Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ListerHayes on January 22, 2020, 09:35:48 am
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First timer here, so please bear with me... LOL.
I just put your Pro Junior Kit together and I have two issues I cant get past (they may be related). First, I'm not getting any signal from the input. Second, my bias voltage at the grid resistor is measuring sky high at 300+. When I connect to the chassis, it's only around 13. This seems backwards from the last time I adjusted the bias (I replaced R29 on my old board with an adjustable bias). The current through the plates is spot on at 3ish.
I'm a long time guitar builder, but have only been modding amps for a short time. To say I fumble through a schematic is no understatement. Any ideas?
BTW, I have checked and re-checked all of my connections, caps and resistors several times. I'm almost ready to just order another kit and start all over.. LOL
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I'm using this for reference;
https://el34world.com/Hoffman/files/Hoffman_Pro_Jr.pdf
with the 84's OUT, what do you read at the socket pin 2 to ground? (both sockets)
what's the "range" when you adjust the bias pot?
DO NOT put the 84's in til this is right, unless you need melted glass for some project
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The bias diode is drawn backwards on the layout! Turn it around. And replace the two 10µF bias caps.
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Did it get edited?, the layout I referenced is correct (now?) :dontknow:
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It's magic! :wink:
The real question is has it been corrected in the amp?
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By the way, the image at the top of each board section on the library page is just a link to the real PDF file layout document.
The layout images are just eye candy links
The schematic on the PDF had the diode facing the correct direction
The layout in the PDF had it reversed
In chrome, you have to clear the files and images to get it to download the most recent files
Otherwise, it just keeps pulling up the files it has on your local machine.
Shift-Control-Delete lets you access a menu and you can clear several things
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I'm using this for reference;
https://el34world.com/Hoffman/files/Hoffman_Pro_Jr.pdf
with the 84's OUT, what do you read at the socket pin 2 to ground? (both sockets)
V3 reads 360.1, V4 reads 359.7
what's the "range" when you adjust the bias pot?
V3 reads 2.81 to 7.88, V4 reads 3.06 to 7.97
DO NOT put the 84's in til this is right, unless you need melted glass for some project
Learned that one the hard way on another bulid...
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It's magic! :wink:
The real question is has it been corrected in the amp?
I had the diode in the correct direction. I compared the PDF to the schematic before I started and opted for the schematic. Not because I know what I'm doing, it just seemed like the stripe on the diode should be relative to the indentations on the caps... LOL
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Also, the normal Pro Junior hum is there and it does go up in volume when I turn up the volume pot. I'm just not getting any guitar sounds...
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Done! That was it. I figured it would be something simple. Thanks for the help! The amp sounds amazing!
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Thanks for the heads up guys on the Pro Jr diagrams
That stuff was all done a while back and not that many people have ordered the Pro Jr boards
I would have expected some errors being that I copy and pasted a bunch of that from the Blues JR Visio file
https://el34world.com/schematics.htm#Hoffman_Pro_Junior_Conversion (https://el34world.com/schematics.htm#Hoffman_Pro_Junior_Conversion)
You may have to reset chrome's cached files to see the new images and PDF