Blown Valve in ARC VT50 - help - diyAudio
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Old 1st January 2007, 11:52 AM   #1
Matt is offline Matt  United Kingdom
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Default Blown Valve in ARC VT50 - help

Recently purchased second hand ARC VT50. Whilst listening in the store a valve blew. It was repaired and all was well. Although told new valves were put in, another has gone! Along with it goes a resistor, which went with a 'bang'.

Whilst not particularly good at electronics I can solder and use a multimeter. Can any one tell me if all I have to do is replace the valves and the blown resistor, or will there be more to such a repair?

I realise this is a, 'it depends' question but as a general thing could I do it? Valves must blow all the time for one reason or another so I wouldn't have thought it would be too catasrophic - would it? This is my first valve amp. Thanks.
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Old 1st January 2007, 12:26 PM   #2
Svein_B is offline Svein_B  Norway
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It depends, on the knowledge of the person who replaced the tube. It could be that the bias for the output tubes was not re-adjusted when tubes were changed.

The pricedures should be in the manual, or here:
http://www.diyaudio.com/forums/showt...12#post1023012

SveinB
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Old 1st January 2007, 12:37 PM   #3
Matt is offline Matt  United Kingdom
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Thank Svein, I am happy with the info James gave but this is about re biasing which I am happy with. Just wanted to know if the repair is limited to the blown valve and resistor and if it is something I could do.

Also when valves go do they tend to take other components or just the biasing resistors?
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Old 1st January 2007, 12:56 PM   #4
Svein_B is offline Svein_B  Norway
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I think you should try to identify the blown resistor. Value? where is it connected? etc.
Also, did you notice any visual or audible abnormalities prior to the failure, i.e. red glowing tube ?

Some of the most likely explanations for a blown tube could be:
- incorrect bias setting
- grid voltage due to a leaky coupling capacitor
- a short circuit somehere
- bad tube

SveinB.
(having absolutely no knowledge on this particular amp)
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Old 1st January 2007, 01:12 PM   #5
Matt is offline Matt  United Kingdom
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Svein

My wife switched the amp on and said that the valve seemed to 'sparkle' just before the resistor blew. The resistor is quite obvious because it shattered. It is next to the valve and each valve seems to have on each. I guess it is the biasing resistor.

I see what you say about why they fail. I think it likely to be a valve failing as the same thing happed at the showroom whilst listening before buying. I am sure old valves were used to cut costs to the supplier and should have perhaps renewed them but they said they were new. Even if they were I know new ones can fail just as easily as old ones.

Don't think it is a capacitor or short as it was working fine last night and just went on switch on - like a light bulb would go.

So you think if I replace the valve and resistor it should be ok pending no other fault! I am inclined to give it a go and see.

Thanks.

Matt
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Old 1st January 2007, 01:19 PM   #6
Svein_B is offline Svein_B  Norway
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Yea, I would give it a go.

Just make sure you carefully adjust the bias, and then use your ears, eyes, and nose to observe any further irregularities

SveinB

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Old 1st January 2007, 02:10 PM   #7
Matt is offline Matt  United Kingdom
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Thanks Svein.

I like the suggestion to use eyes ears and nose! All the senses were assaulted when it went!

I will give it a go. Valves on order and I will measure the resistors left in the unit. Looks simple task to do so fingers crossed. I'll post a reply when done with the result - Whatever it is just so others will know.

Thaks again for your time.

Quite - I can't think why! Too much beer Last night perhaps!

All the best for the new year.

Matt
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