KiT/KaT6550 and fuse blowing inrush current

[Greg.]

I’ve been working on a WAD KaT6550 for a friend to get it up and running after a long time not being used. I have got it working satisfactorily and it sounds great. At my place I had no problems with the well documented fuse blowing at switch on that these amps are known for, however my friend has had these problems on using it in his own home.

I’ve done the research in the archives here and recognise the options of increasing the rating of the fuse (I recognise opinions on the wisdom of this are split) and/or introducing a current inrush limiter. The one recommended originally by John from Farrell has been superseded by this one.
http://uk.farnell.com/epcos/b57238s0...isexcsku=false

My friend wants me to do some other tweaking to his amp and one I intend to do is fit the excellent little simple mains filter with spike suppressor to the rear of the IEC socket.

I need to know if it is satisfactory to fit both the simple filter and the inrush current limiting thermistor together. If this is wrong, how should I approach the desire for a simple mains filter, spike suppressor and an inrush current limiter?

Thanks in anticipation.

[John Caswell]

Hi Greg,
Yes this item is fine and as Farnell says a suitable replacement for the original.
Just remember to mount it on longish leads as it gets pretty hot and contrary to my general advice/comments do make a good mechanical joint on the IEC socket or tag strip before soldering
Yes you can also fit the input filter as well as they both do different jobs and will not conflict.

John

[Greg.]

Brilliant! Thank you John. I’m very much obliged.

PS.We’ve not talked for a while. I hope all is very well with you and yours.

[pre65]

I'm expecting a Kit6550 for examination and repair and have two questions.

Has anyone here ever checked if the anode / transformer resistance of the dual rectifiers is as per data sheet ? Should be 75R according to what I've read.

Also, the first capacitor (s) after the rectifier is 100uf + 100uf in series, so 50uf in total. Data sheet says max 32uf but I assume it's not a problem ?

[Richard]

Hi Philip,
I'd check balancing resistors have been added to the anodes. They'll increase supply impedance and, importantly, help balance the current through the 2 sections of each rectifier, 47R wirewounds of 3W or more should be fine, see the pic in the faq with the resistors in red,

http://www.world-designs.co.uk/forum...ead.php?t=3859

[pre65]

Thanks Richard.

Greg has kindly sent me the literature for me to refer to.

[Greg.]

Hi Philip,

On the amp I worked on I in the end converted it from power to integrated.

I introduced the simple mains filter together with the inrush current limiter as suggested by John. I also did the balanced rectifier input as Richard has suggested.

I also did this.....

http://www.world-designs.co.uk/forum...ead.php?t=7643
Also this.......
http://www.world-designs.co.uk/forum...ead.php?t=5795

I replaced cathode load resistors with Mills. Coupling caps were already Audionote paper in oil and tested good. This amp already had the basic WAD upgrade (Audionote and Blackgate caps) and in addition was fitted with Sowter O/P Tx’s. I tried reducing feedback but it spoilt the bass and returned to original setting.

The owner is totally satisfied with his sound and doesn’t want any further changes now. He loves it. With his Edingdales, he has a fantastic sound that I also like. I should say that in this kit, the Sowters make a massive difference, but these days it’s pretty cost prohibitive unless your client is looking for ultimate sound. If he/she is and is willing to spend, I can’t recommend the Sowter option enough. They really do make a cracking difference to this amp and in my view, actually worth the financial outlay.

I hope that helps,

Greg

[pre65]

On the Kit 6550 amp I am working on the 6550 cathode resistors are 560R, but the parts list says they should be 500R.

That's R35/36/37/38.

Was that a period modification ?

[bob orbell]

Mine too are 500 ohms Phillip, nothing in the FAQ about them being 560, may be John C can help. Bob

[Richard]

Originally Posted by pre65

On the Kit 6550 amp I am working on the 6550 cathode resistors are 560R, but the parts list says they should be 500R.

That's R35/36/37/38.

Was that a period modification ?

Hi Philip, Bob,

Yes it was. The build sheets show 500R but that gave too much standing current with some various mains voltages around the country and the plates glowed red. Whether the build sheets were ever changed I doubt but discussions on here resulted in various options from that 500R to 680R as published in the supplement article about the amp Oct 2001. The relevant bit is attached. Check for red plates in a dark room and if none then 560R sounds good to me