Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: hesamadman on June 07, 2014, 02:16:00 am
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:BangHead: :BangHead: :BangHead: :BangHead:
I know its frowned upon to post a new thread but I really need help finding a solution. Ive tried everything here and this build is still giving me trouble.
2 things:
When tone knob is all the way down i get "motor boating"
I have incredible Hum.
Things ive tried:
replaced pre amp tubes
disassembled the board and re routed wires
tucked in my heater wires better
changed filter caps
installed shielded wire on my input
unhooked all grounds and grinded chassis to ensure a good ground
Im listing everything about this amp right here:
PT= Hammond 269JX (I believe someone at hammond suggested this model)
OT= TR-OT-01 from Tube Depot
Electro Harmonix 6v6EH
JJ 12ax7
Ground Bus connected to chassis. Same point as red wire with yellow stripe on xfmr
Photos may show some wires with alligator clips. Was just trying a few things.
Im including layout, schematic, and photos of amp.
(http://i89.photobucket.com/albums/k205/fratbasher/gretschschematic.jpg) (http://s89.photobucket.com/user/fratbasher/media/gretschschematic.jpg.html)
(http://i89.photobucket.com/albums/k205/fratbasher/layout.jpg) (http://s89.photobucket.com/user/fratbasher/media/layout.jpg.html)
(http://i89.photobucket.com/albums/k205/fratbasher/photo24-1.jpg) (http://s89.photobucket.com/user/fratbasher/media/photo24-1.jpg.html)
(http://i89.photobucket.com/albums/k205/fratbasher/photo34.jpg) (http://s89.photobucket.com/user/fratbasher/media/photo34.jpg.html)
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I just realized my PT doesn't have a center tap for the filament winding. I read about that being an issue. I am not getting 1.5v on each leg. But I am getting roughly 6 volts across filaments. Is any of this a concern?
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If your heaters tap DOES not have a CT and you get about 6v measured across each leg, then you need to create an artificial CT by adding a 100 ohm resistor off each leg to ground.
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Where's the board jumper? Where's the tone cap?
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Where's the board jumper? Where's the tone cap?
I forgot to mention. I took that cap off the tone knob because I was using an alligator clip to try different values there.
My jumper is under the board. Thanks for taking a look at that
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If your heaters tap DOES not have a CT and you get about 6v measured across each leg, then you need to create an artificial CT by adding a 100 ohm resistor off each leg to ground.
I read about that but didn't know how to do it. I'm going to give that a shot.
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The heater CT is a requirement in 95% of cases. It just is. That should do it.
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The heater CT is a requirement in 95% of cases. It just is. That should do it.
I'm so incredibly happy to hear you say that. I've tried SOOO many things. Good thing is I have learned A TON during this build. Which was the point all together.
Just so I have this right guys, I should put a 100 ohm resistor to ground on both filament legs? And then continue to my heater pins on my tubes?
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Look at this schemo, it shows what you guys are talking about.
http://el34world.com/charts/Schematics/files/fender/Fender_BASSMAN_AC568.pdf (http://el34world.com/charts/Schematics/files/fender/Fender_BASSMAN_AC568.pdf)
Brad :icon_biggrin:
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Look at this schemo, it shows what you guys are talking about.
http://el34world.com/charts/Schematics/files/fender/Fender_BASSMAN_AC568.pdf (http://el34world.com/charts/Schematics/files/fender/Fender_BASSMAN_AC568.pdf)
Brad :icon_biggrin:
That's awesome! Thanks you!!
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The heater CT is a requirement in 95% of cases. It just is. That should do it.
More info soon...BUT THANK YOU GUYS!!!! IT WORKS!!!
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Awesome!
Now you will have learned something: when you hear that buzzy hum in the future, you'll know to check that you have a ground reference for the heaters.
I feel silly for not having thought of that when you posted the video clip. I made the same mistake one time, and got the same kind of buzz. It's not a smooth low pitch like you might assume.
I think we were fooled by the location because the hum was everywhere. So when you turn down the volume knob, you don't get the hum injected in the first stage for amplification by the second stage. That makes it seem like the hum is before the volume knob (which it is, but it's also after the volume knob).
Congrats!
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Awesome!
Now you will have learned something: when you hear that buzzy hum in the future, you'll know to check that you have a ground reference for the heaters.
I feel silly for not having thought of that when you posted the video clip. I made the same mistake one time, and got the same kind of buzz. It's not a smooth low pitch like you might assume.
I think we were fooled by the location because the hum was everywhere. So when you turn down the volume knob, you don't get the hum injected in the first stage for amplification by the second stage. That makes it seem like the hum is before the volume knob (which it is, but it's also after the volume knob).
Congrats!
Thanks a ton HBP. I could not have done this build with out you. I am truly grateful for all your time and help. I definitely learned A TON on this one. It was great. Ill keep everyone posted when I house it in an enclosure.
Thanks everyone else for your time and helping me as well!!