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5Y3 vs. 5U4?

3/1/2005 9:51 AM
5Y3 vs. 5U4?
I need a better understanding of the differences between these two rectifier tubes - I know they both drop 50 volts, but the current issues have me confussed. Should I be concerned about burning something up with one vs. the other? I have the 5Y3 in my 5E3X2 amp right now and it sounds great. Is it burning something up slowly or causing a future problem? Should I switch to the 5U4? Will there be any sag differences? Any help is greatly appreciated.
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3/1/2005 10:37 AM
Bruce /Mission Amps

There will be a large difference in B+ because, even though the tubes have similar voltage drops, they occur at two totally different current levels, with the 5Y3GT dropping it's 50v at a MUCH lower current level then the 5U4GB.  
The 5Y3 is rated at this 50v drop with a 125ma load and the 5U4GB is rated at this 50v drop with a 275ma load!!  
That means with the 5U4GB in an amp and only a 75ma to 90ma load, the B+ will be much higher.  
The amp will never load the rectifier tube down enough to see a 50v drop.  
The can lead to "overvoltaging" a smaller low voltage power tube and causing too much current to flow in the tubes at idle possibly damaging them.  
3/1/2005 11:16 AM

So Bruce, if I'm understanding you correctly, I should go with the 5Y3 in this circuit (5E3X2) vs. the 5U4?
3/1/2005 4:26 PM
Bruce /Mission Amps

I used a NOS Svetlana coke bottle Russian 5U4GB.  
The plate to cathode voltage was just a little high. But OK with the JJ6V6 tubes.  
The 5Y3GT is overloaded with two preamp tubes and four 6V6s at +35ma each.  
Another decent tube is the NOS 5R4.  
What do you have now for B+ and idle current of the power tubes?  
3/2/2005 8:41 AM

I'm gonna take the back off tonight and check everything. While I'm checking, (I'm showing how much a novice I really am for asking this) how do I know what the "right" value is for my cathode resistor? I have a variable one installed that I can set anywhere. Right now, it's set to 185 ohms, but I just based that on what I knew it "should" be around. What voltages, calculations, etc, do I need to check to get the right value?  
BTW - I've got a NOS 5R4WGB on it's way just to play it safe.
3/2/2005 10:36 AM
I had a low B+ issue on my 5e3x2 with the WY3GT so on Bruce's recommendation I put in a WR4. Did the trick. Plate voltage to ground is around 380V if I remember correctly.  
I posted the voltages with each in an earlier thread f anyone wants to search.  

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