Hello there! I’ll strongly recommend to unsolder the HV wire underneath the PS board… if you touch those plate brackets by accident is not going to be funny. I have pretty much the same amo, without tubes it shows 2500v, very generous, with tubes on it’s 2400v. That black areas on the tube’s glass is meant to collect the impurities from the elements… when it gets brown like chocolate means the tube is done… Beside the slightly low voltage I don’t think those tubes are bad… If I was you I’ll check the resistors connected on the tube sockets… That’s my advice, I have to say I’m not an expert but I’ve been playing with radios and amps for quite some time. 73’s de w5egm
Eduardo, thanks for the input. Can you explain what you mean by unsolder the HV wire? I have started to test the components. As fir that on tube, not sure if the camera showed it but about half of the getter was gone. Like somebody took a pencil eraser through the middle of it. I don’t remember it being that way. This is my first Amp, so it’s a learning curve. If i can stay alive and keep my tools from being destroyed it will be a good experience. Tim
Literally like that. There’s a wire running between the PS board and the plate choke… lift the end in the PS board (the cable you installed is on white color). Also check the voltaje with another volt meter and see if it reach above 2400. Like I said, mine reach above 2500. I think your amp still as grid-modulated, there’s a few things that can be failing on the grid circuit, but you mentioned it happened suddenly, maybe it’s not the HV but Bias. Those tubes don’t degrade suddenly or loose power easily. When they fail, they will do it badly. Another thing: be careful when you remove the plate clamps. Those tubes are fragile and knocking the glass with a screwdriver can break it and that’s going to be expensive (~$150) for a new pair). Another option can be replacing those tubes for a pair of GI7B. Those are about $60 per pair including shipping. GI7B are modern type tubes, ceramic, the socket is pretty weird but a replacement can be made with some tin can stripes, and the RF deck have to be rebuild in order to make ir work fine. The only three requirements are not to put more than 50w (as per data sheet) drive, and allow 2 minutes minimum for the tubes to warm up, and the filament voltaje is 12v, each filament draw 2 amps.Other than that, they work better than the 572B and push our a little more power.
Yes the Chinese tubes are crap mine only lasted 3 weeks since ten I have reversed engineered my sb 200 and fitted GI7B pulse radar tubes about 50 watts of drive will give about 850 watts pep They are cheap as chips and more rugged than 572b On my QRZ page G4LMZ you can see how they have been fitted it does need some serious surgery to fit them but its better than having a lump of scrap metal sitting in the shack.
I am working on that right now. One rookie mistake is I wasn’t testing it into a dummy load, so I can’t validate the results. The antenna I was using had a failure, but I don’t know if it was faulty when I ran the tests. I do know for sure those tubes are tired, but I will retest probably before I buy new tubes. Thanks.
