6550 valves - longevity?

[Hozepipe]

Hi guys

Been off the radar for a bit, having neglected my hifi due to persistent ongoing problems that I seem to be unable to spend time on fathoming. Hope you all had a good xmas and New Year.

But I may have found the root of my problem so have a question - how long would you expect a quad of SED 6550s to last in an Audio Research amp (a 65watt D70)? I estimate the amp is on 8-10 hrs a week on average at a rough guess.

The persistent distortion seems to have been the power valves all along - I had discounted them because they were only 18 months old before the distortion started creeping in.

I usually get mine from a dealer in the US, cost about 75-100 quid depending on the exchange rate at the time. Assuming the valves were only operating at their best for about 12-18 months does that sound like reasonable longevity? That's quite a maintenance hit every year or so. I'm seriously thinking of going SS...

I'm going to give valves another chance - are there better, more robust valves than the SED/Svetlana or did I just get some duff ones?

[Spectral Morn]

Anthony TD will no doubt give a better answer than I am about to but the life of a set of power valves on a given amp will depend on how hard they are driven. Each designer has a set of specs in which he can design i.e a power envelope that any valve type can achieve in a circuit. There is a consensus that it is better (for valve life/longevity ) not to push power tubes to the limit of what they can do. Keeping power valves operating in a mid point with lower biasing will prolong valve life. However this is a generalization as it can be possible to get more power safely if the circuit design and output transformers + power supplies are as good as can be got/at a given retail cost.

I don't like the sound of the 6550 valve, so I have no direct experience as a long term user of such(and life of the valves will depend on amps used) but I would have said from experience with other types of power valves, that you are not getting as long out of these as I would have thought, this is assuming that the amp is working to spec. I guess you should get someone to check the amp out as there may be a problem with it. The bias may be set to high ? If this has happened with lots of 6550 tubes then it is likely to be the amp at fault,either a fault or design quirk. I have got years out of el 34, kt 66's and 5881 valves. And when I say years I mean, 5 years plus. What type/make of valves are these that you are getting from the USA and are they matched quads ?

Anthony Td is a bit of a valve expert (being a designer and repairer of such kit ) so his thoughts on this should be very valuable, plus if you are close to where he is based he could take a look ?

Regards D S D L----- Neil

[Hozepipe]

Hi Neil

Thanks for the advice - I keep the amp biased at the recommended value (65mA). The amp generates 60wpc (not 65 as I said, I just checked the manual) which I think it the rated value for the valves?? Wonder if I can reduce the bias (or do I raise it?) to run them less hard?

They are 'Winged C' SED aka Svetlana 6550s. matched pairs. There's another 6550 in the PSU. The previous set lasted longer. I've had three sets in the amp (including the ones that came with it) over the past 6-7 years (roughly).

[PS - I really like the sound of the amp apart from the problems - when it works well it's superb. I had Audio Innovations 1000 monos with EL34s which I didn't like at all, too soft sounding. I had a Hiwatt guitar head with EL34s once though, that sounded incredible!

and appols for posting in the wrong forum!]

[Spectral Morn]

Hi Hozepipe

Thats odd I have a set of AI 1000 mk 3 which with the standard EL 34's sound a bit bright and forward (only slightly and relatively to how valves sound normally. There I go again generalizing ). I put a set of Tung-Sol 5881 (military spec el34 )and the sound quality went through the roof. These Tung-sol reissue valves are very good IMHO. Pre - amp used is an EAR (can't remember the model )I'll look later.

You should PM Anthony Td and ask him about your amps and how to Bias them/what he recommends. Ask about the 6550 valves you mention. He has not read this thread yet or I know he would have posted a reply.

Regards D S D L ---- Neil

[Hozepipe]

Hi Neil

I guess it depends what sort of valve sound you're used to - the D70 is not that 'valvey' sounding, by comparison the AI 1000 was soft but I can't remember what valves were in there, I only kept it a few weeks and this is going back a good few years.

I was reading about the Tung Sol 6500s too, maybe I should try those instead of the SEDs... I'll try Anthony tomorrow (don't like to pester people though).

[Gerry]

The suggestion has alway been that these power valves should give some where in the region of 2000hrs use. But it does depend on how hard they are being pushed. i know on the old WAD KT88 amp it was recomended to change the grid resistors so that more life could be got out of the vlaves.
Quite often on power amps it is the resistors breaking down which can cause the problems. Another suggestion given to me which causes them to wear faster is how close the valves are together....heat from each other.
I have the Tung-Sol 6550s....very very nice. I would recomend them. Mine were bought in 2006 and still seem to be going strong in a Beard P101...110W
Regards
Gerry

[Spectral Morn]

Hi Hozepipe

Yes there is a world of difference between the Audio Innovations sound and your Audio research, both are good.

I don't think Anthony TD would mind you PMing him.

regards D S D L---- Neil

[anthonyTD]

hi hozepipe,
i was expecting to see your avatar frozen at the moment
anyway, back on to the 6550's depending on the type, ie, sovtek, svets, chinese etc, if the amp is biased conservatively and the ht voltage is under 500v
they should give at least 5000 hours in a push-pull design, but some will start to loose their tone well before that, the original winged C svets are about the best for reliability and longevity, i run the KT88 version in the soul amps which are single ended, and are run at close to their maximum rating, around 33 watts disapation, i have had amps in that have been left on continously for over a year and they still had full emision! therefore as a safe bench mark i would say 3000 hours is a good expectation. the original GEC KT88 have been known to give up to 40,000 hours of usefull life, thats one of the reasons why they are so sought after, that and the fact that they are the best sonically without doubt.
as the guys have stated, there is always the posibility that the amp has a fault.
regards,anthony...

[Hozepipe]

Gerry: another vote for the Tung Sol then.

Anthony: Ha! Yes it is a bit nippy isn't it. Given my estimate of 1.5 years at 10hrs/week that gives a meagre 780hrs. Not good! Even if I double my estimate that only gives me 1560hrs.

Something is amiss! PM on its way....

[NRG]

Another possible reason given for short valve life is cathode stripping or more accurately cathode poisoning....opinions are divided on this and it's been furiously debated on many forums in the past but the theory goes along something like this:

If the HT is applied in advance of the heaters there is a possibility of ions bombarding the cathode reducing it's emissive capability and so shortening life...with the heaters properly warmed up the electron cloud surrounding the cathode repels the ions.

The old WAD amps with solid state rectification applied HT for approx 30s or more before the heaters had time to reach full temp....if you power cycle such and amp often enough the cathode is subjected to these ion attacks...as I said that's the theory...

The counter argument is that the potential difference between HT on the plate and earthed (effectively) cathode needs to be about 10Kv for this to take place and so at typical domestic valve amp HT of 300~500v it's hardly an issue...

One interesting bit I dug up was that of 'cathode sleepiness' where leaving a valve in an effectively cut off state by removing the heater voltage whilst still maintaining HT for extend periods of time turns the plate into a getter: The gas impurities present in the valve envelope (it's never a perfect vacuum) get attracted to the plate and stop it conducting...