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MV rectifier
MV rectifier
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Old 8th July 2018, 10:14 PM   #1
bluerooster is offline bluerooster
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Default MV rectifier

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. 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.
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Old 8th July 2018, 10:22 PM   #2
trobbins is offline trobbins  Australia
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You don't have a "point", just a doubt. What do you mean by 'indication' - a fuse blowing, or your room lights flickering?
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Old 8th July 2018, 10:34 PM   #3
bluerooster is offline bluerooster
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Quote:
Originally Posted by trobbins View Post
You don't have a "point", just a doubt. What do you mean by 'indication' - a fuse blowing, or your room lights flickering?
I have a "point", that being the rectifier has indicated something has gone wrong with the internals of my amp. (abnormally high current flow) Whether it be due to the mains going wacky and overloading something, or something internal just decided it was time to act a fool. either way, the indicator of abnormal current draw is the rectifier tube.
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Old 9th July 2018, 12:18 AM   #4
trobbins is offline trobbins  Australia
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Ok, so the indication process was initially the lights flickering, and after a few flickers, the amp went off. Then you did some sleuthing which involved stressing the power transformer and mains fuse again, and then went up a stress notch by increasing the fuse value.

Note that the 83 rectifier, like an ss diode, has a steady state peak plate current limit rating, but unlike even a basic 1N4007, the 83 rating is only about 0.7Apk, whereas the 1N4007 is a thermal junction temperature issue as it can allow up to transient 35Apk. Your stress testing may have in some way damaged the 83, but I'm not across MV degradation well enough to have a good view on that.

I'm also not sure if the MV itself may not have been causing the 'fault', due to PIV related short circuiting the power transformer secondary - inserting ss diodes in series with each MV anode (as is typical practise nowadays for common vacuum tube diodes) would help faultfind that scenario. Adding a fuse on the power transformer secondary would reduce stress on the power transformer if ever a fault occurred (such as in an output stage).
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Old 9th July 2018, 04:54 AM   #5
PRR is offline PRR  United States
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Quote:
Originally Posted by bluerooster View Post
...the rectifier has indicated something has gone wrong with the internals of my amp.....
I wonder if the "something wrong" IS the mercury tube.

Mercury vapor can pass HIGH current. And unlike a 1N4007, they sometimes work again after the stress has passed.

(If you have worked in an old-time radio transmitter, or subway power station, you know the cut-outs protecting mercury rectifiers are often rigged to cut back in after a short delay, because sometimes that is enough to break a mercury "fit" and operation may be continued.)
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Old 9th July 2018, 11:43 AM   #6
ethermion is offline ethermion  Antarctica
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You will find an 83 in many Hickok tube testers. They do go wonky, and weird things happen. Hickok mounted them sideways, which is a no-no, which i hope the OP didn't do.

83s are not that expensive, and a swap is probably smarter than an out of spec fuse.
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Old 9th July 2018, 12:04 PM   #7
Kay Pirinha is offline Kay Pirinha  Germany
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Quote:
Originally Posted by trobbins View Post
...inserting ss diodes in series with each MV anode (as is typical practise nowadays for common vacuum tube diodes)...

Is it? Really? That is belts and braces, isn't it?


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Old 9th July 2018, 12:11 PM   #8
trobbins is offline trobbins  Australia
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Yeh, I realised that was a bit too optomistic when i reread it later! Hopefully more will do it as time goes on
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Old 9th July 2018, 04:10 PM   #9
rmb is offline rmb  United States
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MV rectifiers susceptible to arc back, especially with power line surges. Makes no difference whether it's an 83, a 816, 866A, 575A, 872A, 266B, 857B... Transients on the AC probably go right through the power transformer with peak voltage in excess of the PIV..That is what the fuse on your equipment is for. Tried a surge protector??
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Old 9th July 2018, 07:42 PM   #10
bluerooster is offline bluerooster
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What size should the fuse be? Running one 83, two 12sn7, and two 6l6. About 350v on the anodes of 6l6 @~76ma. Looks like closer to 10amp fuse is needed. but a 3amp slo-blo has been doing well for a while now, so going to a 4a should not cause any troubles.

Start up time for MV tubes is a variable. The mercury needs to be vaporized completely prior to HV being applied. If not you will experience arcing, especially at a kv+. There is no set time for this, It's temperature dependent. As the filament warms the mercury, it will condense on the cooler parts, especially the glass. One must give it time to completely evaporate, and the tube to become clear, prior to turning on the HV. So you don't want to use the 5v leg of the HV transformer for the filament of a MV rectifier like you would a 5u4g.

That said. The MV rectifier can be used as a plate current meter, by paying attention to the amount of blue glow. More blue = more current flow, less blue = lower current flow.
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