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Valve amp power failures

Monitor Gold 10 said:
Regarding the part about Valves and TV/Radio, just wondering why this is a potential problem? Apologies for the silly question!
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No problem as such, I’d personally just be reluctant burning through £100s worth of beautiful vintage valves watching Sky News, PMQs, Question Time or whatever! My TV rig is a passive pre, a rebuilt Quad 303 and a pair of Spendor S3/5R, and it does the job as well as it needs doing, as will your Sony and Ditton 15s. A great TV rig IMO.
 
Brian S said:
Accepting that 'all modern KT88' valves are crap, and accepting that I can no longer get my amp to a repair man, I am somewhat anxious about retaining my Graff 50 amp. Anything beyond changing valves and fuses is beyond me. This amp is just as successful driving Spendor D1s as my 300b was driving Audionote Js. Hence, I am not to keen on parting company with it. This amp uses 4 KT 88s and produces only 50WPC. To what extent does this decrease likelihood of valve failure taking out resistors? I have been using Electro Harmonix. NOS valves are out of my range.
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That will make it much more reliable than getting 80-100W from a pair for sure.
 
Out of interest, is the issue for how much power to extract from KT88s dependent on how hard they are driven (i.e. how loud the music is / how (in)sensitive the speakers) or does it just depend on the maximum available output?

For example, would an amp with 80-100w KT88 pairs but driving very efficient speakers and therefore more-or-less idling at well under 1w be any more or less reliable than a 50w KT88 pair driving something a bit difficult?

I'm just wondering about that Mac, with its 75w into 2, 4 or 8ohms and sensitive speakers.
 
JTC said:
Out of interest, is the issue for how much power to extract from KT88s dependent on how hard they are driven (i.e. how loud the music is / how (in)sensitive the speakers) or does it just depend on the maximum available output?

For example, would an amp with 80-100w KT88 pairs but driving very efficient speakers and therefore more-or-less idling at well under 1w be any more or less reliable than a 50w KT88 pair driving something a bit difficult?

I'm just wondering about that Mac, with its 75w into 2, 4 or 8ohms and sensitive speakers.
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It's mainly down to the higher voltages and usually higher dissipation that the valves have to operate at with higher power capability.
When NOS was just new in the 50's and 60's etc, when an output valve data sheet said it could take say 800V and at 580V ish 100W could be obtained from a pair in push pull it was generally true... yeah you'd be thrashing them using them like that and they may wear out in 2-3 years but they would usually be reliable enough.
Most modern versions claim the same figures in their data sheets but they just can't deliver it with any reliability...
Those figures are approx max for KT88 but with modern ones you would be much safer getting just 50-60W from a pair... or less.
Modern power valves are a bit like old British motorbikes (no they don't leak oil!). The fastest could do around 120MPH but if you want to make it from London to Newcastle on the motorway then it's advisable to restrict things to 65MPH!
Luck also plays a part. You will hear some say how they've had "insert new valve make" KT88's or EL34's in a fairly powerful amp for 3-4 years and counting... and no problems. But for every one of those you will usually find 2-3 people who say "well I had a new quad of "X" valves and they lasted 4 months then there was a loud crack and the mains fuse went. Had to get it repaired cos a few resistors had gone. Then I had a replacement set of "Y" valves and they lasted a year and a half then the same thing happened"
 
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Interesting thread, I've had my Puresound 2a3 for over two years now and it's been a solid performer, I know it's about seven years old in total, the only issues I'm aware of are a few power tube failures, but not since I've owned it.
It uses twin valve rectifiers (GZ34) TAD, been reliable so far, looked at Mullard but hard to find.
It's a great sounding amp I must say!
 
O/k loosing gain, I spent loads of time swaping out frame output valves to get the picture to touch the sides, nasty things.

Pete
 
To be pedantic.... gain stays pretty much the same with a triode as it is largely defined by internal feedback, grid positioning and mesh size. As emission drops transconductance lessens and this is the issue.
Valves are great things and can be very reliable. They can do many things which SS can't do or in some cases valves just do it better and more reliably than SS. Hence the valve output stage in many communication satellites even today. Using valves and SS together in a hybrid circuit can give superb results (I do not mean valves followed or preceded by SS but both together as part of the same circuit) and is my preferred modus operandi for a lot of the stuff I do for myself.
 
I used to work on a B&O colour tv that had the usual outpt transistors for each colour driving a valve, it also has the usual black and white frame output valve but used as line output, and a seprate pair of colour output and flyback valves just to genarate the EHT.
Massive dropper resistors all over.

The cabinet was the best thing about it.

Pete
 
Pete MB&D said:
I used to work on a B&O colour tv that had the usual outpt transistors for each colour driving a valve, it also has the usual black and white frame output valve but used as line output, and a seprate pair of colour output and flyback valves just to genarate the EHT.
Massive dropper resistors all over.

The cabinet was the best thing about it.

Pete
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Some of the European sets were renowned for being over complex... Most hybrid sets (which the majority of earlier colour sets were) had transistors for the colour outputs to the guns though, usually mounted on the CRT socket PCB.
 
I recall a few years ago doing some work at a submarine communications facility. The transmitters were huge and were all valve units. The "coax" to the antennas was a steel pipe about a foot in diameter.
 
Monitor Gold 10 said:
I've not heard of this. What does it mean? What are the telltale signs?
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A really good vacuum is said to be a hard vacuum... If any slight amount of air gets in or there is "out-gassing" from the internals then the vacuum is said to be "soft". A pinkish glow that extends beyond the electrodes can be a sign mainly in power valves but don't confuse it with the purple glow you can see through the anode holes in power valves. The days are numbered for a soft valve and it may go into thermal runaway. If the vacuum goes further the gettering (the silver on the inside of the glass) tuns white and the valve won't work.
 
Brian S said:
Accepting that 'all modern KT88' valves are crap, and accepting that I can no longer get my amp to a repair man, I am somewhat anxious about retaining my Graff 50 amp. Anything beyond changing valves and fuses is beyond me. This amp is just as successful driving Spendor D1s as my 300b was driving Audionote Js. Hence, I am not to keen on parting company with it. This amp uses 4 KT 88s and produces only 50WPC. To what extent does this decrease likelihood of valve failure taking out resistors? I have been using Electro Harmonix. NOS valves are out of my range.
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I don't blame you. The Graff 50 was very unreliable as were all Graffs. Their circuit designs had a lot of issues.
 
chris@panteg said:
Interesting thread, I've had my Puresound 2a3 for over two years now and it's been a solid performer, I know it's about seven years old in total, the only issues I'm aware of are a few power tube failures, but not since I've owned it.
It uses twin valve rectifiers (GZ34) TAD, been reliable so far, looked at Mullard but hard to find.
It's a great sounding amp I must say!
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What 2a3s are you running? I have a feeling mine crapped out which has lead to my amp fault, so it would be interesting to see what other brands produce decent ones
 
rescuest3ve said:
What 2a3s are you running? I have a feeling mine crapped out which has lead to my amp fault, so it would be interesting to see what other brands produce decent ones
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I've got a quad of Chinese made 2a3's by GG, a quad of Electro Harmonix golds and a pair of sovtek's.
I feel that the EH and Sovtek's sound pretty much the same, the GG are pretty decent too and all reliable so far.
A couple of the original EH tubes failed with the white getter.
 
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