Smokey Valve Amp!!

[Sligolad]

One of my Art Audio Maestro's began to look very sick last night :-(

I recently revalved both with 2 octets of EL34s and they were working sweetly for over a week but I had seen an ominous sign during installing the valves.
In my rush to hear them with the new valves I just switched off the power switch and the standby switch on both and left them connected to power while I pulled the old valves and pushed home the new ones.
On one of the amps i noticed the little signal valves start lighting up as I was loading the EL34s so immediately pulled the power leads....thought it strange how this could happen with both switches off but maybe there is more to valve amps than I thought!

Anyway last night the one which started lighting during valve installation was running away fine for several hours of sweet music until I went and switched everything off.
Went away for a few minutes and came back and I could hear a little tizzzzing sound and then noticed some smoke starting to waft from said amp....pulled both power cords immediately!!

Certainly learned that leaving old valve amps connected to the mains even with the amps switched off is not as as safe as I thought, probably lucky I was still in the room after switching them off but most likely fried one of the transformers.

Pulled the bottom off this morning (before anyone asks YES the mains was disconnected :-)) and all the capacitors and resistors look good so some fun and games ahead methinks.
Time to pull out the Primare Solid State for a while and see how it sounds until I get to the bottom of this.

Looking back at some old threads I seen the HiFi Clinic mentioned for repairs.....any update on where to go for repairs in Ireland would be appreciated rather shipping back to the UK?

The central heating will have to do a little more work for a while until I get this sorted!!
Cheers,
Pearse.

[giant haystacks]

perse telephone rodney hanna in enniskillen he trades under hanna electrical cross st enniskillen -he works at this sort of thing all the time -and if you arange something with him i have a friend near carrickmacross you can leave with him and i can bring them to rodney -i owe you a favour

[Diapason]

Is it possible you've just fried a signal valve? Do you have any to replace it with?

[terrybites]

You could give Bernard Mothersill a shout at 087 247 7714. He's in Dublin and repairs valve amps for Cloney Audio. Very nice chap.

Terry

[Sligolad]

Thanks for all the offers of help guys but thankfully it was not as bad as first thought.

Lifted off the transformers this evening and powered up with the PCB out and all disassembled and finally tracked down the problem.
There is a Philips MKT Capacitor 1uF 250V across the power switch connections and it was so close to the metal frame it has broken down and power had started bridging to the frame of the amp, hard to believe but this seems to have been the issue, I moved it away from the frame a bit and all is sounding very well again.
Just been listening to Jazz at the Pawnshop for the past 2 hours with a bottle of Pelan 2001 and everything sounds sweet.

Guess the Cap is there to help prolong the life of the switch but not sure, been many years since I was more knowledgeable in electronics, would like to know what those in the know think?

Thanks again and valves are still King!!
Cheers,
Pearse.

[Ivor]

Just been listening to Jazz at the Pawnshop for the past 2 hours with a bottle of Pelan 2001 and everything sounds sweet.

that would work all right!

[JAW]

You should replace that capacitor. It's there to prevent a current surge on switch on. If the capacitor is damaged and goes open circuit you will get a distinctive "thump"
sound from your speakers when you switch on, unless there is some other circuit protection. If the capacitor breaks down and goes short circuit it will
effectively bypass the on/off switch and your amp will be constantly on.

[Fran]

Very strange that they would use that type of cap there - normaly they're X2 rated caps. The second thing is that 1uF is quite big for this, usually its much smaller, from 0.047-0.22uF typically. As JAW says the cap serves a few functions - helps prevent thumps, and also (very imp for valves) it prevents RF and hum pickup from the environment. I had woeful trouble with a valve phono stage until I adopted a pretty rigorous strategy with these caps on the inputs.

A film capacitor could be destroyed when a high
energy pulse is applied and the self healing properties are
failing (self healing is the ability to recover after a
breakdown)

The pulse can happen if it sees more than 250V - which is possible now and again. More than you ever wanted to know here: http://www.vishay.com/docs/28153/anaccaps.pdf


If you need one give me a shout, I'd have some here you can have.

Fran

[Sligolad]

Thanks Jaw and Fran for the feedback, I will look at replacing the Cap in the new year.

I have a system for startup and shutdown which I have been using from day one and it should keep me going until I get to replace it.
I always switch on the power on the back using the switch with the dodgy cap first and let the amp warm up for a minute before I switch in the other switch on the side of the amp which properly fires up the amp.
The switch on the back seems to provide a low enough power to the amps as there is no sign of any flicker or lights dimming, it is only when I hit the switch on the side which I think is the standby switch that the lights in the room take a little sharp intake of breath and the valves really kick in to life.

I follow the reverse procedure on shutdown so I should be OK for another couple of weeks.

Cheers, Pearse.

[Fran]

Nice amps Pearse - those BHC caps are pretty high end, although somewhat unknown - and are about stg£25 each IIRC.

RE: the double turn on - it undoubtedly keep some part of the circuit on - or maybe its just that they had a surplus of swtiched IEC inlets and that its "double" switching.


Fran