Kit 6550

[Paul Stoneman]

Hi Andy,

Thank you for the info, the WAD build instructions say 6.3 volts. I did not measure the voltage prior to taking the original rectifier out which I should have done, I put the old rectifier back as I did not want to damage the ECF80's and guess what it too measured 9.5 volts. Still learning this game. All the new Shotkeys are Ok so I will re do when I have time. I have just re read the measurements on the build instructions and it mentions red glow so yes I should have put the ECF80's in to measure.
Now if I can only get rid of that dreaded loud hum! I think I will also re do the earth point at the back.
Cheers

[bikerhifinut]

Hi again,
When no load on the transformer you will get approximately 1.4 x the Ac voltage when rectified. This will drop according to the load and rating of the secondary. It's not always easy to work out the resistor to drop a voltage, but when you measure the volts on load it becomes a bit more simple.

have you checked the heater wiring in respect of how its routed to the valves and if its twisted? I have been surprised myself what a difference it makes if one follows the principles rigorously. I have a single ended power amp that is TOTALLY silent and it uses ac heaters throughout.

Andy

[bikerhifinut]

does the KIT6550 have an earth lift resistor?

just wondered......................

[bob orbell]

Yes it does Andy, 10R. I had a KAT6550, (my first valve kit) it hummed from the word go and I never did get rid of the hum until I borrowed a PRE III from Peter C, because the PRE III has a floating output, (no connection to it's ground) it was completely silent, but my hum was low and constant, so I don't think this is a fix for Paul, must admit, I am lost on this one. BOB

[Paul Stoneman]

Hi Andy and Bob,

Thanks Andy, I was aware of the increase in voltage after rectification but again it was the under load it tripped up on, not good for an electrician.
I have, I am now sure found the major hum problem, it was a dry joint on the earth / ground cables despite looking nice and shiny and probably a build up of rust, on the earth terminal near the speaker terminals. (when the side of the amp was tapped it sometimes went away ) I scrapped the rust off re soldered the wires into a round crimp style terminal and used some electrical jointing compound and star washers. The only hum I now have is very, very slight and does not increase with volume.
Sounds great with more upgrades now to do.
I also put the Pi filter back in, all this was done yesterday and it has been on for about 7 hours so hopefully that is it.
I do intend to get a Pre3 Bob, but it looks like a new CH boiler may be needed first, oh what fun.
Once again thanks to everyone for the help and motivation.
Kind regards Paul.

[bob orbell]

Let's hope that you have found the problem Paul, finger's crossed