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  #11  
Old 7th May 2021, 08:23 AM
Richard Richard is offline
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Default Re: Kit 6550 refurbishment, guidance please. Help

Hi Tonyo, yes try the balance resistor mod. Bear in mind though that if your 5U4s are tired, or the diode sections were not well matched from new, the diode sections may differ too much for the resistors to cure the flashover problem. It may be that your 5U4s have poorly matched sections from new.

An explanation of whatís happening, and the difference between using 5U4s and 6D22S as John suggests, may help. Apologies to anyone who knows this;

Look inside a 5U4 and youíll see 2 diode plates. Those plates are usually used as separate diodes to make a full wave rectifier (ie just one 5U4 is used). In your amp the 2 diodes of each 5U4 are connected in parallel. This allows them to rectify twice the current of one diode but only if they are both sharing the work equally. If a 5U4 is old, or its diode sections are not well matched when new, one diode will do more work and flash over. This is most usually at start-up when they are filling the big capacitors.

The 6D22S are each just a single diode with each capable of more current than a single 5U4 diode section. Thus there are no flashover problems unless the capability of a 6D22S is exceeded. It is more elegant, cheap and robust. The only thing to be careful of is high voltage on the lead and valve cap so donít remove it without switching off first.

The hum sounds like a mechanical issue so try mounting the choke on rubber washers and change the tightness of the mounting screws for that and the mains TX to see if you can influence the noise. Otherwise loose laminations can be a cause which is tricky as it involves re-varnishing or replacing. A few members have been there with this and may be able to advise more
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  #12  
Old 10th May 2021, 11:44 AM
Dave999 Dave999 is offline
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Default Re: Kit 6550 refurbishment, guidance please. Help

i have the balance resistors in mine and was running black plate winged c Russian diodes 1960s production. i never fixed the issue with those but replacement with much cheaper "grey plate" 1974 versions worked a treat and i have had no further issues. Most on ebay are OTK stamped military spec, and seem legit. although a little stamp can be applied to anything.

i had mine from a supplier in the Ukraine... but Watford valves used to do them and that is probably a route that means you get a matched set, but it will cost a bit more. i do have some GE ones but they just buzzed all the time, something a bit loose inside

presume i had mis matched sections in the older valves. although the intermittent "once a week" symptoms pointed to something worse. loads of hum, 1 valve glowing one valve not, almost as if the heater on one was intermittent. switch off and on and the issue was gone.


new valves sorted it out, but i have since worked my way through the power supply electrolytics as well.

less hum and sounding great. it prompted me to do the caps in the power supply for the phono/pre 2 as well been in there since 2002

mind all capacitors removed looked ok no bulge or discolouration. i might dig them out the scrap bucket and see what they measure.

i started all this a year ago but a few changes and a house move meant life got in the way... Just got all my superfluous gear back out of storage, so the tools and spares are all back in easy reach

i'd forgotten how nice it was to make a small repair or some minor refurbishment and hear the benefit of your work minutes later.

for all of this work i just chose my parts from the RS catalogue good for 105*C, low ESR, high lifetime hours, a brand you have heard of...seems to have seen me on the right track. not the cheapest but not the most expensive similar size, similar mounting, same configuration.
look for the components with the extended data sheet data designed therefore for people who care about that data. not just generic consumer use

it sounded great as a standard build so i was happy to build it back as such...i'll worry about fancy stuff with hi fi name tags if i ever get round to signal path components, but then again technology moves on... what is standard today would have been expensive 20 years ago

Dave
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Old 12th May 2021, 09:16 PM
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Yellow Yellow is offline
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Default Re: Kit 6550 refurbishment, guidance please. Help

Quote:
Originally Posted by tonyo View Post
Thanks gentlemen, first the rebuild as standard.👍
I would say that is the best thing to do. Get it up-and-running again as per the original spec, then start the upgrades with decent signal caps, wiring and better resistors in key places.

Neil.
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