Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: AlNewman on January 15, 2024, 04:15:31 pm
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Hi,
I have a Hallicrafters sx-62a receiver with a blown OT. Should be a fairly simple fix, except the OT had a 500ohm secondary tap... which feeds the negative feedback and tone control.
Using a standard transformer with 4/8/16 Ohm taps, am I able to adjust the voltage divider resistors, R75/R76 at the PI using the 16 Ohm tap to create the same amount of negative feedback and still work with the existing tone control? If so, could somebody walk me through the formula without causing my head to explode?
Thanks
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> could somebody walk me through the formula without causing my head to explode?
No.
SW-4 is a real brain-pain. Is there ONE setting you use, or do you need all four?
Short answer: use 8 Ohm tap and make R75 1K or 1.5K.
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> could somebody walk me through the formula without causing my head to explode?
No.
SW-4 is a real brain-pain. Is there ONE setting you use, or do you need all four?
Short answer: use 8 Ohm tap and make R75 1K or 1.5K.
Haha. Brain Pain. Yes, I know.... I have spent hours just trying to figure out how the switching works, never mind the frequencies...especially how it interacts with the plates of the power tubes.
I've never heard the amp, so I have no idea what would sound best...I would like to get it as close to possible as the original without having to get the transformer re-wound.
I found this thread.
https://el34world.com/Forum/index.php?topic=10163.0
Tubeswell does a good job of explaining that it's the winding ratio that directly influences the voltage divider. So, I was working at 16 ohms, not 8, but I figured it at a 5.6 multiplier in ratio between 16 and 500. Somehow I was thinking I'd have to increase R75 to 56k, but I guess I misunderstood. Would another option be to change the tail resistor, R76? Would that have less effect on the capacitors, or pretty much the same?
I suppose if all else fails, I can get the nfb to stock voltage, and then just dial in the frequencies to where it sounds good for each position in the switch.
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> increase R75 to 56k
8 (or 16) will be a lower voltage. You want a lower resistor.
R76 might act different but has a heavy influence on tube DC bias.
Sorry; had a fuller clip on clipboard and forgot.
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Well, I'll see how it sounds when I get it all wired up... At least I know the transformer will work with some tweaking.
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So, my OT wasn't blown at all. I'm not sure why it didn't make noise when I first fired it up, and maybe since I was measuring ot resistance from the top and not the bottom I read the wrong pins and never had continuity.
Anyways, the radio works, and it works well.
I replaced a shitload of parts, and there are still some paper in oil caps that I need to replace, but I ran out of replacements, and have other projects I need to look at.
I'll revisit this another time. There's about 10 more capacitors that need replacing, then I can give it a full alignment.
Just for reference sake, the best setting on the tone switch is bass.