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Old 12th Jun 2020, 12:28 pm   #1
PsychMan
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Default What causes a rectifier to glow purple?

In this case, an MU14 from a HMV 582 radiogram.

Ive had this radiogram in my "round to it" pile for some time. I had a quick look and thought Id do an initial assessment to see what work it might need.

When powering up via a lamp limiter (60 watt bulb), the bulb stayed bright and immediately the MU14 exhibited the dreaded purple glow.

I'm assuming the rectifier itself is bad, but before I stick it in my AVO 2 panel tester, I thought Id better understand what causes this, before risking any damage to the valve tester.

Is it a case of gases developing inside the envelope? Loss of vacuum ? Or something else?

Needless to say I wont be replacing it with a good one until I can get a closer look at the power supply, or understand how rectifiers die this way.

With the MU14 removed, the lamp limiter goes dim and other valve heaters come up ok from what I can see.

Looking closer, there are signs of this having been "got at", and not in the good way, so I expect this to be a hell of a project when I do start it.

Best Regards
Adam
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Old 12th Jun 2020, 12:37 pm   #2
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Default Re: What causes a rectifier to glow purple?

The rectifier could be gassy or the purple glow could be due to overload - the reservoir capacitor may be excessively leaky or there could be a HT short.

John
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Old 12th Jun 2020, 12:50 pm   #3
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Default Re: What causes a rectifier to glow purple?

Interesting you say could be due to overload. I wondered that, but in this case, as it happens immediately right from switch on, there'd be no heaters and therefore no emission of HT
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Old 12th Jun 2020, 1:20 pm   #4
David G4EBT
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Default Re: What causes a rectifier to glow purple?

I'd guess that it's most likely leaky reservoir/smoothing caps as John suggests.

Could be confirmed (or not) by temporarily disconnecting the existing caps and tacking on two new ones to check. (8uF & 16 uF). Bear in mind that this five-band 1943 radio uses a mains energised speaker in which the field coil acts as a smoothing choke as well as energising the speaker. 1,100 Ohms is rather a lot of turns of very fine wire and makes an expensive fuse if it becomes displeased.

Lots of other caps in their which - if not already replaced - will most likely be leaky and in need of replacement, but maybe that's already been attended to?

Good luck with it Adam.
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Old 12th Jun 2020, 3:11 pm   #5
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Default Re: What causes a rectifier to glow purple?

Quote:
as it happens immediately right from switch on
Then that pretty much proves that the valve is gassy - so it's lighting up like a neon lamp.

You can confirm that by disconnecting the heater so it is sure that it is cold.
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Old 12th Jun 2020, 5:33 pm   #6
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Default Re: What causes a rectifier to glow purple?

If it's an actual MU14 (as opposed to a labelled "approximate substitute"), then it's indirectly heated and would take several seconds to produce HT output- the fact that it happens straight away suggests that it's lighting up because it's gassy, rather than overloaded by a fault down the HT line. The symptoms of bright limiter bulb suggests that it's become a low impedance AC path that's effectively being heavily loaded by the low impedance of the reservoir capacitor. Not a good state of affairs for any length of time, particularly for the transformer, and an excellent illustration of how a lamp limiter can save expensive grief!
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Old 13th Jun 2020, 12:54 am   #7
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Default Re: What causes a rectifier to glow purple?

A possible work-around, if you don't have a spare MU14, is to wire in two series connected 1N4007 in series with each MU14 anode. The ss diodes take on the PIV role, and might allow nominal operation for a while, depending on why and how the valve has gone gassy, and to what degree it has gone gassy.
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Old 13th Jun 2020, 11:44 am   #8
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Default Re: What causes a rectifier to glow purple?

Thanks for the comments folks.

So as the MU14 (and yes it is a proper one) is dead, I could scavenge the base and fit some thing solid state until I know better what the situation is. I wouldn’t be happy with that longer term but it’s a good stop gap

It is easy to get complacent powering things up. I’m very pleased that I use a lamp limiter for everything now, because I hate to think of the what damage might have occurred if I’d given it full mains!
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Old 13th Jun 2020, 1:29 pm   #9
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Default Re: What causes a rectifier to glow purple?

Using just ss diodes could over-voltage stress some circuit components such as coupling and electrolytic caps at power up, as the MU14 was indirectly heated and so would have provided a delayed B+ rise.

As indicated, the existing MU14 may still function (except for PIV capability), so you may be able to get away with adding ss diodes in series with each anode as an interim measure.
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Old 13th Jun 2020, 1:34 pm   #10
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Default Re: What causes a rectifier to glow purple?

And it might get better after a few hours running with diodes in series.
 
Old 14th Jun 2020, 8:49 am   #11
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Default Re: What causes a rectifier to glow purple?

Yes indeed I wouldn’t want to use just diodes for anything but a short length of time.

I must have skim read what you had posted before - the idea of putting them in series is a really good idea if it’s the inverse voltage it’s having trouble with. Much better than just binning the valve if it works - at least for a time.

If it somehow rejuvenates it, even better.
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Old 14th Jun 2020, 9:33 am   #12
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Default Re: What causes a rectifier to glow purple?

The internal arcing is likely to originate each half-cycle as the PIV of each diode increases to a sufficient level. If the cathodes are not damaged too much by previous arcing, then they may well still conduct forward current adequately well. The unknown is why has the valve gone gassy, and is the gas concentration going to keep increasing, or could some local heat from normal operation work with the getter to stabilise or perhaps even help control the gas level.

If you have an insulation resistance meter that can test up to 1kVDC then that could be useful to indicate how poor the diodes are now, and also whether continued use makes the situation stabilise or .... If the IR reading is too high to register (eg. 2000 megohm on the simple handheld meter I typically use) then that's a good start. The MU14 probably has a pretty high PIV of circa 1.8kV.
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