Valve sparking - UK Vintage Radio Repair and Restoration Discussion Forum

humphrey · 29th Oct 2008, 3:04 pm

I am repairing a 1938 Clarville (France) radio. I have changed the e caps but when it is switched on, the set is dead but the valve EBL1 (double diode pentode) flashes with violet sparks inside the glass. There is also a corresponding noise from the speaker. Does this indicate the demise of the valve or could this be the result of something else?
The lineup is EK2 octode,EF9 pentode,EBL1 double diode pentode,1882 fw rectifier.

Steve_P · 29th Oct 2008, 3:18 pm

Well it won't do it much good! Are you sure first of all that the valves are in the right place. Better people have been caught out.

Check everything out thouroughy, especially the output transformer. Also check all your wiring and the wiring on the valve bases. Then bring it up on a variac. And get another EBL1.

Cheers,

Steve P.

paulsherwin · 29th Oct 2008, 3:49 pm

Sadly, it's almost certain to be the valve.

Paul

Nickthedentist · 29th Oct 2008, 4:12 pm

Can I butt in here, please?

If a rectifier valve sparks internally very occasionally, with no other symptoms or signs of distress, is it OK to leave it, or is there a risk of something nasty happening?

Nick.

Steve_P · 29th Oct 2008, 4:13 pm

If a rectifier sparks internally, I would check around it and then change it. But that's me.

Cheers,

Steve P.

paulsherwin · 29th Oct 2008, 4:18 pm

Quote:

Originally Posted by Nickthedentist

If a rectifier valve sparks internally very occasionally, with no other symptoms or signs of distress, is it OK to leave it, or is there a risk of something nasty happening?

I wouldn't leave such a valve in use, even if it only arcs over occasionally. It's likely to get worse, and may eventually fail short circuit causing damage.

Paul

Sideband · 29th Oct 2008, 4:58 pm

Back to the original question....is the EBL1 similar to the EBL31 but side contact? If so it's probably the valve. As this is a high-slope valve, the pentode is driven straight from the detector so it doesn't have the usual pre-amp stage...therefore no coupling capacitor from a previous anode....so no positive volts on the grid due to a leaky cap. I've had several EBL31's give this problem (and EBL21's) and a replacement valve has sorted it. One of them gave a very impressive firework display when tapped!

Rich.

Leon Crampin · 29th Oct 2008, 5:06 pm

The EBL1 and EBL31 are identical apart from the base and the top cap. EBL1s are much easier to get.

Leon.

humphrey · 29th Oct 2008, 6:29 pm

I will be getting a new EBL1. So far I have not discovered the model name as the place where it should be has not been stamped.

squier · 30th Oct 2008, 12:07 am

Its a common problem with the EBL31 (and I presume the EBL1 since its the same valve on a different base) its down to an internal short on one or both of the diodes. You can disconnect the internal diodes and replace them externally with a small signal diode of the 1N914 variety, you can use the pentode section normally. You might experience some drop in output but hey, its a fix. I've done this on a couple of sets which have had faulty EBL31's. Rod Burman on the IOW was selling EBL31's for about £30 (and that was a couple of years ago) so its worth trying the diode dodge.

mark

Steve_P · 30th Oct 2008, 1:25 am

If ANY valve does anything like this, check the circuit thouroughly before fitting a replacement and powering up. It could be expensive otherwise.

Cheers,

Steve P.

Leon Crampin · 30th Oct 2008, 12:16 pm

The potentials associated with the diodes in the EBL1/31 are insufficient to cause visible sparking inside the valve. The electrodes responsible will be either the anode or g2 and shorts here place the rest of the set at some risk - especially the (probably complex) output transformer.

Leon.

geofy · 30th Oct 2008, 2:33 pm

This is a circuit (Philips 650A) of the EBL1 output (with detector AGC diodes) valve V4. The grid coupling cap should be changed if a new valve is fitted.

geofy · 30th Oct 2008, 3:32 pm

Quote:

Originally Posted by Richard_Newman

therefore no coupling capacitor from a previous anode....so no positive volts on the grid due to a leaky cap.

Rich.

A good point, could explain why the grid capacitor in this type of set lasts so well, (my set has the original valves and caps from 1938), but to make up for it the capacitor in part of the AGC circuit is coupled to EBL1 AGC diode from the anode of the previous stage

Geof

Leon Crampin · 30th Oct 2008, 4:22 pm

This is the coupling capacitor which provides the AC feed to the AGC diode - typically 50 pF.

This is absolutely standard pre-war superhet circuitry and the AGC is derived from this point in order to leave just one set of IF tuned circuits (second transformer secondary) out of the AGC loop. This makes the receiver far easier to tune - if you put all of the tuned circuits inside the control loop the AGC flattens the tuning response. As designed, the remaining tuned circuit gives the set a sharp response despite the AGC action.

The coupling capacitor will be either a slilvered mica or a ceramic type. Neither is prone to failure.

Leon.

geofy · 30th Oct 2008, 9:28 pm

Quote:

Originally Posted by Leon Crampin

This is the coupling capacitor which provides the AC feed to the AGC diode - typically 50 pF.
Leon.

Like all Philips sets and possibly the 1938 Clarville which sounds as if it is based on a Philips design, they seem very well thought out and very robust. You are just about spot on, the capacitor is 80pF. Maybe a diode in V3 would have been a better design option though. No doubt there was a reason for making the EBL1 a double diode, as a detector had to be made it was as easy to make two diodes at the same time as the envelope was bigger than V3, EF9 in this case.

Geof

Leon Crampin · 30th Oct 2008, 11:28 pm

There are two main reasons for combining the diodes with the output valve. It is possible to make a very acceptable "short" superhet without an AF preamplifier stage given the high slope of devices such as the EBL1.

A more fundamental reason relates to AGC delay voltage. For an AGC system to operate properly, its action should be delayed until signals of a strength such that overload is about to occur are received. If this is not done, any significant signal will reduce the gain of the receiver. The voltage delay is usually achieved by returning the cathode of the AGC diode to a positive potential so that rectification of the signal to produce AGC voltage will not occur until about the point at which that carrier voltage is developed.

A very convenient source of delay voltage is the cathode bias potential for the output valve. In a short superhet, this voltage usually has to be increased to around 15V for the delay voltage to be optimal, so the bias resistor is split in order to allow the output section of the valve to be correctly biased.

Using the IF amplifier (EF9) would not give any significant delay as its cathode bias voltage is too small. A further problem is that the EF9 is itself AGC controlled, so the delay voltage would shift as the AGC came into operation - not a good idea.

Leon.

geofy · 31st Oct 2008, 12:39 am

Thanks Leon, you give a very good description of the valve operation. I knew about the delayed AGC but not quite the reason for it to be carried out in the output stage diode, apart from it not being a good idea to develope the AGC from the controlled valve, back into the same controlled valve. This would make for a very strange unstable operation. The EBL1 is quite an incredible valve being the whole of the AF stage.

Geof

kalee20 · 31st Oct 2008, 2:40 pm

I'd think that trying to put the diodes in the IF amplifier bottle might cause instability - after all, the diode anodes are handling several volts of IF, and capacitive coupling to the grid of the valve could well cause significant regeneration. Putting the diodes in the output valve envelope would remove this possibility.

I'd expect that the EBF80 exists because post-war, button-based valves have such a lower inter-lead capacitance that the idea is workable.

Finally, hearing that the EBL1 is prone to internal shorts, is this a consequence of it having to be high slope to suit its intended purpose? A fine, close-pitch grid, situated very close to the cathode?

Leon Crampin · 31st Oct 2008, 5:46 pm

To be fair, in my experience I can't say the EBL1/31 is particularly prone to internal shorts. The main problems I've found with this device have been caused by thermal fatigue of the soldered joint on the top cap causing noise (just re-solder it) and loose metallising.

It's quite reasonable to assume that a high slope valve will have close electrode spacings but in my experience, Philips and Mazda both did a good job. To generalise, I would say I've seen more short superhets with their original output valves than standard superhets. This is doubtless because in a short superhet, a leaky grid coupling capacitor biases the valve off with a negative potential from the detector, rather than turning it hard on with a highish voltage from the pre-amp anode.

There was actually an EBF2/32 made. If you use a back bias arrangement to provide the AGC delay, this is quite a workable scheme. The sections of these valves were well screened.

Leon.

29th Oct 2008, 3:04 pm	#1
humphrey Retired Dormant Member Join Date: Aug 2008 Location: Bellac, 87. France. Posts: 357	Valve sparking I am repairing a 1938 Clarville (France) radio. I have changed the e caps but when it is switched on, the set is dead but the valve EBL1 (double diode pentode) flashes with violet sparks inside the glass. There is also a corresponding noise from the speaker. Does this indicate the demise of the valve or could this be the result of something else? The lineup is EK2 octode,EF9 pentode,EBL1 double diode pentode,1882 fw rectifier.

29th Oct 2008, 3:18 pm	#2
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Valve sparking Well it won't do it much good! Are you sure first of all that the valves are in the right place. Better people have been caught out. Check everything out thouroughy, especially the output transformer. Also check all your wiring and the wiring on the valve bases. Then bring it up on a variac. And get another EBL1. Cheers, Steve P. __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

29th Oct 2008, 3:49 pm	#3
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,966	Re: Valve sparking Sadly, it's almost certain to be the valve. Paul

29th Oct 2008, 4:12 pm	#4
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,861	Re: Valve sparking Can I butt in here, please? If a rectifier valve sparks internally very occasionally, with no other symptoms or signs of distress, is it OK to leave it, or is there a risk of something nasty happening? Nick.

29th Oct 2008, 4:13 pm	#5
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Valve sparking If a rectifier sparks internally, I would check around it and then change it. But that's me. Cheers, Steve P. __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

29th Oct 2008, 4:58 pm	#7
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,578	Re: Valve sparking Back to the original question....is the EBL1 similar to the EBL31 but side contact? If so it's probably the valve. As this is a high-slope valve, the pentode is driven straight from the detector so it doesn't have the usual pre-amp stage...therefore no coupling capacitor from a previous anode....so no positive volts on the grid due to a leaky cap. I've had several EBL31's give this problem (and EBL21's) and a replacement valve has sorted it. One of them gave a very impressive firework display when tapped! Rich. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

29th Oct 2008, 5:06 pm	#8
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,874	Re: Valve sparking The EBL1 and EBL31 are identical apart from the base and the top cap. EBL1s are much easier to get. Leon.

29th Oct 2008, 6:29 pm	#9
humphrey Retired Dormant Member Join Date: Aug 2008 Location: Bellac, 87. France. Posts: 357	Re: Valve sparking I will be getting a new EBL1. So far I have not discovered the model name as the place where it should be has not been stamped.

30th Oct 2008, 12:07 am	#10
squier Retired Dormant Member Join Date: Jul 2006 Location: gloucester Posts: 7	Re: Valve sparking Its a common problem with the EBL31 (and I presume the EBL1 since its the same valve on a different base) its down to an internal short on one or both of the diodes. You can disconnect the internal diodes and replace them externally with a small signal diode of the 1N914 variety, you can use the pentode section normally. You might experience some drop in output but hey, its a fix. I've done this on a couple of sets which have had faulty EBL31's. Rod Burman on the IOW was selling EBL31's for about £30 (and that was a couple of years ago) so its worth trying the diode dodge. mark

30th Oct 2008, 1:25 am	#11
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Valve sparking If ANY valve does anything like this, check the circuit thouroughly before fitting a replacement and powering up. It could be expensive otherwise. Cheers, Steve P. __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

30th Oct 2008, 12:16 pm	#12
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,874	Re: Valve sparking The potentials associated with the diodes in the EBL1/31 are insufficient to cause visible sparking inside the valve. The electrodes responsible will be either the anode or g2 and shorts here place the rest of the set at some risk - especially the (probably complex) output transformer. Leon.

30th Oct 2008, 2:33 pm	#13
geofy Retired Dormant Member Join Date: Jun 2006 Posts: 1,798	Re: Valve sparking This is a circuit (Philips 650A) of the EBL1 output (with detector AGC diodes) valve V4. The grid coupling cap should be changed if a new valve is fitted. Attached Thumbnails

30th Oct 2008, 4:22 pm	#15
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,874	Re: Valve sparking This is the coupling capacitor which provides the AC feed to the AGC diode - typically 50 pF. This is absolutely standard pre-war superhet circuitry and the AGC is derived from this point in order to leave just one set of IF tuned circuits (second transformer secondary) out of the AGC loop. This makes the receiver far easier to tune - if you put all of the tuned circuits inside the control loop the AGC flattens the tuning response. As designed, the remaining tuned circuit gives the set a sharp response despite the AGC action. The coupling capacitor will be either a slilvered mica or a ceramic type. Neither is prone to failure. Leon.

30th Oct 2008, 11:28 pm	#17
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,874	Re: Valve sparking There are two main reasons for combining the diodes with the output valve. It is possible to make a very acceptable "short" superhet without an AF preamplifier stage given the high slope of devices such as the EBL1. A more fundamental reason relates to AGC delay voltage. For an AGC system to operate properly, its action should be delayed until signals of a strength such that overload is about to occur are received. If this is not done, any significant signal will reduce the gain of the receiver. The voltage delay is usually achieved by returning the cathode of the AGC diode to a positive potential so that rectification of the signal to produce AGC voltage will not occur until about the point at which that carrier voltage is developed. A very convenient source of delay voltage is the cathode bias potential for the output valve. In a short superhet, this voltage usually has to be increased to around 15V for the delay voltage to be optimal, so the bias resistor is split in order to allow the output section of the valve to be correctly biased. Using the IF amplifier (EF9) would not give any significant delay as its cathode bias voltage is too small. A further problem is that the EF9 is itself AGC controlled, so the delay voltage would shift as the AGC came into operation - not a good idea. Leon.

31st Oct 2008, 12:39 am	#18
geofy Retired Dormant Member Join Date: Jun 2006 Posts: 1,798	Re: Valve sparking Thanks Leon, you give a very good description of the valve operation. I knew about the delayed AGC but not quite the reason for it to be carried out in the output stage diode, apart from it not being a good idea to develope the AGC from the controlled valve, back into the same controlled valve. This would make for a very strange unstable operation. The EBL1 is quite an incredible valve being the whole of the AF stage. Geof

31st Oct 2008, 2:40 pm	#19
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,087	Re: Valve sparking I'd think that trying to put the diodes in the IF amplifier bottle might cause instability - after all, the diode anodes are handling several volts of IF, and capacitive coupling to the grid of the valve could well cause significant regeneration. Putting the diodes in the output valve envelope would remove this possibility. I'd expect that the EBF80 exists because post-war, button-based valves have such a lower inter-lead capacitance that the idea is workable. Finally, hearing that the EBL1 is prone to internal shorts, is this a consequence of it having to be high slope to suit its intended purpose? A fine, close-pitch grid, situated very close to the cathode?

31st Oct 2008, 5:46 pm	#20
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,874	Re: Valve sparking To be fair, in my experience I can't say the EBL1/31 is particularly prone to internal shorts. The main problems I've found with this device have been caused by thermal fatigue of the soldered joint on the top cap causing noise (just re-solder it) and loose metallising. It's quite reasonable to assume that a high slope valve will have close electrode spacings but in my experience, Philips and Mazda both did a good job. To generalise, I would say I've seen more short superhets with their original output valves than standard superhets. This is doubtless because in a short superhet, a leaky grid coupling capacitor biases the valve off with a negative potential from the detector, rather than turning it hard on with a highish voltage from the pre-amp anode. There was actually an EBF2/32 made. If you use a back bias arrangement to provide the AGC delay, this is quite a workable scheme. The sections of these valves were well screened. Leon.