DIY Audio members
Lets talk about the PL519.
A bit of background .... from the 80s to the early 2000s we had an audio manufacturer in South Africa that used PL519 tubes in all his amplifier models. When these amplifiers work they sound great. The biggest downfall of each and every one that I had the displeasure to work on is the PL519 tubes going bad.
Over the last few years I have noticed a clear trend on how the PL519 tubes behave. They are unpredictable.
Two major issues are:
1 - they lose vacuum via pin/glass junction due to thermal expansion and/or cracks
2 - grids sag due to thermal expansion and causes havoc
Now the local manufacturer got quite ingenious on how to keep the tubes with circuits that automatically adjusted the bias to accommodate drifting tubes. But these systems could only adjust the tube current 'so' much and then fireworks happened.
The G2 resistors almost always go up in flames when a tube goes bad.
The PL519 is becoming a thorn in my side because no other tube goes as bad as these do.
The reason for this post is to engage with other unfortunate PL519 users and see if my findings does calculate.
On the bench is a pair of very expensive monoblocks that run 4 pairs of Pl519s in PP configuration for nominal 300 watt output. Plates at 300 vdc. Cathode bias at 57.2 vdc with a 1K cathode resistor per tube. Low voltage, high current.
I would like to hear from some people here. Thank you.
Lets talk about the PL519.
A bit of background .... from the 80s to the early 2000s we had an audio manufacturer in South Africa that used PL519 tubes in all his amplifier models. When these amplifiers work they sound great. The biggest downfall of each and every one that I had the displeasure to work on is the PL519 tubes going bad.
Over the last few years I have noticed a clear trend on how the PL519 tubes behave. They are unpredictable.
Two major issues are:
1 - they lose vacuum via pin/glass junction due to thermal expansion and/or cracks
2 - grids sag due to thermal expansion and causes havoc
Now the local manufacturer got quite ingenious on how to keep the tubes with circuits that automatically adjusted the bias to accommodate drifting tubes. But these systems could only adjust the tube current 'so' much and then fireworks happened.
The G2 resistors almost always go up in flames when a tube goes bad.
The PL519 is becoming a thorn in my side because no other tube goes as bad as these do.
The reason for this post is to engage with other unfortunate PL519 users and see if my findings does calculate.
On the bench is a pair of very expensive monoblocks that run 4 pairs of Pl519s in PP configuration for nominal 300 watt output. Plates at 300 vdc. Cathode bias at 57.2 vdc with a 1K cathode resistor per tube. Low voltage, high current.
I would like to hear from some people here. Thank you.
What about the Russian 6П45С? It's quite sturdy and for 4 years of average daily use, no single one ever failed on me.
The only culprit I found is that the emission tends to slowly decrease, doesn't stabilize for a long time. So you have to check the bias (if fixed) recently, let's say two times per year at least.
The only culprit I found is that the emission tends to slowly decrease, doesn't stabilize for a long time. So you have to check the bias (if fixed) recently, let's say two times per year at least.
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That's odd. Well, I'm using mine in class A SE amplifiers, with 40W total dissipation (plate +g2). The good news is, these can be bought locally here for almost nothing at flea markets.
G2 drive has been discussed before, if you can find the topics. Tubelab did a lot of interesting stuff with mosfet followers, and CCS loaded drivers.
About the grid voltage on the PL519, i must be confusing myself with the EL36. That tube has VG2 =150V in the application notes. The E130L aswell doesnt like much VG2.
I noticed that tubes like the PL508 and 504 have very low anode current up to VG2= 100V
The VG2 max of the PL519 is specified at 275V, however i think this is is largely due to its intended purpose in pulse mode, in that way the screen grid has time to cool between cycles.
About the grid voltage on the PL519, i must be confusing myself with the EL36. That tube has VG2 =150V in the application notes. The E130L aswell doesnt like much VG2.
I noticed that tubes like the PL508 and 504 have very low anode current up to VG2= 100V
The VG2 max of the PL519 is specified at 275V, however i think this is is largely due to its intended purpose in pulse mode, in that way the screen grid has time to cool between cycles.
In sweep mode the tube conducts most of the time and is open at flyback only. I guess that is less than 10 % of the cycle.
Best regards!
Are these failing tubes operating in triode mode?
The peak (2X B+) plate voltage then has to exist between grid2 and grid1 support posts on the micas. (<1 mm separation) If the tubes were not competently evacuated (diff. pumped to remove Argon, then N2 back-filled and diff. pumped again), then non-getterable residual Argon ions will continuously bombard the mica insulator between g2 and g1, causing eventual insulation failure. ( g2 resistor burns up as described)
Do the tubes have big grid 1 cooler fins up top? (to avoid heat warping the grid) The 6KG6 has dual base pins for most elements.
Are the extra base pins all connected to a thick copper wire to provide cooling? (specified on some datasheets) Does the socket have cooling air circulation from below? (and the bottom of chassis has air vents, maybe a quiet fan)
Large (magnoval) pins have greater issues with thermal expansion mis-match with the glass, did the manufacturer use the best thermal matched pin feed-thru material? Are the tube sockets those cheap Chinese ones that "fit" both Novar and Magnoval pins? (tubes forced into the sockets?)
(Do the problems primarily occur with certain tube brands? )
If running in pentode mode, what is the g2 voltage and dissipation? What is the max plate and g2 dissipation if over-driven? (overdrive limiters?) Does the OT have over-voltage protective devices? (MOV or spark gap) Did the user operate at max power without a load? Is the amplifier stable without a load? Are the power supplies, or at least the g2 supply and g1 supply, regulated against power line variation? Or using EFB bias? A g2 supply current limit? Power line surges frequent?
Chances are the manufacturer did not take all design precautions. And current production tubes are not up to the standard of the old tubes for abuse.
The peak (2X B+) plate voltage then has to exist between grid2 and grid1 support posts on the micas. (<1 mm separation) If the tubes were not competently evacuated (diff. pumped to remove Argon, then N2 back-filled and diff. pumped again), then non-getterable residual Argon ions will continuously bombard the mica insulator between g2 and g1, causing eventual insulation failure. ( g2 resistor burns up as described)
Do the tubes have big grid 1 cooler fins up top? (to avoid heat warping the grid) The 6KG6 has dual base pins for most elements.
Are the extra base pins all connected to a thick copper wire to provide cooling? (specified on some datasheets) Does the socket have cooling air circulation from below? (and the bottom of chassis has air vents, maybe a quiet fan)
Large (magnoval) pins have greater issues with thermal expansion mis-match with the glass, did the manufacturer use the best thermal matched pin feed-thru material? Are the tube sockets those cheap Chinese ones that "fit" both Novar and Magnoval pins? (tubes forced into the sockets?)
(Do the problems primarily occur with certain tube brands? )
If running in pentode mode, what is the g2 voltage and dissipation? What is the max plate and g2 dissipation if over-driven? (overdrive limiters?) Does the OT have over-voltage protective devices? (MOV or spark gap) Did the user operate at max power without a load? Is the amplifier stable without a load? Are the power supplies, or at least the g2 supply and g1 supply, regulated against power line variation? Or using EFB bias? A g2 supply current limit? Power line surges frequent?
Chances are the manufacturer did not take all design precautions. And current production tubes are not up to the standard of the old tubes for abuse.
The amplifiers were built to a questionable level and if one compares the circuits with that of the GEC tube amplifier design book you'll notice great similarity.
Tubes are connected in UL configuration. Low voltage and high current. The g2 resistors most always fail with devastating effect.
Plan now is to convert the amplifiers to Pentode connection scheme and feed the g2 network from a stable supply and see if the amplifiers behave.
Got guts?
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4x pairs of 519s on a chassis
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Family portrait
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Looks like the grids are wound on sheet metal stampings. Never seen that before. (sensitivity to microphonics, mechanical jolts? Heat warping?) How accurate can they hold the thickness of the dents in the sheet metal, versus the diameter of conventional support rods (for spacing of grids)? (sheet metal stampings spring back some when released from the stamping press, not to mention distorting in shape and flatness some) There could be un-resolved issues in such a tube design. I would call that an immediate --RED FLAG--. Suggest trying another tube brand with real grid support RODs. Plus adding safety precautions to the amplifier design.
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