Daf96
 

Hi As some might remember I have had a set of battery valves with no filament continuity. It was suggested that a close examination of the valve might show up an o/c filament. The DAF96 shows this most clearly. I dont think that it can be remade ;)
I have got higher resolution images if anyone is interested.

Malcolm

Re: Daf96
 

Yeah, that's what I meant (quite dramatic in this case, too). Makes you weep doesn't it!

It also shows the mayhem that an over-volted LT line could do in an IF or output valve - imagine burning out the filament which then goes Pyongg (breaking at the chassis end rather than LT+ end), the fine wire then contacts the anode, the healthy HT supply (or bypass electrolytic) then dumps energy via the anode's transformer into the LT line, zapping the other valves which have so far survived, and burning out the IF or outpout transformer as it does so.

Has this ever happened to anyone?

Re: Daf96
 

That's quite dramatic Malcolm. Before replacing the valves, it would be very wise to check that there are no shorts between HT and LT. Mind you it could just be the slip of an Avo prod......:o



SB

Re: Daf96
 

SB, oh yes I'll give the set a through check out before I plug in any new valves. Just a thought, are 'normal' 6.3v heater valves more robust? I'm thinking of the EXX3x series and the more modern eXX8x (read ECH34 or ECL80)

Malcolm

Re: Daf96
 

I imagine indirectly heated mains valves are much more robust as the heater is thicker, spiralled, insulated and tucked inside a cathode sleeve. Also, they're not under tension as directly heated filaments are so a breakage won't wreak as much havoc as a battery valve could.
Cheers, Pete:thumbsup:

Re: Daf96
 

When fault-finding on battery valve sets, if you find the need to dig around in the chassis with probes, I would recommend temporarily adding a 100mA fuse in the HT +ve lead and (and assuming 1.4v LT) three forward biased 1N4001 diodes in series across the LT leads. This will usually protect the valve filaments in the event of briefly and accidentally shorting HT to LT.

Given the cost of a set of valves, this could be a worthwhile permanent modification

I was taught to be cautious - by experience!!

Re: Daf96
 

Yes - very much so. You'll be very hard pressed to burn out a 6.3V filament accidentally. Only likely scenario is a metallised valve that you can't see through, on a valve tester set to a massively wrong heater voltage. Anything else - you'll see the problem before anything catastrophic happens.

Re: Daf96
 

I was thinking of an accidental short from anode to filament On a fully isolated set would the filaments 'float' to the HT as they are isolated? how about an AC/DC set where the filaments are not isolated?
Malcolm

Re: Daf96
 

These battery 25ma filament valves are notoriously delicate. As has been said a slipped probe can blow all four valves with ease from a good HT battery or HT eliminator. A 100uf cap charged with 90v will easily blow the filaments in a '96 series valve.
One thing though not to panic about is the audio output transformer or IF transformers, these will withstand much higher currents for short periods than the valve filaments.

Being careful is my motto, use a prod that is insulated right down to a small tip, that will lessen the chance of an accidental short, also cold checks are often enough to trace most faults.

Re: Daf96
 

It may even be worth building a set of special probes, say using pins insulated right down to within a mm of the end? I used to use these to probe through insulation on 12V electrical wiring to save breaking the wire to measure the voltage. Could be frowned upon I suppose as it does damage the insulation albeit very slightly...I wouldn't use this technique on high voltages.
Alan.

Re: Daf96
 

That's what I did early this year - 100µF HT electrolytic, set switched off, no bleed resistor, a bit later a moment of carelessness, bright flash from the DK96! :dunce: :dunce: :dunce:

Stupid thing is that I had planned to add a bleed resistor - just not done it yet.

The LT was decoupled with 600µF of capacitance, so at the instant of the HT/LT short, I would have got 100/700 x 90V or 12.8V on the LT line.

Other valves survived.

Re: Daf96
 

This is a bit scary so all 3 of my battery valve radios are going to get 3 silicon diodes in series across the heater line.