Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Mike_J on March 26, 2021, 02:54:33 pm
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I can't remember if it is common to have continuity between the ground and hot wire at the output jack. Going through the third go through with the marker on this amp and noticed this. It is been a long time since I did a start up on an amp and I just don't remember if this is okay. Seems like there must be a problem to me but I don't remember in this case.
Thanks
Mike
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I can't remember if it is common to have continuity between the ground and hot wire at the output jack.
Thanks
Mike
Output jack of a amp= speaker secondary = very low ohms 1.5 -3 ohms
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An OT secondary is a few turns of thick wire, hence it has pretty low resistance, especially in the case of a 2ohm secondary.
So as one end is connected to the chassis / 0V common, the other end should have a very low resistance to it.
Bizarrely, an output signal can still develop across its inductive reactance.
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I can't remember if it is common to have continuity between the ground and hot wire at the output jack.
Thanks
Mike
Output jack of a amp= speaker secondary = very low ohms 1.5 -3 ohms
Unfortunately the ohm reading is the same as it is when I hold the multimeter leads together which is somewhere near .3 ohms.
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An OT secondary is a few turns of thick wire, hence it has pretty low resistance, especially in the case of a 2ohm secondary.
So as one end is connected to the chassis / 0V common, the other end should have a very low resistance to it.
Bizarrely, an output signal can still develop across its inductive reactance.
In terms that a retired Certified Public Accountant can understand does that mean I shouldn't worry about it? My MM is reading the same across the transformer as it does when the leads are held together but it is not exactly your top of the line Fluke either. There is no reason it shouldn't work because it was making noise before I started this mess but this is confusing.
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Used another piece of garbage MM that I bought something like ten of for some experiment I was doing. Think I spent $3 for it new. It is reading 1.3 ohms and I trust it more that I do the other one.
Thanks
Mike
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The very low resistance is to be expected.
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Next concern reading across the 27K resistor in the feedback circuitry. Put my MM across it and it reads exactly what the presence pot reads across it. Not even close to 27K. Guess I need to lift one of the legs and measure across it and see if there is a problem or only a problem in the circuit. Have other 27K resistors to put in.
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The very low resistance is to be expected.
Thanks much for that reply.
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The 27K ohm problem was not a problem. Pulled it out and it read 27.00K ohms. Got a chance to circle something else on the old schematic. At this pace every component on the schematic will be circled at some point.
Thanks
Mike
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From a resistance measurement perspective, the 27k feedback and 4k7 pot are in parallel.
Due to the very low resistance of the OT secondary.
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From a resistance measurement perspective, the 27k feedback and 4k7 pot are in parallel.
Due to the very low resistance of the OT secondary.
Thanks pdf64, that explains why the readings seemed so strange. Does the 27K feedback resistor have an impact on the tail resistor for my vision, no matter how delusional, of balancing the PI on this amp if only for experimental reasons. If you look at the schematic posted a few replies back the tail resistor and the 27K resistor in the feedback circuit seem to branch off one way to the presence pot and the other way with the 27K resistor to the tip of the speaker jack. The ground of the speaker jack, at least in the case of my amp goes to the negative side of the PI ecap and then on to the CCW side of the presence pot. I was somewhat concerned the 27K resistor would somehow interfere with my balancing plans but I still don't know if it will and if it will, how?
Thanks
Mike
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Hmm, good question, I’m not sure.
Certainly it needs including if a full, detailed circuit analysis was being done.
But whatever, it’s been shown that the positive feedback effect it provides that acts to improve the LTP’s balance. So it’s perhaps a question mainly of academic interest only.