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5AR4 @ 250mA
5AR4 @ 250mA
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Old 24th February 2012, 11:49 PM   #1
scott17 is offline scott17
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Default 5AR4 @ 250mA

What about running a 5AR4 at max 250mA? Specifically the Sovtek variety. Anyone have any experience with this? Will the tube provide this current for a reasonable amount of time before failure?
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Old 25th February 2012, 12:54 AM   #2
tomchr is offline tomchr  Canada
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The 5AR4 is rated for 825 mA RMS - and over 3 A peak... I doubt it'll fail at 250 mA.

~Tom
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Old 25th February 2012, 01:06 AM   #3
Eli Duttman is offline Eli Duttman  United States
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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.
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Old 25th February 2012, 01:34 AM   #4
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Quote:
Originally Posted by Eli Duttman View Post
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.
Would the diodes be in series with the 50R resistors that I need for the series resistance factor?
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Old 25th February 2012, 01:36 AM   #5
Tubelab_com is offline Tubelab_com  United States
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Quote:
Both Russian and Chinese current production 5AR4s have weaknesses.
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!
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Old 25th February 2012, 01:37 AM   #6
Tubelab_com is offline Tubelab_com  United States
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Would the diodes be in series with the 50R resistors that I need for the series resistance factor?
Yes.
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Old 25th February 2012, 02:13 AM   #7
Eli Duttman is offline Eli Duttman  United States
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Would the diodes be in series with the 50R resistors that I need for the series resistance factor?
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.
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Old 25th February 2012, 02:18 AM   #8
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Originally Posted by Eli Duttman View Post
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
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Old 25th February 2012, 02:23 AM   #9
Tubelab_com is offline Tubelab_com  United States
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Quote:
Authoritative 5AR4/GZ34 data sheet here.
Great data for it's time, 1958, but virtually none of todays production could survive 400+ AC volts into a 60 uF cap. I have seen other data sheets recommend a 47 uF. Even that might blow up a new tube. Most of my new builds have used a 33 or 39 uF.
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Old 25th February 2012, 02:27 AM   #10
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Originally Posted by tubelab.com View Post
Great data for it's time, 1958, but virtually none of todays production could survive 400+ AC volts into a 60 uF cap. I have seen other data sheets recommend a 47 uF. Even that might blow up a new tube. Most of my new builds have used a 33 or 39 uF.
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?

Last edited by scott17; 25th February 2012 at 02:52 AM. Reason: current limiter suggested by George
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