Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: draperjojo on August 12, 2018, 08:54:47 pm
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I did look thru the "read this first". I checked things off as I installed components over the last month or so on the Tube Depot JTM45+ kit. I looked on the internet for something showing voltages at all test points, but both links were dead and 5 to 7 years old. I did take readings with all tubes but power tubes in. When I plugged in power tubes and flipped the power on, I saw the pilot light dim some, so there is some heavy current draw/bad solder joint?. The amp is plugged into a dim bulb tester, and the dim bulb tester is plugged into a variac. Up until I plugged in power tubes, the dim bulb tester showed everything coming up normally. Tomorrow after work I'm going to start back thru it, hopefully with a voltage chart to double check test points. I'm including some notes and pics. If you have suggestions for things I could look for I would be grateful as this is my first attempt at building an amp, usually I'm working on an old jeep or harley, and those are my comfort zones. Thank you!
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https://el34world.com/charts/Schematics/files/Marshall/Marshall_jtm45_lead_45w.pdf
take these, yours, add up, divide by 2 and you should be in the park :icon_biggrin:
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Pull all tubes. Plug the amp straight into the wall. Now measure the bias voltage at T41 or V4 and V5 pin 5. You must have approx. -40 to -50V. You show +11V!
***DON'T PUT TUBES BACK IN UNTIL THIS IS FIXED!***
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Thank you, I will check that tonight!!
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Thank you! The diode at T62&T64 was reversed. :BangHead: I now have -41V at T41 and pin 5 on V4 and V5 when the amp is plugged into the wall. I did connect a 1X12 cabinet and a guitar, but had no output, other than I could hear a swishing sound as I dialed the presence knob up and down, and that was with all the other controls dimed including MV.... so I still have an issue.
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Bear in mind that your bias supply caps have been subject to significant reverse voltage (assume that regular ecaps have been used).
The bias supply is 'mission critical', do you want to leaving potentially 'walking wound' parts in there?
The swooshing noise when adjusting the presence control is normal with that arrangement, ie Vdc on the pot.
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No output with a new build is almost always a wiring error. Compare your actual build to your layout (Sometimes it may take multiple comparisons to spot an error.) Verify all under board jumpers are in place. Verify all tube filaments are glowing. Look at input and output jacks. Check voltages for all tubes and post your findings. Include zero volt pins also. There may be a clue in the voltage readings.
I agree with pdf64. C17 and C18 have been stressed. A lot of expensive components rely on those caps being healthy. I would replace both.
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After I changed out the diode, I went back and got the guitar to play through the cab but it has hum, and when I crank the bright volume past 3/4, it gets squealy. I’ll check those components and report back in a day or so. Thank you again!!!