KIT6550 Blowing Fuses

[bob orbell]

Isn't our Matthew a star . Bob

[Richard]

Yes I’d get those caps from Matthew whilst he still has them. High voltage electrolytic caps are disappearing from commercial suppliers now and especially axial types.

Yes Evox Rifa were low ESR, which helped their very long lifetimes, and were made for the auto and telecoms industries amongst others. Now owned by Kemet, it seems they and Vishay have mopped up many of the old brands. Matthew’s will be decent standard quality and should last 10+ years of regular hobby use.

Bob, if your amp is still on its original valves it will have done less than 4000 hours. I ran 6550s at similar dissipation in Kit88 for several years and went through a set every 12 to 18 months working from home and using the amp during the day. It got so bad that the last 6550’s I bought were a cache of 22 from a supplier which I matched into 3 good quads on the AVO and returned the rest.

Remember valve amps runs hot – I checked inside the chassis of the Kit88 once with a heat probe and it was 48C. That will mean ¼ the life span for the electrolytic capacitors inside compared with running at a warm ambient 28C (lifespan doubles/halves for each 10C down/up). https://www.illinoiscapacitor.com/pd...apacitors.pdfR

However, for lighter use of a few hours during a few evenings a week the valves may well last for 10 years. Reckon on 3000 hours before they start to sound weak if run in Class A at near full dissipation and far longer if run in AB at lower dissipation. Keep a set of new ones handy as you will need them eventually and they can be tried in every few months to check the condition of the old ones.

Coming back to the thread, we don’t know how much use Clipper’s amp has had. It will be over 10 years old and even if it hasn’t been listened to a lot there was a fashion (urban myth as John will say) to keep the amp powered up 24/7 as it would “sound” better. It may do but it won’t last long and the valves and electrolytic caps will have shorter lives. Instead, 20 minutes will see it “on song” then switch off afterwards and save the valves, and capacitors, when not using it.

[bob orbell]

Richard, the 6550 has been is in a friends use for a long time, I have said many times, we need to change some things, but he still uses it, if you remember, I had four Svetlana 6550 valves from you way back, all very well matched, but they are still here in the boxes, waiting . Bob

[Richard]

Hi Bob, yes I remember, glad it's still going well
The point about the 4000 hours is that the valves will be noticeably tired after 4000 hours use so it indicates that if your friends amp is still sounding good on the original valves it hasn't had that much use.
An amp left powered up permanently would reach 4000 hours use in less than 6 months.
An amp played 2 or 3 times a week for 3 hours each time would take over 10 years to reach that

[nneilboss]

Originally Posted by John Caswell

One of the biggest power problems I strike is that nowadays pretty well all power transformers are wound for 115/230 power lines. Now a lot areas in the UK are sitting regularly at 250v and local to me some at 253v. This is within the supply co's ±10% but no help to us.
Although the fuse will be totally satisfactory for 230v it is getting edgy for 250v so an increase rating is necessary, usually up to 2A T (Trage =Anti surge) but in some cases, especially with big toroids, up to 2.5A.
As pre65 says an inrush thermistor (or two) will ease the situation, but ideally a multi-tapped primary is best but this costs more money.
You pays your money............
For those of you that use 2 off 5U4G as the HT rectifier (which are now becoming expensive) is is a relatively simple mod to replace both with 6D22S, which are a TV boost diode. Main advantages are, indirectly heated slow warm up (30 secs or so) so should reduce the fuse blowing syndrome pretty near bombproof, and most importantly cheap. Although they are a 6.3V heater they run beautifully on 5V as I can testify.

John

Hi
Recently acquired a 2nd hand KaT6550 and although sounding great after a complete revalve (using Watford Valves kit), I have noticed a tendency of the unit to make alarming "whump" noise on start-up , sometimes accompanied by a blue flash on the rectifier tubes. No fuse-blowing, but this cannot be good. I noticed the various pieces of advice regarding the addition of thermistors or replacement of the 5U4Gs with a different rectifier. Can someone provide more information on the rectifier replacement - is it a straight swap, or would I need to undertake additional surgery? Many thanks for your assistance.

[Richard]

Hi mneilboss,

The switch-on whump shouldn’t be a problem so long as it doesn’t continue after switch-on. It’s only the transformer energizing.

The rectifiers shouldn’t flash blue though as this indicates an imbalance of current being passed by the diode sections and they may fail.

Fit resistors as per the drawing in the FAQ (47R 6W wirewound) which will help balance the current through the diode sections and increase the supply impedance a little to give the rectifiers and first cap an easier life, http://www.world-designs.co.uk/forum...newpost&t=3859

It would also be a good idea to change the first caps C1 and C2 if they are a few years old as they take a hammering from ripple and will cause hum as they start to fail.

[nneilboss]

Thank you