New and innovative ways for a valve to die and take out your amp!

[AlexM]

Oh dear... I was listening to some music, when I suddenly heard a loud crackle from one channel and saw a flash from inside my CJ LP125sa. Not good. I powered down quickly, and started to try and find the cause of the problem - I found this rattling around inside one of the KT120s

https://farm3.staticflickr.com/2932/13913905913_645655c753_c.jpg (https://flic.kr/p/ncwptx)2014-04-17 17.31.52 (https://flic.kr/p/ncwptx) by alexmastersuk (https://www.flickr.com/people/14548229@N07/), on Flickr

It is one of the top mica supports had apparently detatched itself and dropped offf into the valve base, causing a short. You can see the failed spot weld in the centre of the mica where the support finger dropped off.

https://farm8.staticflickr.com/7380/13890471296_4560258055_c.jpg (https://flic.kr/p/nasibq)20140417_173358 (https://flic.kr/p/nasibq) by alexmastersuk (https://www.flickr.com/people/14548229@N07/), on Flickr

After swapping some valves between channels, I have established that the plate fuses appear not to have blown. Unfortunately there isn't any music coming out of that channel, so I fear some damage has been caused.

I'll let you know what I find after some further troubleshooting. It's a bugger that it has happened over the Easter weekend... I guess I'll have to disintere one of my backup amps... hopefully there is still one working up in the attic!.

Regards,
Alex

[Firebottle]

That's a bugger. Not even as if the support finger was at a particularly hot part of the valve.

I hope it's not taken out the output trannie :eek:

:cool: Alan

Good pics by the way :)

[agk]

Thats a pisser man.
It sounds like you have probably lost a transformer, not too pricey but a pain.
Mind you, at least not self inflicted. I had a nice valve amp until I took out an el34 with a poorly placed freekick. Caught fire and did a fair bit of damage with shorts including an output transformer.
Never play footie in the house children!

[AlexM]

God, I hope not. The output transformers on this thing are massive very high quality items, and they are guaranteed to be very pricey!. Hopefully it is something less drastic. Will have to get my meter to test resistance side to side.

Regards,
Alex

[StanleyB]

Parts do fall off valves at times. I used to just change the valve and try the item again. Engineers like me used to be called bottle swappers in those valve days.

[Firebottle]

I swap a few bottles these days, empty ones for full ones normally :lol:

One of the latest:

http://i601.photobucket.com/albums/tt97/Paramotorpilot/P1030843.jpg

:cool: Alan

Sorry for the drift, i'm quite chilled................

[anthonyTD]

Could just be the screen grid supply gone down, is the amp ultra linear, or conventional Tetrode connected ?
A...

[AlexM]

Hi Anthony,

Ultralinear fixed bias, no triode mode. Interestingly, the channel that was making the noise was the left channel, the tube with the loose part appears to be on the right channel. I can't be confident that the bad valve isn't the one photographed above. Is it safe to replace the one with the loose part and power it up, or do I need all of the valves testing before reuse?. I'd appreciate any suggestions.

Regards;
Alex

[anthonyTD]

Hi Alex,
First of all, don't replace that valve, you never know what parts it may short out next!
I think what has happened then is the HT supply has gone down, if its on both channels, its got to be something on the raw side of the HT supply, are there any other fuses inside the amp ?
A...
Hi Anthony,

Ultralinear fixed bias, no triode mode. Interestingly, the channel that was making the noise was the left channel, the tube with the loose part appears to be on the right channel. I can't be confident that the bad valve isn't the one photographed above. Is it safe to replace the one with the loose part and power it up, or do I need all of the valves testing before reuse?. I'd appreciate any suggestions.

Regards;
Alex

[walpurgis]

Hi Anthony,

Ultralinear fixed bias, no triode mode. Interestingly, the channel that was making the noise was the left channel, the tube with the loose part appears to be on the right channel. I can't be confident that the bad valve isn't the one photographed above. Is it safe to replace the one with the loose part and power it up, or do I need all of the valves testing before reuse?. I'd appreciate any suggestions.

Regards;
Alex

Is it possible that you have more than one dodgy valve? Maybe they are from a batch that was below standard.

[anthonyTD]

I think what we are trying to establish now is what damage has the valves in question caused to the amp, which is what i am most concerned with at the moment.:)

[AlexM]

Hi Anthony,

The amp has a main fuse, and a plate fuse (internal) per side. Helpfully, there is an indicator LED that illuminates if the plate fuse is blown. Neither the left or right channel plate fuses are illuminated.

The situation is somewhat confusing as the faulty tube shown above appears to be on the right channel, but the noise is from the left. The valve numbering is somewhat unhelpful as the KT120 output valves n the left channel are numbered V2 ,4,6 & 8 and those i. The right are v, 7,9 & 11. V1 is the 12AU7 phase splitter and V2 and v3 are the left and right channel 6n30pi drivers.

V9 is the faulty valve with the loose bits from the right channel, but the noise was from the left speaker. So my quandary is having replaced V9 with a spare, and then putting them into the left, I still have no sound. I haven't swapped left to right as there may be a faulty valve there too. I'm not sure I now have eight good output tubes, so I am loathe to cause any potential further damage.

I have put the four output tubes from the right channel into the left. Heater supply is up as all of them appear to be glowing, but it could be an HT rail down - does each channel ypically run it's own HT supply?.

Thanks in advance for any advice.

Alex

[anthonyTD]

Hi Alex,
Does sound like the main HT supply has gone down, do you have a circuit schematic, if not, i will try and find one, i really need to look at that before i can give you any more pointers.
Hi Anthony,

The amp has a main fuse, and a plate fuse (internal) per side. Helpfully, there is an indicator LED that illuminates if the plate fuse is blown. Neither the left or right channel plate fuses are illuminated.

The situation is somewhat confusing as the faulty tube shown above appears to be on the right channel, but the noise is from the left. The valve numbering is somewhat unhelpful as the KT120 output valves n the left channel are numbered V2 ,4,6 & 8 and those i. The right are v, 7,9 & 11. V1 is the 12AU7 phase splitter and V2 and v3 are the left and right channel 6n30pi drivers.

V9 is the faulty valve with the loose bits from the right channel, but the noise was from the left speaker. So my quandary is having replaced V9 with a spare, and then putting them into the left, I still have no sound. I haven't swapped left to right as there may be a faulty valve there too. I'm not sure I now have eight good output tubes, so I am loathe to cause any potential further damage.

I have put the four output tubes from the right channel into the left. Heater supply is up as all of them appear to be glowing, but it could be an HT rail down - does each channel ypically run it's own HT supply?.

Thanks in advance for any advice.

Alex

[Spectral Morn]

God, I hope not. The output transformers on this thing are massive very high quality items, and they are guaranteed to be very pricey!. Hopefully it is something less drastic. Will have to get my meter to test resistance side to side.

Regards,
Alex

If its the CJ then sadly I don't see a new transformer being cheap, but fear not it may just be a resistor blown under the valve base. Go get it checked out it may not be as bad as it appears.

Edit I see plenty of advice from Anthony - should have read full thread first lol


Regards Neil

[AlexM]

It's under warranty, happily. Should have it back early next week.. I'll update once I've got a report back