hi all, I am building my first amp. it is a marshall 1987 circuit modified to run on el84 tubes. I am using mojotone 18w transformers. I did not have a specific schematic to work from to make the changes to el84's so I took some clues from various sources on the net to make what I hope would work. I also knew that I would probably have to make modifications as it probably wouldn't work (although I had hope that it would) work the first time powering on.
well I tried to power it on the other night without tubes using a variac to slowly bring up voltage and when I got to about 80volts the screen resister going to the el84's started smoking so I imediately shut it down before anything else happened. the resister still checks out at 100ohms. I have seen schematics where there were and were not resistors attached here. at the moment I do not know of anything else that might be a problem but I want to solve this one first before I apply power again. I used a 3watt resistor thinking it would be big enough but many schematics show these resistors as being 5watts. do ya'll think this might be as simple as replacing this with a 5watt resistor or maybe the circuit needs to be changed to improve things. attached is the schematic for the circuit I am currently building but again I also know I am not a designer so things may have to change some to make this work. thanks for any help ya'' can give.
dju wrote:I tried to power it on the other night without tubes using a variac to slowly bring up voltage and when I got to about 80volts the screen resister going to the el84's started smoking
Confused by this. You tried powering up WITHOUT tubes, and the screen resistor started smoking. With no tubes, the screen resistor is an open connection, and no current will flow through it. What am I missing?
dju wrote:I tried to power it on the other night without tubes using a variac to slowly bring up voltage and when I got to about 80volts the screen resister going to the el84's started smoking
Confused by this. You tried powering up WITHOUT tubes, and the screen resistor started smoking. With no tubes, the screen resistor is an open connection, and no current will flow through it. What am I missing?
This is the problem I am trying to figure out. I was talking with a friend and lunch telling them about this and it made me think I need to take a closer look to make sure nothing accidentally got grounded somehow.
thanks for looking at my post.hopefully this will get straightened out quickly and easily.
well I checked my amp out again and I found that the wire going to pin9 is grounding on the nut next to it. Unlike all of my other wires going to socket connectors from the outside towards the center, I wired this one from the center going outward. thus the wire core was pointing at the nut and I didn't notice that it needed to be trimmed more. problem should be solved.
There is an 18watt Plexi layout and schematic over at 18watt.com in the downloads section. Might save you some time and effort; el84 power section is 18watt based (1974) not JMP50 (1987).
statorvane wrote:There is an 18watt Plexi layout and schematic over at 18watt.com in the downloads section. Might save you some time and effort; el84 power section is 18watt based (1974) not JMP50 (1987).
thanks for the info. I have been unable to get my account problems resolved when I tried to enroll so I can't look at their website. there is no contact information on the homepage for when you have problems so I sit in limbo.
In the future you should crimp the wire on the socket pin so it is mechanically secure then solder it. This would also prevent the lead from flying off the pin and inadvertently touching stuff around the socket.
thanks for the schematics. I will save them and see if I can adapt them to my project. as it is now I powered it up yesteday and nothing happened (no smoke or sound). one of the first things I am looking at is my B+ voltage seems very high for what I was expecting. my power transformer is supposed to be 120v-->290-0-290. I am measuring 120-->312-0-312.
after the rectifier I am getting ~423Vdc.
but then again I am a beginner at building amplifiers and my electronics math is very rusty. going for an electronics degree and then working on computer firmware doesn't help keep the hardware side very sharp. I'll get it figured out with some help.
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