KAT 6550 problems

[Richard]

Yes, 500R was revised very early on as it made the plates glow red but did get printed on some build circuit sheets. 680R was the value settled on in the supplement article (attached) but I think some kits/builders used 560R. 20 years on the mains voltage has dropped a little (in my area 236V down from 246V is now typical) so you may get away with 560R these days. Do the maths and keep plate dissipation under 32W.

The cathode resistors themselves dissipate less than 4W so 7W or 10W will be fine. The cement in the white cased ones does sometimes disintegrate – lots of thermal cycles I suppose. The Welwyn ones are better, being welded construction inside a fused vitreous case.

There may be a case for not over-rating cathode resistors too much in that they will then act as a fuse if the valve draws too much current, thus preventing possible OPTX damage. I’ve not seen it stated as such but think this was the case with the Quad II which used slightly under-rated 3W cathode resistors. I can’t think this was an accident on Quad’s part; customer is a music lover who hides the ugly valves in a cabinet. Eventually one channel dies. Takes the amp to his local agent who changes the valves and the cathode resistors and bypass caps. Job done for another 5 years. Even into the 90’s our local Quad agent had a workshop in the back room. Quite nice to see the scope sat there idly pinging a dot across the screen when I visited

[bob orbell]

Thanks Greg and Richard, I still have the build sheet for mine and they are 500, don't have the amp. now but it is still going. Bob

[gerrygwm]

Hi to everybody who helped me.
The amp is up and running perfectly.. I used the caps and resistors from RS as
advised. Very little cost and received next day as promised. Good job I am not counting the time! I wasted two evenings before I asked for help.(Thanks John and all) I forgot how good it sounds. I started using an early Cyrus amp as a temporary fix whilst I mended the valve amp. It took 5 years!
Been listening to Vinyl for the last four hours.. Say no more. Cheers
PS. How do I post a picture of the amp?

[John Caswell]

Hi Gerry,
Excellent news, I always thought the 6550 was one of WADs better amps along with the KEL84.
I believe Richard is the photo posting expert here, so over to him.

John

[Richard]

Great result Gerry well done

Firstly, to get your fullest posting options Click User CP, (user control panel) top left of screen when logged in,
then in Settings and Options click Edit Options
scroll to bottom and in
Miscellaneous Options > Message Editor Interface select Enhanced Interface pull down
then Save Changes
before returning to World Designs Forum using link top left.

Then you can either,
host a picture on your own web space and include the URL link to it in your WD message using the Insert Image icon (pale yellow square with mountain range) or
have WD host the image by clicking Attach Files > Manage Attachments in Additional Options that is now enabled under the message box. That lets you browse and upload certain file types.
For pictures jpg is most popular and file size limit is 1024 x 1024 pixels from memory.

[gerrygwm]

Thanks all.
Images of the amp all fixed on test in the workshop.

[Richard]

Very nice did you have it chromed or is it stainless?

[bob orbell]

I think I will say stainless , not ideal though for valves, it looks like the whole chassis has been remade, not difficult with CNC. Bob

[Intenso]

Lovely amp Gerry and well done on getting it going again.

May I ask what tubes you are using?

[gerrygwm]

Hi Richard and Bob.
When I first bought the kit in about 2003 I had the enclosure polished and chrome plated before I built the amp.
I also made the feet on the lathe in aluminium. I had them anodised to match the facia plate. I did make a new facia but changed it back to the WAD one some time later.
The metal work for me is the easy bit as it is my day job. The electronics is the puzzling hobby!
The 6550 valves are newly fitted Electro Harmonix. Up until this week I have been using Svetlana 6550C. These did not cost much money in 2006!
I only really fitted the new ones (at great expense !) in case it was a faulty valve that caused the problem with the caps and resistors.
I have only had one chance to listen to the amp once since I fitted the new valves, and that was with three other people in the house invading my space self bloody isolating!
Maybe I,ll get a proper listen soon and report back.
Keep well, Gerry