Both Russian and Chinese current production 5AR4s have weaknesses. The Russian stuff can deliver the rated 250 mA. of B+, but get into trouble at the high end of the rated voltage. OTOH, the Chinese tubes definitely can take Volts. However they get into trouble trying to deliver 250 mA.
Perhaps the best thing to do is use current production Russian 5AR4s and protect them, voltage wise, with UF4007s in the lines leading to the anodes.
Perhaps the best thing to do is use current production Russian 5AR4s and protect them, voltage wise, with UF4007s in the lines leading to the anodes.
Would the diodes be in series with the 50R resistors that I need for the series resistance factor?
So do JJ's. The problem seems to be an uneven cathode coating. The cathode heats up quickest where the coating is the thinnest. This area will try to conduct all of the current while the rest of the cathode is still warming up. These will usually blow, often spectacularly, at turn on. If you find a tube that survives start up a few times it will usually be OK for a year or two.
Using an inrush current limiter, or placing silicon diodes in series with each plate lead helps solve the start up issues.
I have an old Sylvania that has been in my SSE for nearly 7 years. That amp runs 230 mA. No issue with the tubes, but the power transformer gets really HOT!
The SS diode cathode leads get soldered directly to the tube socket. Be sure to use a hemostat or other clip on heat sink to protect the "sand", while soldering.
You might not need any series resistors whatsoever. It depends on the DCR of the power trafo's rectifier winding.
Authoritative 5AR4/GZ34 data sheet here.
I have already calculated that 50R series resistance is required for the PT that I am using. Thanks for the info.
Scott
I'm using 30uF. No problems. No diodes either yet, but I'm only running at 200mA. I will increase to 250mA in the future when I get my new PT.
A question, do I even need the protection diodes? I'd hate to "sand up" an already sand-free design unless it was absolutely necessary.
Maybe just an inrush current limiter as suggested by George?
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They are not absolutely needed, but several users including myself have observed a reduction in brand new rectifier tubes dying at first turn on by adding the diodes. As I said before if you find a tube that survives the first few turn ons without diodes, it should be OK for a while. I know it can be unnerving to watch two or three brand new tubes spark like Chinese fireworks before finding a good ones.
The silicon in this location will not add the usual diode noise created with pure silicon rectifiers because the conduction characteristics of the tube rectifier dominate. The diodes remove the PIV stress on the tube that causes a tube arc to continue once it starts.
I'm taking the "OK for a while" to heart. I don't like that scenario. I suppose I'll bite the bullet and use the diodes. It's not like they're in the "circuit" so to speak.
A nice way to implement it. UF4007 or better will be preferred as mentioned.
I first read of Kevin Kennedy advocating it and using it for years with great success and then found this nice drawing. It apparently came from Kevin @ dynakit I've been spreading it around on the forms! Never any negatives to it down the road!
Click on for full size. (too big to upload here)
AudioKarma.org Home Audio Stereo Discussion Forums
Randy
I first read of Kevin Kennedy advocating it and using it for years with great success and then found this nice drawing. It apparently came from Kevin @ dynakit I've been spreading it around on the forms! Never any negatives to it down the road!
Click on for full size. (too big to upload here)
AudioKarma.org Home Audio Stereo Discussion Forums
Randy
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Is the Pope Catholic? Yes, NOS 5AR4s are scarce and VERY costly.
Even the Mastushita tube (quite nice) made with Mullard machinery commands a high price.You can get completely anal and install a reverse recovery spike filter (RRSF), ala "Buddha" Camille, in between the composite rectifier and the power trafo. Connect a 0.1 μF./2500 WVDC film cap. across the ends of the rectifier winding. Insert 10 Ω inductive WW resistors in the lines leading to the composite rectifier. Connect a 0.01 μF./2500 WVDC NPO ceramic cap. across the lines leading from the resistors to the rectifier. It is possible to use an o'scope to examine what's happening across the power trafo secondary, while tweaking the value of the snubber cap. wired across said secondary. "Buddha" stated it is possible to completely eliminate the switching noise generated by UFnnnn diodes.
I am one... I don't know what the turn on draw was, but at idle, pulling less than 200ma. I went through four or so new production tubes in a few months... Finally added diodes and haven't had any more trouble.
In this application, would 1N4007s work as well? I've got about a million, give or take, of those.
Don't have to sign in to see this one. They show IN4007 which I haven't tried. Others have recommended UF4007 or better as quieter. Try the IN4007 till you get better ones and compare. Maybe in some implementations like PP you won't notice more noise.
tube rectifier diode mod
Randy
tube rectifier diode mod
Randy
So thats where they all went. I only have a few hundred left....That's what I have been using and I haven't noticed anything nasty yet. I also find that it helps to float the 6.3 volt heater winding 30 to 50 volts above ground with a 0.1 to 0.5 uF cap to ground.
They are not cheap any more. Beware that some NOS Westinghouse in the usual blue and white box may be relabled Chinese. I bought some from a reputable tube seller who told me up front that they were Chinese even though it doesn't say that anywhere on the tube or the box. None have blown up yet, but I haven't put them into an SSE yet.
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