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  #1  
Old 21st January 2011, 03:58 PM
hooger hooger is offline
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Default When to change valves?

I am sure this must have been asked before (not that I can find it mind) but are there any obvious signs of when to replace valves? I have a KEL84 which I built 5 years (ish) ago with the original valves. I am sure it's not sounding quite like it used to but am unsure if it is the valves. Are there any tests I can do? Thanks in advance.
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  #2  
Old 21st January 2011, 06:10 PM
John Caswell John Caswell is offline
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Default Re: When to change valves?

Hi Hooger
Generally small signal valves last for up to 5-10 years used a few hours daily, but I tend to advise looking at them/replacing after 5 years.
Output valves are a different kettle of fish and I usually recommend replacing at the latest after 2 years, once again used a few hours daily.
As to how you can tell, experience is a good start, but of course that doesn't help newbies etc.
I suggest that you measure the cathode voltages of the EL84s in your amp and compare them to what the manual says, they should all read the same (within 10%) any that are wildly different from each other or the stated voltage in the manual tend to be knackered. Look at the gettering (the internal silvering) if it is whitish or a transparent brown change the valve(s).
Personally after 5 years I would be changing the EL84s for a new matched quad set.
Hope this gives you some ideas. Any questions ask here, I or others will try to confuse you

John

john
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  #3  
Old 22nd January 2011, 09:27 AM
Richard Richard is offline
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Default Re: When to change valves?

Yep, agreed with John and would expect your EL84s at least to need changing if they've been used regularly.

How to tell is a good point and experience isn't much good if the valves are unknown etc. Get them tested with a valve tester is one way. Another is to buy a complete new set for the amp. You'll then always have spares for when they do eventually fail and have them to hand to pop in to check any difference as they get older.

Basically they sound weaker, like you turn down the colour on a TV and it gets washy, the sound picture gets less dynamic with less bass and treble.

It also creeps up on you slowly and I've found myself tweaking the system only to realise the OP valves were tired! So get new valves ready.

On valve life I was reflecting recently that running 300Bs is actually quite cheap. They have a directly heated cathode which last over twice as long as indirect types and after several years with a few different ones now I've yet to actually wear one out! I can hear/compare them and test them. In that light it makes running the 300BPP cheaper than a KT88 amp and the 300BSE about as cheap as an EL34 or 84 PP. Frankly the 300B is a bargain and far from expensive the only downside is size and weight!

New valves seem to go through stages btw, sound bright for 100 hours or so then settle for a year or two then fade slowly, and might hum just before running away completely and going pop...
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Old 22nd January 2011, 10:05 AM
hooger hooger is offline
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Default Re: When to change valves?

Thanks John and Richard, good advice. I won't start the question of which EL84s to go for but are there any suggestions suppliers for a quad set?
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  #5  
Old 22nd January 2011, 12:18 PM
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Ali Tait Ali Tait is offline
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Default Re: When to change valves?

I liked the Philips 6bq5 (I think!) that Langrex sell in matched quads for I think 64 quid. Very nice to my ears and not silly money like Mullards etc. Search the bay for seller "yitry"
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Old 22nd January 2011, 03:09 PM
Richard Richard is offline
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Default Re: When to change valves?

I was impressed with EI EL84s when I had an ST20, almost dead ringer for Mullard construction, got them from Maplin several years ago. They were in when Ali had a listen once

Very powerful open sound, could have been a tad sweeter but they were new and crisp and it may also have been the amp.

There used to be a helpful chap called Troy (I think) at EI, he'd know the ones they suppled to Maplin, originally Yugo I believe but whether that's Slovak or Serbian valve collection now I don't know, drop him a mail,

http://www.edicron.com/collections.html
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Old 22nd January 2011, 04:04 PM
hooger hooger is offline
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Default Re: When to change valves?

Decisions, decisions. Thanks for the suggestions. I will be buying one or other very soon and report back on my findings.
Hooger
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Old 23rd January 2011, 12:47 PM
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Ali Tait Ali Tait is offline
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Default Re: When to change valves?

Yes Rich, that was a nice amp.You made a lovely job of it.
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  #9  
Old 2nd February 2011, 12:42 AM
Revoli Revoli is offline
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Default Re: When to change valves?

Oh dear. Does that mean I should be changing my wad6550 valves that are 10 years old.......

Can anyone recommend what is worth trying in this amp in terms of valve upgrade. I knOw it's somewhere in the forum....
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Old 2nd February 2011, 04:14 PM
John Caswell John Caswell is offline
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Default Re: When to change valves?

Hi Revoli,
I tend to use Svetlana 6550C as they seem to be the best of the current bunch both in results and reliability. Have not been very happy with Sovtek ones. Personally I would not be seduced by offers of NOS GE etc as they are liable to cost lots of money and very often not even warranted for the nominal 90 days. But as usual it is up to you for the final say.

John
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