Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jacodaro on March 01, 2016, 11:39:53 pm
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Haven't posted in a long time but stop in to read and learn. Bassman 100 that I completely rebuilt typical blackface style. Both channels. Very quiet and works great except for this: I'm hearing a subtle fuzztone on top of a note which became obvious when playing a low note on a guitar. It occurs on the initial attack and continues as the note decays but immediately drops off below a certain amplitude (correct term?) as the note then continues to decay. Tubes ruled out as source of problem. Using a Fluke meter probe to increase the noise, I tracked it down to one side of the PI from the pin/plate through every point up to the grid/pin 5 of both 6L6's on one side So I switched the OPT leads, and of course, the secondary speaker leads, and the problem went to the other side of the PI, right on through from the PI pin/plate to every point on that side. So it's following the OPT plate lead. A little background on this amp: it had been dropped hard and broke/bent the OPT mounts which were flimsy bent metal plates not part of the bell covers. Dropped so hard that the mid 1970's stapled head cabinet build loosened at the joints. No surprise there. It didn't pull/stress any of the leads. Massive OPT by the way. I fabricated four individual, full length, bent steel brackets to properly secure it. Proper OPT resistance check and there doesn't seem to be any obvious shorts. Fuzztone on top of the note only happens with a signal, if that means anything. Do I have a bad OPT or what? Where do I go from here? I have a 2x6L6 OPT to sub in and test if that would be the next thing to do. Thanks in advance for any help.
EDIT... Changed grey font color to default black... sluckey
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schematic? you said black faced mods/rebuilt. many versions of blackface. which one?
symptoms read like the PI has oscillation problems. did you install 47pF cap across PI plates?
--pete
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Oh ok. AB763 straight on through, including the bass channel.
So, two "normal" blkface channels with no mods to that basic circuit.
All voltages are correct.
No 47pf across PI plates but will be the first thing I try as it's the easiest.
Then swap in a known good OPT. If it is determined to be bad it looks like Doug has a suitable Twin Reverb replacement.
If it's a PI oscillation, it occurs on which ever side I connect the one OPT plate lead.
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Just an update that the OPT was bad. I did have another 4x6L6 unit and all is good. I suspect that there was probably internal damage to at least a few windings from the amp being dopped. Thanks.
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Damn....you beat me to it ...lol. I had my Hot rod deluxe doing the exact same thing a few years ago, and it was the OPT