martin H
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Thought I'd mention a long running amp problem that I’ve just solved so perhaps someone else won’t go through the same process:
A couple of years a go I set to restoring my 50W JMP Marshall 1987 head. I replaced all the smoothing capacitors, and got a complete set of tube to replace the mismatched bits and pieces that had gradually replaced the stock tubes. After reading a number of reviews, I chose Tung-Sol’s for their “bright detailed almost hi-fi tone.”
When I was finished, I noticed that the amp seemed to hum more than it used to. No amount of checking my work revealed any problem. The high voltage rail was clean. I even tried putting the old smoothing caps back in, with no success.
I finally took it Ito my local tech who kept it for a month, and announced he could get it to behave a little better by moving wiring but that, the hum was a design flaw in the Marshall because it used AC heater current.
(As a quick primer, the heater is the hot glowing wire in the tube. It typically requires about 6-12 volts across it to make it glow properly. Ac or Dc current will produce the same effect, but AC is a little cheaper to use, circuitry wise. I knew that older HI FI amps always used DC heater current in the front stages because the AC current gave off a field in the tube that could induce 60 cycle hum into the signal path. )
However I was still faced with the fact my Marshall, with AC heaters, used to not hum, and now it did. After a good deal of research, I found out the hum induced by an AC heater can be reduced considerably by the way the heater coil is wound physically inside the tube. “General” application tubes are usually wound this way, while “hi-fi” tubes are not , beause ther assumption is that, in a HI FI amp, the heater current will be DC
I replaced the Tung Sol preamp tubes with Mullards, and the hum, was gone. Later research showed that the Tung-Sol’s didn’t have heaters wound for AC current , and the amps they were tested in for the review all used DC heater current
Moral – if you change a pre amp tube, and the amp hums, it may have the wrong sort of heater winding
A couple of years a go I set to restoring my 50W JMP Marshall 1987 head. I replaced all the smoothing capacitors, and got a complete set of tube to replace the mismatched bits and pieces that had gradually replaced the stock tubes. After reading a number of reviews, I chose Tung-Sol’s for their “bright detailed almost hi-fi tone.”
When I was finished, I noticed that the amp seemed to hum more than it used to. No amount of checking my work revealed any problem. The high voltage rail was clean. I even tried putting the old smoothing caps back in, with no success.
I finally took it Ito my local tech who kept it for a month, and announced he could get it to behave a little better by moving wiring but that, the hum was a design flaw in the Marshall because it used AC heater current.
(As a quick primer, the heater is the hot glowing wire in the tube. It typically requires about 6-12 volts across it to make it glow properly. Ac or Dc current will produce the same effect, but AC is a little cheaper to use, circuitry wise. I knew that older HI FI amps always used DC heater current in the front stages because the AC current gave off a field in the tube that could induce 60 cycle hum into the signal path. )
However I was still faced with the fact my Marshall, with AC heaters, used to not hum, and now it did. After a good deal of research, I found out the hum induced by an AC heater can be reduced considerably by the way the heater coil is wound physically inside the tube. “General” application tubes are usually wound this way, while “hi-fi” tubes are not , beause ther assumption is that, in a HI FI amp, the heater current will be DC
I replaced the Tung Sol preamp tubes with Mullards, and the hum, was gone. Later research showed that the Tung-Sol’s didn’t have heaters wound for AC current , and the amps they were tested in for the review all used DC heater current
Moral – if you change a pre amp tube, and the amp hums, it may have the wrong sort of heater winding
