kel84 in a puff of smoke :(

My kel 84 just blew something.
Ive been running it for a year or two.

Took it apart and its blown c12.

Im thinking there was 2 possible reasons.

1st, i was trying some old untested mullard valves. I was warned one might fail, go red and blow something. I didnt really notice the valve as i turned it straight off but the nearest valve (v5) was looking a little red as the glow subsided.

At the time it blew i was changing my cartridge, amp was on, volume was down, phono was selected. Could shorting 2 cartidge tags have blown c12?

No big deal to replace the cap but i'd quite like to know why it blew and if anything else is likley to have been damaged?
I dont really understand what does what so i cant tell for myself.

Everything looks ok, took me a while to even find what had popped.

What do you think?

While im at it, i was thinking of upgrading some caps anyway.

Had a search and this seems to have been the recomended formula
C6 & 7 Blackgate 1000uF 16v
C8/9/10 & 11 Jensen paper in oil 0.22uF 400v
C12/13/14 & 15 Blackgate 100uF 50v


Where's best to get these and how much should they cost? I have absolutely no idea!!

[richardcooper2k]

standard blackgates are £2-3ish each in the values you want from hifi collective

the .22 coupling caps i wouldn't bother with paper in oil unless you want to go for mundorf silvers or audio note coppers at £20ish each. the soniqs sax are £4ish each from wd

as for why it went i doubt it was anything to do with shorting cartridge tags. bad valves are more likely i would think

Hi richard.

I thought it was much more likely to be the valve.

Made a good sound though, like a chestnut popping under the grill

Ill get some caps ordered.

Dont bother replacing c8/9/10/11 at all then?

Actually, ill start a new thread about cap upgrades

[colin.hepburn]

When you took the amp a part did you take that cap out as you may have put it back in wrong way round then it would go with a pop

Havent has the base off since i built it.

Just took the base off to see what popped and caused the smoke. c12 has opened up so i guess thats the culprit.

Not removed anything yet.

[Richard]

Originally Posted by graeme

Havent has the base off since i built it.

Just took the base off to see what popped and caused the smoke. c12 has opened up so i guess thats the culprit.

Not removed anything yet.

Hi Graeme,

Well, it seems too coincidental with trying the old valves so I think C12 is the victim rather than the culprit .

What happens is the dodgy valve draws grid current across R16/17 causing a positive bias voltage on the grid at pin 2 instead of it being 0v.

This "turns on" the valve so it draws more current through it and hence across cathode resistor R24/25. In turn this extra current causes extra cathode voltage at pin 3.

This cycle continues, in bad cases, into thermal runaway and something has to give out first.

If you're lucky the cathode voltage at pin 3 rose higher than C12 35v rating and that gave up first (it was trying to hold back more than 35v from reaching ground). Do check R24/25 as it will have been thrashed.

Drop me another pm if you still want those valves matching to some of mine and I'll send my address.

Oh, nothing to do with the cart tags btw.

Rich

[The Moose]

Hello mate

C12 is the cathode bias capacitor for Valve - V5. To blow that it is almost certainly the valve. It could well be worth checking the cathode bias resistor (as alreaded mentioned) - R24.

If you need some shipped out from us a World-Designs, drop me a PM soon and I will get them out to you. If you need the resistors too we can do that.

Look forward to hearing back from you.

Cheers

The Moose

Ok, thanks.

I need to see if i can find my multimeter, searched a few weeks ago and ive no idea where it is.

Ill find it, then check the resitors then order up what i need.

Richard, i PM'd you the other day