|
Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
|
Thread Tools |
28th Aug 2012, 2:01 pm | #1 |
Dekatron
Join Date: Aug 2006
Location: Madrid, Spain / Wirral, UK
Posts: 7,498
|
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?
__________________
Regards, Ben. |
28th Aug 2012, 3:43 pm | #2 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,966
|
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.
|
29th Aug 2012, 1:29 pm | #3 |
Octode
Join Date: Nov 2004
Location: Hebden Bridge, West Yorkshire, UK.
Posts: 1,885
|
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... |
29th Aug 2012, 9:40 pm | #4 |
Dekatron
Join Date: Aug 2006
Location: Madrid, Spain / Wirral, UK
Posts: 7,498
|
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.
__________________
Regards, Ben. |
29th Aug 2012, 9:58 pm | #5 |
Octode
Join Date: Oct 2003
Location: Newbury, Berkshire, UK.
Posts: 1,770
|
Re: overdriving valves - likely damage?
I've never seen those before either - interesting. My guess would be they are caps.
__________________
Chris |
29th Aug 2012, 10:06 pm | #6 |
Dekatron
Join Date: Feb 2005
Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW
Posts: 3,518
|
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.
__________________
Engineers make things work and have spare bits when finished |
30th Aug 2012, 2:45 pm | #7 |
Triode
Join Date: May 2006
Location: Near Bern, Switzerland.
Posts: 48
|
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). Regards, Walter |
31st Aug 2012, 6:13 pm | #8 |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
|
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. |
1st Sep 2012, 10:01 am | #9 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,536
|
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.
__________________
....__________ ....|____||__|__\_____ .=.| _---\__|__|_---_|. .........O..Chris....O |
19th Sep 2012, 10:44 pm | #10 |
Dekatron
Join Date: Aug 2006
Location: Madrid, Spain / Wirral, UK
Posts: 7,498
|
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.
__________________
Regards, Ben. |
20th Sep 2012, 12:08 am | #11 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,536
|
Re: overdriving valves - likely damage?
Could be an open circuit inside on the cathode or anode? Or just no emission.
__________________
....__________ ....|____||__|__\_____ .=.| _---\__|__|_---_|. .........O..Chris....O |
21st Sep 2012, 8:58 pm | #12 |
Dekatron
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,195
|
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.
Ed |
22nd Sep 2012, 1:21 am | #13 |
Heptode
Join Date: Feb 2011
Location: Washington DC, USA
Posts: 619
|
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.
__________________
David |