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Old 28th Aug 2012, 2:01 pm   #1
ben
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Default overdriving valves - likely damage?

Related to this thread.

I am currently trying to revive a recently-acquired 1952 Victrola (110-115v, used via 125v stepdown tranny). I suspect (but have no way to be sure) it has had 230v shoved up it. I have replaced one waxy before powerup as precaution (40+80uF 150v) but get no sound, just a barely audible hum.

Valve lineup : 12SQ7 (triode detector) and 50L6GT - both octal. Plus a mini 35w4 rectifier. No obvious burned resistors. This is a transformerless design with valve heaters wired in series. Mains connection appears to enter on pin 4 and 5 of the rectifier and on pins 3, 4, 5 and 8 of the 12sq7.

I suppose I'm asking what the likely consequences of excess mains would be on the valves. The heaters all light (presumably also on the black 12sq7 as this is a series arrangement.) I would have thought that I'd see o/c heaters due to overvoltage? Any pointers?
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Old 28th Aug 2012, 3:43 pm   #2
paulsherwin
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Default Re: overdriving valves - likely damage?

If it has been run at 230V there will almost certainly be one or more open circuit heaters. I suspect the problem is elsewhere.
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Old 29th Aug 2012, 1:29 pm   #3
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Default Re: overdriving valves - likely damage?

Depends how long it was run like that. I've left 6.3 valves running at 15 volts for half an hour and had them work better afterwards in my evil valve boosting experiments in the past. Some actually did boost some got worse soon afterwards.

Other problem is likely that the H-K voltage will have been exceeded and may have welded heaters to cathodes but again I've abused plenty of valves in this way both deliberately and accidentally and most have survived the ordeal.

If it were me I'd check that they are all actually lighting up and check that the voltages along the heater chain are as they should be when running on 110V in case something has happened. Next check the rectifiers are working as they should and HT is where it should be

Then follow normal procedure for any new radio...assume everything is broken an start at the output stage and work backwards...

A 40+80uF waxy - are you sure? Would be bigger than most radios...
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Old 29th Aug 2012, 9:40 pm   #4
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Default Re: overdriving valves - likely damage?

A few pics.
The wax cap is shown, it is (was) by far the largest component there.
I cut the end off with a view to restuffing, but the innards were stuck solid to the thin outer paper case and it would have been destroyed totally had I pursued it.

There are some strange 'domino' looking items in there with coloured dots on which I have not come across before, and what looks like a big red dropper resistor made flameproof.

Going to trACe the HT rails next and measure.
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Old 29th Aug 2012, 9:58 pm   #5
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Default Re: overdriving valves - likely damage?

Quote:
Originally Posted by ben View Post
A few pics.
There are some strange 'domino' looking items in there with coloured dots on which I have not come across before,

Going to trACe the HT rails next and measure.
I've never seen those before either - interesting. My guess would be they are caps.
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Old 29th Aug 2012, 10:06 pm   #6
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Default Re: overdriving valves - likely damage?

They are moulded mica caps, with RMA markings.

Its a simple American set - check for OC resistors on anode feeds. Be aware that the set might have an isolated -HT connection, if the caps were connected with the negative to chassis there wouldnt be a lot of HT around....

Most of the caps will be fine in the set.
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Old 30th Aug 2012, 2:45 pm   #7
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Default Re: overdriving valves - likely damage?

I've seen such "waxy" caps in some Philips radios. They were marked as electrolytics and were all bad.

For the "domino" looking caps: These are Micamold brand caps and often mica caps as Sean stated. But Micamold also sold paper caps in the same appearance and i guess that you have such in that radio (the mica caps are mostly smaller).

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Old 31st Aug 2012, 6:13 pm   #8
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Default Re: overdriving valves - likely damage?

Ahhh, Micamold capacitors as used on the BC348. If you are looking for a wire link these work perfectly but as a capacitor they leave alot to be desired as they tend to spit boiling wax out of each end.

If they are low values you may be lucky and they could be Mica.

A 4 valve USA set which had evidently been fired up on 240V had 2 blown heaters and another with a hk short.
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Old 1st Sep 2012, 10:01 am   #9
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Default Re: overdriving valves - likely damage?

Sounds like the 12SQ7 isn't conducting. The click would be the 50L6 amplifying the the transient caused by probing the 12SQ7 anode with a meter.

Try a pin cleanup on the 12SQ7. Maybe its grid capacitor is leaky and the lower resistance to chassis is causing the problem- the circuit relies on the high value 4.7M grid leak for its biassing.
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Old 19th Sep 2012, 10:44 pm   #10
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Default Re: overdriving valves - likely damage?

Still battling on. Changed the 4.7M And the 0.01uF grid cap - no joy.

Unless there is something I'm missing, it seems weird that the valve is receiving HT and has intact heaters but is seemingly dead.
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Old 20th Sep 2012, 12:08 am   #11
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Default Re: overdriving valves - likely damage?

Could be an open circuit inside on the cathode or anode? Or just no emission.
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Old 21st Sep 2012, 8:58 pm   #12
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Default Re: overdriving valves - likely damage?

Hi Ben, in the absence of a valve tester you could do a lash-up with the valve and psu's to see if there is any emission/ oc electrodes.

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Old 22nd Sep 2012, 1:21 am   #13
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Default Re: overdriving valves - likely damage?

Hi Ben,

I lashed up a sort of valve tester with the attached diagram. You will need to put in an ammeter and a voltmeter and then you can run a simple test.
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