KIT6550 Blowing Fuses

[A Stuart]

6p20p perhaps? The thing looking like an n may be П, essentially same as the Greek letter Pi.
There appear to be 6Д20П (6D20P), and a 6П20С (6P20S), if that helps.

[colin.hepburn]

Originally Posted by A Stuart

6p20p perhaps? The thing looking like an n may be П, essentially same as the Greek letter Pi.
There appear to be 6Д20П (6D20P), and a 6П20С (6P20S), if that helps.

Hi Alastair
Yep the Greek letter П thinge is likely correct. but I think most of us just refer to it as n like in the 6n2p valve for example but a search for the 6D20p brings up the same damper diode as I have but they also come under the code EY81
The version i have is the 6Д20П and what I will use in my new single ended 5751/12B4a Headphone amp PSU and I may even do a WDPSU3 build using them as I have a box of them a slow start up PSU3 that may be an idea now a project to do for the next lockdown
Link for the 6D20p/EY81
PS note just to bring it back to the KAT6550 upgrade you will need the 6D22s version for power amps use.

https://www.ebay.co.uk/itm/Lot-of-12....c100010.m2109
Colin

[John Caswell]

Hi all,
I have tried 6d22s with a great deal of success, and they run very well on 5V, and with a warm-up time of about 30 seconds are a great help to fuses and elco capacitors.
I have also used EY84, but not at 16,000ft just at ground level. They are probably less tolerant of running on 5V but as they only take 1A of heater current whereas the 5U4 takes 3A there will be a rise in heater volts so they may work very well. I will try them out on my amps to see what results I get.
I haven't tried the EY81 but cannot see why they would not work very well as they are a TV boost/reclaim diode - 6.3v equivalent of the PY81.
I try to avoid directly heated valves 'cos of their almost instant warm up time, which of course means you need to rate the following capacitors to at least 1.5 times the AC in.

John

[Richard]

All interesting stuff but we should remember there are lots of Kit6550s etc out there which do not need re-designing to work fine

Kit 6550 uses a regular-type EI mains TX which is easier on start-up than a toroid.

By all means raise the value of the fuse a little, to 2.5A Slow Blow is reasonable. If it still blows there may be a problem such as failing rectifier valves. Change them and try again.

The balancing resistors are suggested as a way of evening-up the current through the diode sections to help prevent one section doing more work than another. Often they are not needed but will do no harm and may prolong the life of the rectifiers.

[Richard]

Clipper, consider also that the amp will be several years old. Hopefully the mains TX is ok, and if the rectifier valves are also ok, the next suspect would be failing first smoothing capacitors C1, C2, which may be going short with switch-on surge. Electrolytics over 10 years old in a valve amp of unknown history should really be changed if you're looking to keep it.

[Greg.]

Yes, I agree with Richard. Those power supply electrolytic caps were not particularly good quality (IMHO) in the first place. Recently I replaced them all on a friends 6550 and the outcome was a considerable improvement in sound quality with reduced buzz from the amp chassis/mains Tx. A job well worth doing.

[John Caswell]

Greg wrote
"Those power supply electrolytic caps were not particularly good quality (IMHO) in the first place"
Unfair comment, this is how urban myths start. I am sure that the manufacturers of the capacitors would not be happy with this comment and it does lead people to believe the wrong thing.
The were standard commercial grade and well up to the job - at the right cost.
"Boutique" capacitors and other parts could have been supplied, but the kit cost would have rocketed and then it would not have sold in sufficient quantities to justify the extra cost.

John

[Greg.]

Not an unfair comment. I qualified it with IMHO. I am entitled to hold that opinion and readers can choose to agree or disagree (as in your case) with it. The same PSU electrolytics were fitted to my 2A3PSE which I replaced with commercial Evox Rifa to provide a much better sound performance. So my experience with two different amplifiers is the particular original caps used were, for me, lacking in satisfactory quality. Boutique caps never came into the equation.

[Matthew Snell]

N.O.S. 100uF; 450V; Axial; 85º; Marked "HL"

Just in case anyone's interested, I have a decent stock of the original part. I have just pulled one out at random and got 99.1uF! I don't think they were too shoddy ... rather, as John says, a reasonable commercial standard.

If anybody wants a few, they can have some for just the cost of the postage. We don't use axials in anything now.

Matthew

[bob orbell]

Originally Posted by Greg.

Not an unfair comment. I qualified it with IMHO. I am entitled to hold that opinion and readers can choose to agree or disagree (as in your case) with it. The same PSU electrolytics were fitted to my 2A3PSE which I replaced with commercial Evox Rifa to provide a much better sound performance. So my experience with two different amplifiers is the particular original caps used were, for me, lacking in satisfactory quality. Boutique caps never came into the equation.

Greg, I agree with John, my KAT 6550 is still running with all the original capacitors and valves the Evox Rifa may well have had a lower ESR, hence the improvement in sound. Bob