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#1
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Reduced Heater Voltage
I have a push-pull 6V6 amplifier that uses a series heater resistor to
run the 12AX7 phono stage and the following 12AU7 multiple-input stage at 4.73 volts when the rest of the amp is at 6.30 volts. What is the purpose of this? Brian |
#2
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To wreck the cathodes faster ;-)
-- Gregg *It's probably useful, even if it can't be SPICE'd* http://geek.scorpiorising.ca |
#3
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Brian wrote: I have a push-pull 6V6 amplifier that uses a series heater resistor to run the 12AX7 phono stage and the following 12AU7 multiple-input stage at 4.73 volts when the rest of the amp is at 6.30 volts. What is the purpose of this? Brian Some amplifiers used a selenium or similar (copper oxide?) rectifier to supply filtered DC to the early stages, reducing hum. As these rectifiers start to fail, their effective resistance increases and output voltage decreases. It could be that's what's going on. What make/model amplifier is it? Having a look at the schematic might provide more insight. Cheers, Fred -- +--------------------------------------------+ | Music: http://www3.telus.net/dogstarmusic/ | | Projects: http://dogstar.dantimax.dk | +--------------------------------------------+ |
#4
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In article , Fred Nachbaur
wrote: Brian wrote: I have a push-pull 6V6 amplifier that uses a series heater resistor to run the 12AX7 phono stage and the following 12AU7 multiple-input stage at 4.73 volts when the rest of the amp is at 6.30 volts. What is the purpose of this? Brian Some amplifiers used a selenium or similar (copper oxide?) rectifier to supply filtered DC to the early stages, reducing hum. As these rectifiers start to fail, their effective resistance increases and output voltage decreases. It could be that's what's going on. And other amplifiers with selenium rectifiers for the heaters were designed to put only 5.0 volts or so on the heaters of the low level stages when the rectifiers were new. Same thing with tuners that ran the audio stages from a 5.0 volt winding on the power transformer, so I don't think it is simply a matter of rectifier aging. Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ |
#5
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Some amplifiers used a selenium or similar (copper oxide?) rectifier to
supply filtered DC to the early stages, reducing hum. As these rectifiers start to fail, their effective resistance increases and output voltage decreases. It could be that's what's going on. There's a 1.6-ohm power resistor in the filament line, Fred. This was done on purpose. It's hard to believe that it is for reducing hum, as it amounts to a reduction of just 2.5 dB. This trick is done to diodes in radio circuits to lower contact potential and improve rectification, but I don't see the connection with triodes. Maybe it lowers white noise. If so, I'll bet more than 2.5 dB. Next time I have the amp apart (a no-name console pull), I'll short the resistor while listening and see if I can hear any difference. Brian |
#6
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Brian wrote: Some amplifiers used a selenium or similar (copper oxide?) rectifier to supply filtered DC to the early stages, reducing hum. As these rectifiers start to fail, their effective resistance increases and output voltage decreases. It could be that's what's going on. There's a 1.6-ohm power resistor in the filament line, Fred. This was done on purpose. It's hard to believe that it is for reducing hum, as it amounts to a reduction of just 2.5 dB. Well, I suppose that 2.5 dB could be considered significant. This trick is done to diodes in radio circuits to lower contact potential and improve rectification, but I don't see the connection with triodes. Maybe it lowers white noise. If so, I'll bet more than 2.5 dB. Next time I have the amp apart (a no-name console pull), I'll short the resistor while listening and see if I can hear any difference. That's the ticket, the good old "scientific method." Ideally you'd actually measure the hum/ noise, but a listening test would be better than nothing. One thing to watch out for - changing the filament voltage can also change the transconductance, and hence stage gain. So to prevent being misled, it would be a good idea to adjust the input signal for the same output voltage under the two different conditions, before removing the input to measure the hum and noise. Let us know what you find, this could be quite interesting. Cheers, Fred -- +--------------------------------------------+ | Music: http://www3.telus.net/dogstarmusic/ | | Projects, Vacuum Tubes & other stuff: | | http://www.dogstar.dantimax.dk | +--------------------------------------------+ |
#7
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Let us know what you find, this could be quite interesting.
I dug into the amp and fired it up. The lower heater voltage drops the hum 2 dB, and only on the phono position. The signal level is not affected, nor is the hum for high-level inputs. I had to rezero the hum balance pot, but the net result was 2 dB change. Brian |
#8
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Brian wrote: Let us know what you find, this could be quite interesting. I dug into the amp and fired it up. The lower heater voltage drops the hum 2 dB, and only on the phono position. The signal level is not affected, nor is the hum for high-level inputs. I had to rezero the hum balance pot, but the net result was 2 dB change. Brian Well, that seems significant enough to warrant the added resistor. Learn something new every day... another technique for the bag of tricks. Cheers, Fred -- +--------------------------------------------+ | Music: http://www3.telus.net/dogstarmusic/ | | Projects: http://dogstar.dantimax.dk | +--------------------------------------------+ |
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