KAT 6550 C11 PIO did sweat !!!

[LSfarzo]

Hi all,

New problem are arising on my great KAT 6550.
That is to say it is getting 10 yo and maybe something is getting tired...

Some days ago while I was listening music a grey smoke did start from the right of my KAT... I suddenly turned it off and opened it.

My KAT is equipped with various improved components, among them there are 4 Audio Note PIO caps 0,22 uf, 630V as C11, C12,C13,C14.

I have found that C11 did sweat a lot! Meaning that I have found its own oil (at least I think it was) on the bottom of my KAT and around the edge of the cap.
Giving a better look to C12 I have discovered that it has a little sweat too but it has not leaked on the ground of the amplifier as C11 did.

I turned the AT on again and found that the right channel, especially the high frequencies, sounds bad. Everything is right about the left channel.

With that said I can tell that I am quite upset about the situation especially because those caps are really expensive!!

So I have some questions I would kindly ask to someone asking for your help.
1) which voltages should I check to understand if the caps are gone or if something caused the caps to die?
2) do you think this is the normal "expiration" of a PIO cap?
3) if I found the cap has died, could you please suggest me some other cheaper brand/type of cap I could use in place of it or, worst of all, at the place of all 4 of them?

Thank you
Luca

[Richard]

Hi Luca,

Yes they can leak and if there's oil on them it's time to change them all or risk your output valves and transformers.

If they become electrically leaky as well you will find a positive voltage on the grids of the 6550s at pin 5 where it should normally be 0V. That bias turns the valve on and it may glow red or draw so much current the primary of the OPTX breaks.

The best coupling caps for the money imho are WD Soniqs Polypropylene from Matthew here http://www.world-designs.co.uk/sax.htm . They have a fuller mid than the usual polyprops and a nice bass and treble which is cleaner than PIO. I'd get the 630V 0.22uF ones at £5 each and see how you find them

[Greg.]

Hi Luca,

You are not the first to find that AN PIO caps start to leak their oil after several years use. I think you should now regard all four of them as failed components that need replacing, because there is every likelihood the others will also start to leak oil. If you particularly want to replace them with other PIO caps, Jensen are good. You can keep costs down by buying directly from them, they will take small orders and their service is very good. Just email them your requirements and they will quote you a price.

http://www.jensencapacitors.com

Some Russian ex military caps sold on eBay are PIO so it is worth a search there as they are cheap to buy.

Personally I prefer the sound of polypropylene capacitors. There are many to choose from at varying prices but these from WD are very good value and sound equally as good.

http://www.world-designs.co.uk/sax.htm

I hope that helps.

Greg

[Greg.]

Ah, Richard and I posted around the same time so I did not read his post before I sent mine. Essentially and independently, we are saying the same thing

[LSfarzo]

Thank you very very much for your so quick answers!!

Some additional(sad) news from my ket. I checked the required (by instructions) test voltages and found that:
1) Valve 7 pin8 is supposed to be 42V and I found 167V !! Looking better I found R35 burnt!!!
2 ) V4 p 1 is 95 V while it should be 125 V
3) V4 p3 is 78 V while it should be 95V
4) V4 p6 is 93 V and should be 130V
5) v4 p7 is 1,85 an should be 1,6
6) v4 p8 is 7,25 while it should be 6,5
7) v3 p8 is 9,36 while it should be 6,5

I will replace R35, too but what do you think about V4???

Thank you!

Luca

[Richard]

Hi again,

1) 167V on that cathode means it was passing around 300mA and R35 was trying to dissipate 45W Probably the 0.22uF coupling cap is electrically leaky and had turned the valve on hard. Let us hope V7 is ok but watch it carefully and check voltages as soon as you switch on after repairs.

At this age I would change all the 0.22uF coupling caps, 6550 cathode resistors and 6550 cathode bypass caps.

2,3,4,5,6) the ECF80 voltages look off but are often like that without it being a problem. The voltages of both sections of V4 correspond with them passing a little more current than specification; the voltages at the anode pins are lower and the voltages at the cathodes are higher. So both sections are working fine and there will be little difference in performance. John Caswell checked several similar stages and found no problem with performance when the valve is working like this. If you see these small pentodes on a valve tester you often get "lively" ones and many Russian types in particular read consistently higher than the nominal characteristics. This isn't a problem usually. If you can get a couple of Mullards for V3/4 you should find the voltages are better.

7) same answer really, it isn't far off enough to be a worry if it sounds ok otherwise but try Mullards if you'd like to see better figures.

[John Caswell]

Hi Luca, just as an aside,
Just get any capacitors that meet the specification ie 0.22µF/630V cheap ones and get the amp working then replace them with whatever you fancy. Though funnily enough orange drop types are very very good. If you need some pm me and I will stick some in the post.

John

[LSfarzo]

Hi Richard and John,

thank you very much for your kind support, I appreciate.

As far as I have understood I will apply Richard' suggestion by substituting: "all the 0.22uF coupling caps, 6550 cathode resistors and 6550 cathode bypass caps".
In order to do this the better way before buying some very expensive coupling caps I'll follow John suggestion trying some very cheap coupling caps then checking if voltages are correct and if there are no additional "additional faults".

To start I would kindly ask if I got the components to change right:
1) coupling caps: C11, C12, C13, C14 (0,22uF,400 V)
2) 6550 cathode resistors: R35, R36, R37, R38 (560R 7W)
3) 6550 cathode bypass caps: C15, C16, C17, C18 (100uF, 100V)

If the list is correct I would ask :
1) Concerning the 6550 cath. res. (R35-R38). Which type of resistor do I need? I think they are of ceramic type. Is it right? Are there some brand I should prefer ? Do you know some online shop where I could buy them (i.e. Farnell or RS or something else)?
2) Concerning the 6550 cath.bypass caps (C15-C18) I am currently using some Black Gates and they are not properly cheap to be replaced. Do you think I could use some cheaer model to see if everything is fine? If so, which type/brand should I use?

Thank you very much and excuse me for the lenghty questions.

Regards,
Luca Sfarzo

[Richard]

1) coupling caps: C11, C12, C13, C14 (0,22uF,400 V) RS 755-4664 0.83p each
2) 6550 cathode resistors: R35, R36, R37, R38 (560R 7W) RS 206-1035 0.42p each
3) 6550 cathode bypass caps: C15, C16, C17, C18 (100uF, 100V) RS 315-1072 0.51p each

Yes those are the ones. As John says try something like the RS ones above, they will all work well, the Nichicon pet coupling caps should be fine, the ceramic case wirewounds are original type and the Panasonic FC caps are very good. They are in stock in Italy and come in 5's so if that valve is bad and damages them you will have a spare.

[Greg.]

What an excellent explorative response. Nice one, Richard

I would be inclined to remove the Blackgates and test them before replacing. Surely if they test well they could be continued with. I don't think we yet have caps that can equal their quality unless of course, others are better informed than me which is highly likely.