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In Reply to: RE: Finally ........... it's done. posted by vinnie2 on May 30, 2017 at 08:28:03
Maybe one of these days I will learn to quit while I am ahead. It may not have been as pretty, but it was working fine and had no hum at all. Since making the changes it now has low level hum and the filaments of the 813's are not glowing evenly. I think they were before. Now the right side is brighter.
I had to compromise on the changes because it would have been just too much disassembly required to make all the changes that would put all the wires going to the plate connections on top of the 813 tubes inside the chassis. Instead I moved the resistors to the tube sockets and tied the rest of the wires together at the plate cap with no resistor.
It looks better now, but I now have low hum and the 813 filaments on the right channel are glowing brighter than those on the left channel. They were all about even before I think. Anyway, it still sounds good, but now I will have to flush out the gremlins. Bah, humbug!
I decided to switch some 813's around and I also put several twists in the wires going up to the plate connector. That seemed to help quite a bit (not sure which one) as the hum level is way down again now. The filaments are still not even, but that is probably due to the fact it was difficult to get an accurate measurement of the AC voltage coming off the small switchers. I will just have to live with it for now and hope it isn't high enough to do any real damage.
The other problem is I have is that I have no way to test 813's except by plugging them into the amp. None of my testers are set up for transmission tubes. I guess I will just plug my extras in one at a time in a side that I know is working right and see what happens.
I think I am just going to run it for awhile now and see if anything else turns up before going on to measure the power output and the operating points. They should be close because I did not change any thing from when it was bread boarded.
Comparing your photos in you first post and the photos after you messed with the amp, it appears that the two on the left side are now dimmer than in the first post. Check that the caps for the AC switchers on the dimmer side are still connected. If they are not, you would be getting lower voltage and some hum.
It probably is not the answer but it is a possibility.
The surprising part is that both tubes seem to be the same brightness on each side, which leads me to believe it might be the mains feed, but I will have to check all possibilities. Thanks for the tip.
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