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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Fri Oct 05, 2018 10:41 pm 
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Holy Ghost
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Joined: Sun Mar 13, 2005 4:10 pm
Posts: 7430
Location: Brighton, Canada
Yes, see if moving wires around the power tubes changes anything.

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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Tue Oct 16, 2018 10:48 pm 
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Novice
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Joined: Sun Sep 18, 2016 2:54 pm
Posts: 64
So today I spent most of the day with one of the professors at my school and he helped me a great deal with my amp.
First we build a tube tester circuit to check that the output tubes weren't completely fried. We found that the tubes should be good so we decided to continue with troubleshooting.
We then next disconnected the rectifier, pre-amp tubes, and power transformer from the circuit. We then supplied our own 6.3v to the heaters and then replaced B+ with a power supply that we had set to 94v (All voltages were dc). We than measured the voltages at multiple pins on the output tubes. We found that the V5 tube grid, pin 2, was 8v while the V4's grid was .685v This confused us so we unsoldered the jumper from the turret board to the tube socket and the resistance reading read perfect. We reconnected this and the problem was still present. This had us stumped until we went out on a limb and tried removing the resistor and the jumper from the turret board and soldering them in the air. This miraculously worked and we saw milivolts on the grid, however the V4 tube then started to overheat. So we did the same thing and it started to work.
We reassembled the amp completely and turned it on. All our reading looked normal and there was no noticeable red plating. There was a noticeable amount of sound coming through the amp with no guitar hooked up. Also if I turned up the normal channel volume the noise clearly got louder. Later when I had a guitar I played it through the amp. It sounded wonderful! I started to crank up the volume until the sound cut out and when I looked up there was smoke coming out of the amp.
I didn't catch exactly what had been smoking on the underside, but I believe it to be the tube socket due to some burnt looking plastic.
So I think I need a set of tube sockets. I want to get some ceramic ones so I don't have to worry about them melting. Is there a brand of ceramic ones you'd recommend? Also, what's the best way to find my grounding issues? (to eliminate the ambient sound and the sound from the normal channel)

It's almost done,
Cooper C


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Mon Nov 12, 2018 10:56 am 
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Joined: Sun Sep 18, 2016 2:54 pm
Posts: 64
So I got the ceramic tube sockets in and they worked wonders. My red plating in the V5 tube has completely stopped!
I can now leave the amp on without worry that it will quickly burn out the tubes. When I turn the amp on there is a noticable amount of sound coming through it when I don't have my guitar connected. Also whenever I turn up the volume on the normal channel there is an increase in the level of sound. I suspect that these are coming from a poor connection to ground somewhere in the preamp for the normal channel and in the output stage. As I was cleaning up some solder joints I ended up overheating the V1 tube socket till one of the posts started to slip through the plastic out the top of the amp, I fixed it but am unclear if this may have caused damage.
How do you recommend that I go through the amp to find any and all grounding issues?
Also seeing as the ceramic tubes have already solved one of my issues, should I go ahead and replace the other sockets with ceramic ones?

Cooper C


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