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Vintage threads from the first ten years

Re: How 'bout a 5Y3 ?

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8/27/1999 6:32 PM
Mark	Re: How 'bout a 5Y3 ? I agree on the 5v4 but I believe the 5r4 will drop your voltage as much or more than a 5y3 although it is a more rugged tube than the 5y3.

8/27/1999 8:34 PM
Michael Cameron	I have found 5R4's dirt cheap at hamfests and have bought used, tested excellent ones on ebay for a couple bucks each. Michael

8/27/1999 9:09 PM
Trace	Cool! I'll have to keep an eye open for those hamfests. I have a 5R4WGY in an amp now....It's a huge thing! Trace

8/29/1999 5:02 AM

I installed the 5ar4 and rebiased to 26.5ma and the amp sounds great. Plate voltage is at 422 but drops to 380 when played. I tried biasing to 30ma and got the voltage down to 415 but I didn't like the sound. The tubes run hot at that bias but they dont seem to be overheating. They are NOS RCA's. I tried the 5v4 but I didn't like the sound. It was lacking richness that the 5ar4 had but the voltage dropped to 412 with the bias at 23 and would sag like the 5ar4 to 380v. It would do in a pinch but If I have the choice its the 5ar4. I didn't even try the 5y3 cause of what you guys said about the mushiness. If I start to go thru tubes I can always go back to the 5u4.

M/J

9/2/1999 1:05 AM

OK Guys, I just tried something else that all of you all have talked about. That is the installing of 1000 ohm screen resistors in place of the 470's that were in there. This is supposed to prolong tube life but it did something else that I don't understand and was hoping that one of the gurus would be able to explain. 1) Just how does this mod prolong tube life? 2) Why did it drop my plate voltage from 422 to 414v and it also dropped my bias from 26.5ma to 25.3ma? Now I am right according to spec on the schematic and the amp sounds better than ever. 3) Why didn't Leo put 1000's in there to start with? 4) Is there any particular wattage ratting that you recommend? I put the 1 watters in there. Thanks again guys.

M/J

9/2/1999 1:37 AM

I use 5W for the 1K screen resistors, just habit and that's what I use for the Marshalls. (fewer parts to order!)

It prolongs tube life by limiting screen voltage on big swings. While we worry about plate dissipation, the thing we often can't track so well is how hot the screens are getting. The most common tube failure mode (IMO) is the screen shorting.. it is after all, only a wire swaged in place on a couple posts. It gets hot, sags, and boom - you're outta biz. Keeping the screens cool is a great way to keep your tubes alive. The screens can glow bright orange on overdrive (it's easier to see on EL34's where you can peek through the plates) check it out.

Glad you've found a good setting for your DR btw!

9/2/1999 2:09 AM

Why did the plate voltage drop?

I can see the screen voltage dropping a a volt or two and the idle current dropping a bit too (if the measurement was done with a cathode resistor method).

The plate voltage dropping is really odd to me.

With less current demand, I would think the B+ voltage would rise very very slightly. Not an 8 volt change though and that looks like yours went the other way anyhow.

Bruce

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