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MV rectifier
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Old 10th July 2018, 12:07 AM   #11
bluerooster is offline bluerooster
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Knowing your MV diodes are properly warmed up. Easy to see:
All pics are 872 MV diodes, full wave setup, 2kv power supply feeding an 813. (but any mv rectifier would show similar)
First pic is cold, prior to lighting the filament. Second pic is beginning to warm, Hg is condensing on the glass. Third pic HG is totally evaporated, and ready for HV to the anode.
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File Type: jpg DSCN1126 (Copy).jpg (130.0 KB, 203 views)
File Type: jpg DSCN1128 (Copy).jpg (148.4 KB, 201 views)
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Old 10th July 2018, 12:20 AM   #12
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here's what happens when HV is first applied, which immediately settles to a 20ma draw, followed by a 280ma draw when the xmit button is pushed. You can get an idea of current draw by paying attention to the blue glow.
Note the difference between the two diodes. That is also the difference between the Hg content of each. Guess which has more Hg.
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Old 10th July 2018, 01:03 AM   #13
Miles Prower is offline Miles Prower  United States
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Schemo please?

Quote:
Originally Posted by bluerooster View Post
I built the power supply for my 6L6 amp using an 83 rectifier tube. Good thing too. It's been working just fine for quite a while now. but just today, I noticed something different.

As you all should know, MV rectifiers glow blue when current is flowing, and more blue equals more current. When HV is first applied there will be a very brief flash of blue, then almost none at idle.
This, right here, suggests a problem. It looks like you're operating the 83 into a C-input filter. I've seen the same phenomenon with SS diode power supplies and C-input filters. Hg diodes don't have the Isurge capabilities of vacuum diodes, and are not to be used with C-input filters, but rather L-input to slow down that Isurge into discharged filter capacitors. Hg diodes are typically found in applications requiring high voltage and high steady state currents where good voltage regulation (the forward voltage of a Hg diode doesn't change very much with variations in current -- much like a SS diode) is required. This would be the case for transmitters or audio rigs that can fill a stadium with sound or plate modulate a high power AM rig. The really big ones such as a commercial AM transmitter will run off 3-phase AC (six Hg diodes).

Quote:
Well, today, I was in the shop, working on the T-Buzzard, listening to some music, when the power went funky. Lights flickered, then went off, then back on. The amp was on the other side of the building. It seemed to be playing ok. Then, at the last flicker, it went off, and didn't come back on. I shut it down, and pulled the plug. I checked it's fuse, (3a Slo Blo) and it was blown.

I replaced the fuse with a 3a regular fuse (all I had in the 3a range) and re-fired it.
I warmed the filaments for a few minutes, then hit the HV switch. Rectifier flashed, and settled to it's normal barely noticeable glow as usual. Turned on the music, and went back to work. Well, after a bit, the mains went wacky again. And fuse popped almost immediately. Well, dang.

I have a 4a slo-blo so I'll give that a go. (what's an amp give or take?) when I hit the HV, rectifier flashed bright blue for the whole tube, and stayed that way for a bit. IOW something's drawing a lot of current and not letting up. I suspect a bad cap, but not sure. I'll check into it tomorrow. But my point, is that I doubt SS rectifier would show any indication before the majik smoke escapes.
This time of year, with the heat and lots of air conditioners running, mains drop-outs become rather common events. It's bad for electronics as it's the same as power cycling. Again, Hg diodes don't like hot starts like that. Who knows what happened here? Hot start and/or a high voltage transient that over volted and poofed a filter capacitor? Arc over inside the 83 that poofed something? Straighten that out, and if summer drop-outs are common in your area, consider investing in a surge protector/power failure backup.
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Old 10th July 2018, 09:51 PM   #14
PRR is offline PRR  United States
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Quote:
Originally Posted by Miles Prower View Post
....Hg diodes don't have the Isurge capabilities of vacuum diodes, and are not to be used with C-input filters....
Not disagreeing with a dislike of merc with cap-input.

But the '83 datasheet specifically calls out a condenser-input condition, with small series resistance and reasonably large (huge for the day) C value.
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Old 11th July 2018, 12:14 AM   #15
bluerooster is offline bluerooster
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Quote:
Originally Posted by Miles Prower View Post
Schemo please?



This, right here, suggests a problem. It looks like you're operating the 83 into a C-input filter. I've seen the same phenomenon with SS diode power supplies and C-input filters. Hg diodes don't have the Isurge capabilities of vacuum diodes, and are not to be used with C-input filters, but rather L-input to slow down that Isurge into discharged filter capacitors. Hg diodes are typically found in applications requiring high voltage and high steady state currents where good voltage regulation (the forward voltage of a Hg diode doesn't change very much with variations in current -- much like a SS diode) is required. This would be the case for transmitters or audio rigs that can fill a stadium with sound or plate modulate a high power AM rig. The really big ones such as a commercial AM transmitter will run off 3-phase AC (six Hg diodes).
Running an LC input filter/voltage divider. Yes the immediate flash is the caps charging. (what's supposed to happen) What happened during the power anomaly was the steady higher than normal current draw, as indicated by the glow of the rectifier. Probably around 100ma+ instead of 76ma. It finally settled down after a while. No majik smoke escaped, and all seems to be well now.





Quote:
This time of year, with the heat and lots of air conditioners running, mains drop-outs become rather common events. It's bad for electronics as it's the same as power cycling. Again, Hg diodes don't like hot starts like that. Who knows what happened here? Hot start and/or a high voltage transient that over volted and poofed a filter capacitor? Arc over inside the 83 that poofed something? Straighten that out, and if summer drop-outs are common in your area, consider investing in a surge protector/power failure backup.
I've noticed from time to time, that the mains will fluctuate from 120v to 130v RMS. I called a guy that I know who is the engineer for this area at the power company. He told me that it's not unusual to find anywhere from 117 to 127 or more, depending on demand, and location.
Dropouts are not all that common, but wind, and trees seem to cause fluctuations in the power, from time to time.
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Old 11th July 2018, 12:56 AM   #16
bluerooster is offline bluerooster
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Here's the PS with the 83:
I'm no artist, and forgot the resistor values, but I can get them in a day or two.
Sorry I didn't include it earlier.

Oh, the secondary on the hv xformer is 450v. With also 6.3, and 5v. 5v is unused, and 6.3 feeds the heater on the 6L6.
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Last edited by bluerooster; 11th July 2018 at 12:59 AM.
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Old 11th July 2018, 08:37 PM   #17
hpeter is offline hpeter  Slovakia
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Click the image to open in full size.

time delay + charge limiter works nice ! (1000f - you better have protection ..)
fet is warm in beginning , no cooler needed 1kv ixys
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Last edited by hpeter; 11th July 2018 at 08:41 PM.
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Old 11th July 2018, 10:01 PM   #18
trobbins is offline trobbins  Australia
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hpeter, all that 'protection', but no secondary fuse or ss diode in series with valve anode.
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Old 13th July 2018, 09:40 AM   #19
Kay Pirinha is offline Kay Pirinha  Germany
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Due to the hybrid bridge rectifier design, there actually is a SS diode in series with each MV diode!


Best regards!
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Old 13th July 2018, 09:48 AM   #20
trobbins is offline trobbins  Australia
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Unfortunately not in the sense of adding a series ss diode with a valve anode to avoid collatoral damage if a valve anode arcs to cathode (or other anode in a dual diode valve).
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