Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: camsna on September 19, 2016, 12:52:55 pm
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Here's how I have it wired up. Brings the bias stupidly way up (200+mv). Do I just need to disconnect the blue wires?
Thanks!
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Cam
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The bias should not change if done correctly. You don't need the blue wires. Do you still have the wire labeled "S" connected to the board and the intensity pot?
After installing the MV mod pull the output tubes and test for approximately -40v to -50v on pin 5 of the output tube sockets. Don't put the output tubes back in until you have this negative voltage on pin 5 of BOTH tube sockets.
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Thanks!
Yes, I still have "S" connected to the board and intensity pot.
I'll yank the blue wires and re-check. Are they merely unnecessary, or are they problem-causing?
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The blue wires bypass the MV pot, so it won't work as expected.
This is important. Pull the tubes. Check the voltage.
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The blue wires bypass the MV pot, so it won't work as expected.
Roger that.
"This is important. Pull the tubes. Check the voltage."
Sorry. Didn't mean to make it seem like I was ignoring this bit of wisdom/direction. Am not. Will do. :)
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I only mention the bias because it will only take a few seconds to destroy those KT66s with 200+mV on the bias test points. Since the MV is directly in the bias path to the control grids I highly suspect that's the problem. Pulling the tubes may save you a bunch of money.
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Disconnected the blue wires -- same thing. I'm going to take apart and totally re-do the PPIMV. I'll let you know how it went in a couple of days.
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It may not be wired correctly. I guess you didn't do this...
After installing the MV mod pull the output tubes and test for approximately -40v to -50v on pin 5 of the output tube sockets. Don't put the output tubes back in until you have this negative voltage on pin 5 of BOTH tube sockets.
Turn the tremolo off and see if it gets happy.
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Sorry. Did precisely that -- with no change (after pulling the blue wires) :-/
There's a gremlin in the trem circuit, too, that I have to chase out. I'm convinced that the two are related.
Can't thank you enough for your help and wisdom!
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Let me ask a different way. What voltage do you measure on pin 5 of each KT66 socket with no tubes in the sockets?
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Derp. I'm thick.
I can't remember what they were (now that I'm at work) -- but whatever they were, they didn't change pre/post blue-wire-removal. I'll re-wire the thing, re-test, and make sure to be getting sufficient -voltage on the power tubes.
AND -- if I'm still having trouble, I'll post what the voltages are! Y'know. So helpful people can actually help. . .instead of having to wonder why the dummy won't just tell them what they need to know. . .Sorry 'bout that.
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That's a good plan.
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Woo! It all works!
Trem was a result of a (seriously) backwards-made LED. No crap, flat-side positive.
Completely redid the master volume from the ground up. Worked on the first try :)
Can't thank you enough for your help and guidance! Amp sounds really really great. My dad is so happy with it!
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:thumbsup: