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Valved mono. blowing mains fuse. Any idea of cause?

Mike Reed

Mike Reed

pfm Member
Last winter this monoblock (EAR 509) blew its fuse. Eventually another slow-blow fuse was acquired and tried and all seemed well for 6 months. A couple of weeks ago, in the middle of an LP, there was a loud glitch/bang. I thought it was the record until I realised that I'd lost a channel. I did fear for my ESLs, but testing yesterday suggested it was okay.

All valves are operating normally and the LED biasing lights are too, so it's obv. connected to the mains side. Before blowing the second fuse yesterday it emitted a few pops through my 'default' testing speaker during the 5 to 10 minutes it was on.

It'll have to go back to EAR shortly as there's no way I could repair the fault, but I'd like to know what could have caused this; twice now, in effect. The other amp is and has been fine. Both amps fully serviced May '18 and used sparingly (max once a week for 4/5 hours).

Any ideas?
 
Most likely a valve has an intermittent short - perhaps a tiny flake of metal floating-around inside the envelope or a saggy filament, &c. If you have the time and can live with the suspense, swap one tube position from each amp - start with one PL509 - and wait and see if the fault follows the valve to the other side.

Alternatively, just get a replacement matched set of PL509s - they are agreeably cheap after-all - from a reliable vendor. If the fault persists, swap the small signal valves around one-by-one and see if the fault follows them.

IME EAR amps are well-constructed, so I'd be a little surprised if it was a fault with the amp itself. Not impossible, but seems less likely.
 
My 861 is recently back from EAR, as it too was blowing its fuse. As earlofsodbury suggests, it was an internal short on one EL509.
Replacement valve was £75+vat
 
earlofsodbury said:
Most likely a valve has an intermittent short -
Click to expand...
s1h1 said:
My 861 is recently back from EAR, as it too was blowing its fuse
Click to expand...

Thanks for hat and I appreciate your similar narratives. I was going to phone EAR and get them to collect but as I have 2 x unused 509s (from EAR) and spares of the ECCs it's worth experimenting. However, I don't have any slow-blow fuses left so shall have to order. Alternatively (as suggested?) I could swap valves one by one with the other amp, but this would be equally laborious and still require fuses. I've plenty of fast-blows (2A) but they soon blew when I had the initial fault; however, can't see any harm in trying; can you?

Been with hifi for 55+ years (and initially valves) but for some reason, age maybe, I'm loth to embark on this elimination business, which will take me well over a week to complete as (I gather from the hand-book) it's bad news to not run these amps for at least an hour.

If a faulty valve was the initial fault in Dec./January, that valve has performed perfectly since. All looks good with them and they light up well and the LEDs suggest all is well; then the fuse blows, for no apparent visual reason.

Hmmm!
 
I've had a similar experience with a valve amp that was caused by a tiny flake of metal in an 8-pin small signal valve - it was profoundly random, with long spells between problems then one or two brief firework shows in succession (always too brief to spot which valve was the culprit!). I also had a Unison Research amp which used some odd SETs (forget which, but none of the usual suspects) which was always fine unless someone heavy-footed (me, then...) walked-across the rather bouncy suspended floor, which caused enough movement for the filaments to short against nearby metalwork with an alarming and terminal outcome for the VERY expensive valve. Up close you could see the near white-hot filaments visibly bag and distort on warm-up - just bad design and no doubt solid reason why said SETs are vanishingly rare.

Frustratingly, there are a lot of reasons why valve amps might lunch fuses - hope yours is easy and cheap to correct!
 
earlofsodbury said:
Frustratingly, there are a lot of reasons why valve amps might lunch fuses - hope yours is easy and cheap to correct!
Click to expand...

You used the correct adjective (frustratingly) and I could think of a few more! Your and S1h1's posts have stopped me in my tracks (from contacting EAR) for the mo' and I'll replace the 2 x 509s as the more likely suspects (?)* as I've found a small assortment of T (slow-blow) fuses I didn't know I had.

*I'm assuming the output valves shorting would create a bigger bang than the input valves going but this could be utterly ignorant/flawed logic.
 
Just an update, Earlofsodbury and s1h1. Following your advice, I transposed the 5 valves from the good 509 to the fuse-blowing one, stuck an old T1.25A fuse in and everything is fine. I did have to balance (bias) the 2 509 output valves though (understandable.

I've now got to make a choice of whether to replace just the 509 output valves (I have 2 new spares) or use old ones which I THINK failed (wouldn't bias properly) soon after buying them from EAR or change just the signal valves. I can't see the point of swapping all 5 valves back to the other amp now everything's hunky-dory (????) in that one.

All the 5 valves taken out of the fuse-blowing amp look in perfect shape at close range under strong light; nothing rattling around, so how one finds a faulty valve I don't know.

Any advice on the next steps above? Thanks.
 
Remember anything electronic can fail. Just put a new pair of 509's in and re-bias them up. Don't change or swap the small signal valves as it is very unlikely they will have failed, or are responsible for blowing fuses. This will only take a day or so if you go to a good supplier of valves. If it still blows fuses then get it back to EAR.
 
^ This ^ - 509s are remarkably cheap compared to a lot of more in-demand o/p valves - just buy a nice matched quad or matched pairs from a reliable source, sit back, enjoy some great music.
 
G T Audio said:
This will only take a day or so if you go to a good supplier of valves. If it still blows fuses then get it back to EAR.
Click to expand...

earlofsodbury said:
This ^ - 509s are remarkably cheap compared to a lot of more in-demand o/p valves
Click to expand...

I do have a pair of unused 509s from EAR (bought a full spare set a few years ago). I also have spare signal valves but I'm not sure of them. I'll put the new 509s in the other amp tonight with the signal valves from the fuse-blowing amp (easier that way).

Those 509s were not cheap; about £75 each if I recall, but I've no idea of valve availability/costs nowadays as the last time I had valves was 2 x RCA mono's in '82. A friend has a valve tester but can't so 509s; I guess EAR are the obvious choice for this. I've at least 6 x 509s which need testing, incl. 2 new ones from EAR which didn't last long (I think).
 
Output valves should be cheap as chips for these! The raison d' etre of the thing is that it uses PL509 TV line output valves which were once available from any high street TV repair shop and vastly more common at a fraction of the price compared to EL34's, 6L6's, KT88's. PL509's are not a particularly good choice but the cheap as chips and available anywhere bit made them very attractive. The EAR 519 used... surprise surprise... PL519's. A bigger TV line output valve and once present in just about every colour TV in existence. They used valves for line output, EHT rectifier and frame output even well into the solid state era and such TV's would have been transistorised (and even had IC's in them, TBA120 IF amp etc) apart from these 3 valves.

As others have said, it's almost certainly a PL509 that's responsible for the fuse blowing.
 
Arkless Electronics said:
As others have said, it's almost certainly a PL509 that's responsible for the fuse blowing.
Click to expand...

Thanks for more confirmation, Jez. My 509s can take PL519s as well, though I've never seem these. I started off with Tungsram as fitted when I bought the amps, with Tungsram spares but after a time all refused to bias up. Changed to EAR branded 509s (as said, @ £75 a pop) on (I think) my first service maybe 3 years later and got 4 spares, two of which didn't last long, or at least, refused to bias up.

I just wish it was obvious when a valve blows because without testing, how does one know? I've been going off (power) valved amps the last 9 months, although that was partially my ignorance, as I hadn't had a fuse go on anything since '83 and even that reset itself (and was my fault) That's the reason I've been looking at class A (?) s/s replacements (if you remember my threads) but so far the general opinion is that I'm going to have an uphill struggle even matching, let alone improving on my 509s and taking a punt is not really my thing at my age. Of course, ESLs need to be considered in that choice.

The bang/glitch/whatever which happens when a fuse blows is quite distressing. Although I understand that exc. protection is built into my 2905s, unlike a coil speaker, I'm not sure whether I'd realise if a panel had suffered; initially at least. Not quite ready to revert to my Dansette of the fifties, but you get the gist.:(
 
I had a mk1 pair of 509s. Quality left a bit to be desired, always ran noisy even after a full EAR service. The screws for the bottom panel were slightly different lengths, if you got them mixed up a long one could touch a pcb track with hilarious results, not.
 
Mike Reed said:
Thanks for more confirmation, Jez. My 509s can take PL519s as well, though I've never seem these. I started off with Tungsram as fitted when I bought the amps, with Tungsram spares but after a time all refused to bias up. Changed to EAR branded 509s (as said, @ £75 a pop) on (I think) my first service maybe 3 years later and got 4 spares, two of which didn't last long, or at least, refused to bias up.

I just wish it was obvious when a valve blows because without testing, how does one know? I've been going off (power) valved amps the last 9 months, although that was partially my ignorance, as I hadn't had a fuse go on anything since '83 and even that reset itself (and was my fault) That's the reason I've been looking at class A (?) s/s replacements (if you remember my threads) but so far the general opinion is that I'm going to have an uphill struggle even matching, let alone improving on my 509s and taking a punt is not really my thing at my age. Of course, ESLs need to be considered in that choice.

The bang/glitch/whatever which happens when a fuse blows is quite distressing. Although I understand that exc. protection is built into my 2905s, unlike a coil speaker, I'm not sure whether I'd realise if a panel had suffered; initially at least. Not quite ready to revert to my Dansette of the fifties, but you get the gist.:(
Click to expand...

I've known a few output valves play silly games where it seems the physical shock and vibration etc of removing them and putting them in the other channel seems to temporarily clear the faulty valve and have you thinking it was something else! Not often though thankfully!

I don't see why you should not find a SS amp that meets your requirements. I think I suggested a few in fact in your other thread.
FWIW I rebuilt a pair of Papworth M100 monoblocks a few years ago and these are basically the EAR 509 but with four EL34's per side and I was not particularly impressed when I put them in my system. It's not the 509 of course.... just virtually the same circuit... I've heard EAR509 sound very good elsewhere before.
 
sq225917 said:
I had a mk1 pair of 509s. Quality left a bit to be desired, always ran noisy even after a full EAR service. The screws for the bottom panel were slightly different lengths, if you got them mixed up a long one could touch a pcb track with hilarious results, not.
Click to expand...

Yes, I remember the latter and can confirm it also. I was at a dealers in the 1990s when the salesman went to switch one of the 509s on and got a shock as the chassis was live. He wasn't too impressed. As you said the longer chassis screws were touching the board and the mains...
 
Arkless Electronics said:
I've known a few output valves play silly games where it seems the physical shock and vibration etc of removing them and putting them in the other channel seems to temporarily clear the faulty valve and have you thinking it was something else! Not often though thankfully!
Click to expand...

Possibly on this occasion as well, Jez but I'm not prepared to blow any more fuses just to try them in again. I've pressed my last 2 new EAR 509s into service on the other amp with the signal valves from the other. Biased them up for 5-10 minutes and all is good. I've used a throw-away speaker for testing purposes but tomorrow I'll fire up the whole system again. Can't wait !

Need to get some more slow blow (T) fuses and am surprised that the ordinary glass ones on eBay range from 10 for under £3 posted and £3.50 each and beyond. Isn't a case fuse just a case fuse (barring foo ones)?
 
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