to pass through the rectifier and it's your "gate keeper." Comparing rectifier voltage drop is somewhat tricky. Ignoring difference between different manufacturers, tube manuals don't specify the voltage drop at 100 mA, for example. Instead you have to look at the curves and guess. Perhaps we could get an Ampager to conduct tests in a couple of amp power supplies on various rectifiers and report their findings (would still have a small sample size of each rectifier and somewhat skewed results).  
 
Still, if you've got the 6.3v current capicity you could add a damper diode in series for more "tube sag."  
 
GW
 
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