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  #16  
Old 03-10-2017, 08:53 PM
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dtvmcdonald dtvmcdonald is offline
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"Hum" pickup is often not hum but buzz ... 180 or 240 Hz and up, up, up.
This is often pickup from wires connected directly to the power transformer
or direct to the line.

If the filter capacitors are grounded will away from the
transformer it may well be unfixable keeping the radio near
original. I've fix it by completely redoing the power supply with
1N4007s and two modest electrolytics and two modest series
resistors, right at the transformer, and letting the resultant somewhat
filtered DC go around the radio to the tube rectifier and filter caps.
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  #17  
Old 03-11-2017, 01:59 PM
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jr_tech jr_tech is offline
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Quote:
Originally Posted by dtvmcdonald View Post
"Hum" pickup is often not hum but buzz ... 180 or 240 Hz and up, up, up.
This is often pickup from wires connected directly to the power transformer
or direct to the line.

If the filter capacitors are grounded will away from the
transformer it may well be unfixable keeping the radio near
original. I've fix it by completely redoing the power supply with
1N4007s and two modest electrolytics and two modest series
resistors, right at the transformer, and letting the resultant somewhat
filtered DC go around the radio to the tube rectifier and filter caps.
So you apply the partially filtered DC to the plates of the rectifier tube, instead of AC directly from the power transformer? Or do you connect it to the cathode/filament of the rectifier and leave the plates out of the circuit?

jr
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  #18  
Old 03-11-2017, 07:39 PM
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dtvmcdonald dtvmcdonald is offline
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It depends on whether you want to use the rectifier as a dropping resistor.
This is unwise if you have unreliable heater-cathode rectifiers prone to shorts.
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  #19  
Old 03-12-2017, 09:43 AM
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init4fun init4fun is offline
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Quote:
Originally Posted by dtvmcdonald View Post
"Hum" pickup is often not hum but buzz ... 180 or 240 Hz and up, up, up.
This is often pickup from wires connected directly to the power transformer
or direct to the line.

If the filter capacitors are grounded will away from the
transformer it may well be unfixable keeping the radio near
original. I've fix it by completely redoing the power supply with
1N4007s and two modest electrolytics and two modest series
resistors, right at the transformer, and letting the resultant somewhat
filtered DC go around the radio to the tube rectifier and filter caps.
Having run into a similar situation in the past of the power transformer located on the other side of the chassis from the rectifier tube and filter caps , and having a hum that just couldn't be eliminated , I took a somewhat different approach . I installed the power transformer I had taken from one of the Philco radios that has the rectifier tube mounted directly on top of the transformer and an appropriately sized filter cap right there on the B+ lead coming from the transformer/rectifier unit and then ran the resultant B+ across the chassis to the original (restuffed) filter caps . With no signal the radio was dead silent , not one bit of detectable hum . Yes indeed the originality was of course gone but a usable radio was preferred in this case rather than an original one . The diodes idea is a great one and if I didn't have that spare Philco transformer I'm sure the diodes would have worked just as well in my application .
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  #20  
Old 03-12-2017, 12:57 PM
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jr_tech jr_tech is offline
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Once, I bent up a piece of 3/8 copper tubing and threaded the leads from the power transformer to the rectifier tube inside the grounded tube... I will try the diode trick if I run into a stubborn hum problem again... thanks for posting!

jr
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