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Old 8th May 2019, 8:04 pm   #1
GrimJosef
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Default How much anode glow is acceptable in a KT88 ?

I'm currently working on a pair of Leak TL/50 Plus amplifiers which use KT88s as the output valves. The output stage circuit is as shown here http://www.44bx.com/leak/Leak/Circuits/tl50Plus.gif although in my amps the earlier stages are slightly different. I have an anode-cathode voltage of 440V and a cathode voltage of a little over 50V, corresponding to a cathode current of 84mA. So each valve's internal dissipation (anode plus screen grid) is 37W.

This is within the GEC datasheet's design maximum of 40W http://www.r-type.org/pdfs/kt88.pdf, and yet there is some red glow from the anode seams - see attached picture (my phone struggles to get clear pictures in the low light needed to see this glow - apologies). Both of the valves are glowing, as are the two in the other amp that I have. I've tried a pair of modern production Svetlana KT88s and they glow too, although less than the GEC valves.

The datasheet includes some recommendations for installation of these valves, saying that the socket centres should be not less than 4" apart and that pins 4 and 8 of each valve should be in line. Leak followed neither of these recommendations, with the centre spacing being 2.5" and pins 2 and 7 (or 3 and 6 if you like) being in-line between the two valves.

So should I be surprised that the valves are glowing a bit (someone who once owned a pair of these amps has told me that he remembers his glowing) ? And should I be worried ? I came across this used TT21 (the top-capped version of the KT88) in a box of random valves recently. It was a little while before I noticed the hole melted through the anode (just above the GEC sticker in the pic) ! The opposite seam is just the same and the holes go right through - at the right angle you can see daylight through them.

Cheers,

GJ
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Old 8th May 2019, 8:38 pm   #2
bikerhifinut
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Default Re: How much anode glow is acceptable in a KT88 ?

The voltages seem correct GJ.
84mA is well within the KT88's specs I think, I have seen a fair few circuits that show a cathode current of 100mA.
I got a bit paranoid about a glow from a set of EL34 that were running well inside their parameters, but this was only noticeable in very low ambient lighting.
I wondered if the anodes are actually a bit thin and so kind of a bit transparent? bad way of describing it I guess.
The only other thing is, i wonder if your grid coupling caps are even very slightly leaky? Myself I'd have removed any suspect coupling caps and that means ANY PIO regardless of vintage, bagged em up to put back with tha amp for the owner, and replaced em with a set of decent plastic film jobs. I realise this is a teaching granny to suck eggs moment, but if it was one of my amps I'd be straight on to checking the DC on the KT88 grids.
Another thing is, would the owner notice if you dropped the grid leaks to 220k, this may result in a slight power reduction but would it be so audible? That might lower any stresses on the valve?
The problem if there is one, has to be in the output stage and if all voltages measure correctly then I'd just put it down to the valves. Might be worth checking screen grid current too?
Apologies in advance GJ, I realise your experience eclipses mine but those are the things I check on my amps.
yeah the valves are packed a bit tightly, again on homebrews I keep these big bottles separated by the specified distances. I think the pin orientation for vertical mounting might have something to do with noise pick up? There are also rules for horizontal operation on KT88 and I think also EL34 which is to do with heat radiation/dissipation?.

A.

Andy.
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Old 8th May 2019, 8:56 pm   #3
GrimJosef
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Default Re: How much anode glow is acceptable in a KT88 ?

I don't think I'm worried about control grid voltages Andy, which can of course rise if the coupling caps are iffy (the caps are brand new, so fine in fact) or the grid leaks too high in value. The reason we worry about raised grid voltages is that they raise the quiescent current through the valve, which causes it to overheat. But the current is exactly right, at least according to the numbers on Leak's circuit, so it can't be being raised by anything going on at the grid.

The question really is just about the valve I think (at least, after all the goings on in the other thread, I know the amp isn't oscillating ).

Cheers,

GJ
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Old 8th May 2019, 9:02 pm   #4
davebms
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Default Re: How much anode glow is acceptable in a KT88 ?

my small experience , many new production valves ,cant take the
the power rating as advertised, I've lowered the power rating on
2/3 amps, not really noticeable at all

dave
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Old 8th May 2019, 9:38 pm   #5
Half a Mullard
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Default Re: How much anode glow is acceptable in a KT88 ?

I'm not sure if this really helps but I've just cranked up my Carlsbro PA which uses 4 KT88 - they are all GEC. I'm not sure about the current but I think these are running with about 700 volts on the anodes in fixed bias. Not the same as the Leak obviously but I thought I'd see what they look like. Its a bit difficult to photograph and get the light at the right level. I've also just noticed a little bit of a blue glow in one of the valves, I wonder if it might be a bit gassy?
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Old 8th May 2019, 11:06 pm   #6
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Default Re: How much anode glow is acceptable in a KT88 ?

For myself I dont like anode glow, having said that at a radio station I worked at the 807 pre drivers for the modulator ran bright red for years . I worked there long enough to replace the modulator valves twice, (4CX15000 ) but the 807,s were still in service when I left.

I have had LOTS of trouble with Svetlana and Sovtek KT88,s. In my experience they ALL glow red. Its a long time since anybody agreed to pay for GEC versions, IF I could even find some. They are silly prices.

The blue glow that half a mullard mentions is normal. In fact its usually the best valves with the deepest vacuum that glow blue. Reddish purple glow is a definite sign of gas.

Joe
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Old 9th May 2019, 6:11 am   #7
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Default Re: How much anode glow is acceptable in a KT88 ?

Is it acceptable? If GEC KT88's we're plentiful and cheap like EL34's it wouldn't matter perhaps, but as things are perhaps best to run 'em lower, doubt if it would make much difference to specs or sound. Whilst they can probably take it and will be fine do you want to take the risk of premature failure? It's a bit like running around in a vintage car at maximum speed all day, whacking it round the corners; all the drivers of vintage cars I see pootle along and take it steady.

To pile on the metaphors, let 'em live out their dotage by the seaside in the deckchairs instead of on the dodgem's : )

Tother Andy.
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Old 9th May 2019, 9:18 am   #8
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Default Re: How much anode glow is acceptable in a KT88 ?

The valve orientation when horizontal is specified to reduce the effect of element sag when hot?
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Old 9th May 2019, 9:29 am   #9
GrimJosef
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Default Re: How much anode glow is acceptable in a KT88 ?

I think it could be. The direction between pins 4 and 8 is parallel to the long directions of the grid windings, so if these were prone to sagging then they'd do it least if the 4-8 direction was vertical.

The corollary to that in a beam tetrode is that the hottest parts of the anode box lie either side of this line, and the maximum radiated heat comes out from these parts. So if we align the 4-8 directions when the valves are mounted vertically then the maximum radiated heat won't be cooking the next door valve. Instead it will be heading at right angles to that direction, 'out' of the valve line-up if you like.

Cheers,

GJ
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Old 9th May 2019, 9:43 am   #10
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Default Re: How much anode glow is acceptable in a KT88 ?

I endorse your 807 comment, I used to run regulated power supplies and the 807s spent their entire long lives glowing like electric fires. I knew they had ceramic bases for some reason.................
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Old 9th May 2019, 11:40 am   #11
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Default Re: How much anode glow is acceptable in a KT88 ?

As an aside, anybody that has looked at my "creations" will have noted that ALL valves I use are mounted into a chassis that has 12 holes around each valve, Be they OA2's or KT88's. The reason is pretty simple to a simpleton like me, it allows air to flow around the valve. For lil bottles ( 12a*7 series) they are 3mm in diameter. For KT88's they are 8 mm in diameter. For larger valves the holes become correspondingly larger.

Yes valves run hot, BUT they were all designed with "free air flow". Jammed into a tiny radio cabinet was fun when I was a kid and it was minus 18C outside, I moved 3000 kliks north, by road, (about 1500 straight line) to avoid this problem. Most of my equipment tends to be quite reliable, and I believe that its due to the fact that I do "let em breath".
Im a "very average" engineer, but I do read the design criteria and take this into account with design. Like output transformers!!. A large core of ordinary 5% Si GO core material is far superior to a small core of expensive unobtainium material. Cooling is the same story.
Its many years since I played in the sand pit, but my amps with transistor output stages are very easily recognisable by the HUGE heatsinks, and their age, MOST of what I did back then is still working.

Joe
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Old 9th May 2019, 12:31 pm   #12
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Default Re: How much anode glow is acceptable in a KT88 ?

It might be worth bearing in mind that as a general rule with all things electronic, heat and high voltages reduce the reliability and life of such things.

Al.
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Old 9th May 2019, 3:26 pm   #13
Half a Mullard
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Default Re: How much anode glow is acceptable in a KT88 ?

Quote:
Originally Posted by joebog1 View Post

The blue glow that half a mullard mentions is normal. In fact its usually the best valves with the deepest vacuum that glow blue. Reddish purple glow is a definite sign of gas.

Joe

Thanks Joe, that's reassuring to know
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Old 9th May 2019, 4:41 pm   #14
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Default Re: How much anode glow is acceptable in a KT88 ?

Quote:
Originally Posted by joebog1 View Post
For myself I dont like anode glow, having said that at a radio station I worked at the 807 pre drivers for the modulator ran bright red for years . I worked there long enough to replace the modulator valves twice, (4CX15000 ) but the 807,s were still in service when I left.

I have had LOTS of trouble with Svetlana and Sovtek KT88,s. In my experience they ALL glow red. Its a long time since anybody agreed to pay for GEC versions, IF I could even find some. They are silly prices.

The blue glow that half a mullard mentions is normal. In fact its usually the best valves with the deepest vacuum that glow blue. Reddish purple glow is a definite sign of gas.

Joe
I don't think I'm going to buy Sovtek KT88's again. Out of a matched quad, one had a anode glow. We call it "red plating". The other three were fine! The thing runs so hot, it even discolored the red Sovtek logo. The same valve glowed red, no matter what socket I put it in.
The is in a Harmon Kardon Citation II amp. I even reworked the fixed bias source.
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Old 9th May 2019, 11:56 pm   #15
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Default Re: How much anode glow is acceptable in a KT88 ?

I once worked on a very expensive Japanese amp that used KT88's. Its circuit was a very curious one for me. Basically a Parry cath-amp, but with a twist. 3 primaries all centre tapped. One was obviously the plate load running around 450 volts. The second was for the screens with its own power supply, NOW !!! it used 500 volts on the screens. Third winding was for the cathodes in the normal manner. The origional GEC KT88's that were fitted at the factory were very tired, so I recommended they be replaced. The customer did not want to spend that much on GEC's, so I suggested using Sovtek "coke bottles" as they are called here. I had to buy 20 before I got four that didnt red plate.
One in particular after about five minutes actaully burnt a hole right through the plate !!.

Their EL34 are also pretty poor examples, and regularly redplate, but as the bottle is the same size as the origional Mullards, they do get rid of the heat better.
The "coke bottle" variety are the same shape as GEC's but about 2/3rds the origional size.
In one of our Electronics magazines ( ETI )there was published a design entitled
" 140 watts of Rich, Warm Valve Sound" ,using the origional maximum voltages namely 750 volts on plate and 225 volts on screens, obviously in pentode mode. I can highly recommend that you dont build it. Even with hand selected Mullards, they have a life of a few hours. I would not even dare to try Sovtek/Svetlana/JJ .

Joe
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Old 10th May 2019, 12:13 pm   #16
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Default Re: How much anode glow is acceptable in a KT88 ?

Indeed, my GEC applications notes for the KT88 do say that vertical mounting on a chassis with FREE AIR ventilation will not cause the anode temperature to be exceeded at maximum ratings, however the Leak design does not seem to allow for this, I assume there are no ventilation holes in the chassis around the valveholders, and the valves are less than the recommended minimum distance apart.

A simple test to prove whether ventilation is the culprit would be to use a fan to train cool air onto and past the valve envelopes and then see if that reduces the problem.

I used to have an RF power amplifier using a pair of TT22, mounted in a screened enclosure which had holes around the valveholders allowing cool air from a blower to flow up past the envelopes, I don't remember seeing any "red-plating" when squeezing more than the recommended maximum number of watts of AM out of them.
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Old 10th May 2019, 2:03 pm   #17
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Default Re: How much anode glow is acceptable in a KT88 ?

The anodes are in vacuum, so the only path of escape for heat is via radiation. The rate of radiation of energy is proportional to area and proportional to temperature raised to the fourth power. Anode current times anode-cathode voltage gives the total anode dissipation.

Two things can go wrong from the point of view of the anode colour temperature: Something in the circuitry can go wrong and the anode current can go up, or the heat may not be well distributed across the radiating area. The former can heat the whole anode to red and beyond, the latter can create hot spots.

KT88 are essentially beam devices. There was the well known legal footsie over names like kinkless Tetrode versus Beam Tetrode, but the things were rather similar. Heat is not therefore created uniformly across the anode metalwork. Artful design uses extra metal to form fins rooted in the areas where the beams hit, but the anode metal is thin and lateral thermal conductivity is not wonderful. Valve designers want anode metal to be cool everywhere to reduce secondary emission. Suppressor grids don't work perfectly, so avoiding emission at source is valuable.

The KT88 anode's active areas are not that much bigger in area than many substantially lower-powered valves, so it is going to be a particularly hot anode even if run at its allowed conditions.

The envelope temperature is due to its interception of some of the radiated power. Reliability means keeping the glass temperature within bounds. Specified separation from other valves is imortant, as is space for convection, better still some airflow can be deliberately engineered by the chimney effect as Joe has described, or by blowers.

Some valves re-purposed in hard-run transmitters like amateur radio 'linear' amplifiers using line output valves, were notorious for heating the glass until the vacuum sucked it in. Intentional RF valves went into pyrex envelopes to get the plastic point up. Extreme ones went for alumina and eventually beryllia.

Some redness in the beam landing area can be expected in a KT88 run at its ratings.

It's a valve just on the boundary of where giant size or extreme measures would be employed.

The hot glass will radiate some heat back at the anode, but mostly it relies on air cooling from the outside. I'm not sure that improving airflow round the glass will have much effect on the temperature of the actual anode, though. It will keep the glass better cooled and that is very important in itself.

David (suffisiently old that valves were in the A-level and degree curriculae)
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Old 10th May 2019, 5:45 pm   #18
John Caswell
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Default Re: How much anode glow is acceptable in a KT88 ?

Hi all,
My amplifiers are the GEC KT88 50Watt design from the 1950s, and when built I tried several brands of KT88 including some NOS GEC ones I happened to have handy (as you do) When it was all set up several makes ran red plating on the anodes, whereas the GEC ones didn't.
I am of the opinion that modern KT88s are definitely not as "good" as the GEC ones but of course are much, much cheaper. I tried KT120 but found because of their construction they tended to vibrate physically, no doubt caused by the power tx magnetostriction. Damping them with some lead collars reduced the problem quite a lot, and I stuck with the KT120 until KT150s came along. I then changed to them and all was good. I suspect that the KT150 will last a long time as they are running at about 2/3rds max dissipation.
Another thought, when I started in '58 I was taught by an engineer/mentor that power output valves should ideally have 2 to 3 fingers width between them to avoid mutual heating, and I have always stuck to that wherever possible. I note that a lot of amplifiers that I get in for repair have the output valves very close to each other, I suppose it is all down to cost.

John
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Old 11th May 2019, 3:16 am   #19
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Default Re: How much anode glow is acceptable in a KT88 ?

Certainly worth keeping the amp 'unenclosed'. The modern day placement of valve amps 'front-and-centre' in a room is a win-win for aesthetics and cooling!

KT88's were certainly used in dense packed arrangements.
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Old 11th May 2019, 10:47 am   #20
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Default Re: How much anode glow is acceptable in a KT88 ?

One thing worth remembering with valves is:-

When the anode is 'black' it acts as a black-body-radiator, which is very good at radiating heat. Black is also very good at absorbing heat - which is why you don't want flat shiny surfaces near your valves to reflect the heat back at them, nor do you want the valves close to each-other so heat radiated from one anode is absorbed by the other!

When the anode starts turning red, its efficiency as a heat-radiator actually _falls_ compared with when it's black and at the same temperature - so making it likely to get hotter, and redder, and so even less-effective as a radiator, so it gets hotter still. Thermal runaway, and a pain in the wallet.

Some high-power transmitting valves used Graphite as the anode; this has the advantage that it won't melt - though it can be disconcerting to see the anodes glowing bright orange with an intensity that fluctuates slowly in time with the modulation.
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