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Old 11th December 2011, 04:41 PM   #11
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Both my SP-P and SSE have been operating for hundreds of hours now on their original (cheapo) Chinese 5AR4's. No sparks; no arcs. (Sound of knocking on wood!) I'm thinking the currently available tubes are probably all over the map for quality /reliability, so it's luck of the draw.
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Old 11th December 2011, 06:22 PM   #12
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A tube that has manufacturing defects bad enough that it is going to spark out, will usually do so the very first time you turn it on. If they have lived this long, they should be OK.

As the emission dies off with age the possibility of a sparking death increases. Again an uneven cathode coating or improper alignment will cause uneven cathode wear.
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Old 12th December 2011, 03:47 AM   #13
anatech is offline anatech  Canada
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Quote:
A tube that has manufacturing defects bad enough that it is going to spark out, will usually do so the very first time you turn it on. If they have lived this long, they should be OK.
Absolutely!
I have to say that it's been a very long time since I have seen any that bad. The situation where the cathode has been unevenly coated is another real quality problem. You may also see the cathode oxide material flaking off or maybe even rolling around in the tube. Pitch those in the bin. If the quality of the tube is that poor, I would expect all kinds of problems. Low vacuum and solder failure in the base pins are other signs of shoddy workmanship. Even the appearance of the internal elements may not be parallel with others too. Never use stuff like that.

I'm agreeing with George that your rectifier tube was / is very probably a good one. Arcing and other forms of discharging within a good tube can wreck it in no time. There is a possibility that your tube was damaged. Do you have another piece of equipment you can plug this tube into to test it?

I have seen more than my fill of badly designed power supplies in equipment with low secondary winding impedance and silly large filter caps. Using a high capacitance filter that is above the maximum rating in the tube manual is just plain wrong. This doesn't even buy you anything in the way of performance.

-Chris
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Old 14th May 2016, 06:35 PM   #14
rmyauck is offline rmyauck  Canada
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Can't find the mod on site I linked to anymore so try this.

Diodes On Rectifier Question


Diode mod.gif

Last edited by rmyauck; 14th May 2016 at 06:44 PM.
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Old 14th May 2016, 06:40 PM   #15
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Hi rmyauck,
I think you're much further ahead if you replace the diodes with the resistors as suggested in an RCA tube hand book.

-Chris
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Old 14th May 2016, 06:46 PM   #16
rmyauck is offline rmyauck  Canada
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As George says they take the load off the tube helping older or new production last. It should help if the amp is also short cycled also!

Some amps (big ones) load the tube rectifier near or to the max so it would of course help there the most such as for example it is a popular mod on the forms for the Dynaco ST-70

Last edited by rmyauck; 14th May 2016 at 06:49 PM.
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Old 16th May 2016, 02:13 AM   #17
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Hi rmyauck,
The resistors reduce the peak currents to safe levels. They also reduce the "hot switching current" as the tube runs normally.

Know how to replace a tube rectifier? Use a diode, Zener(s) and resistance in series. The diode does what you would expect it to do. The zener provides the voltage drop seen across a tube in normal operation, and the resistors replicate the resistance of the rectifier tube. You should therefore end up with the same B+ voltages and close to the same performance. You're almost there now, why even leave the tube in circuit?

The delay in B+ the tube offers only helps the capacitors with their peak voltages, and if you are using a filament type rectifier (5U4 - say), there is no advantage to using a tube.

-Chris
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