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Brian
 
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Default 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
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Gregg
 
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To wreck the cathodes faster ;-)

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Gregg
*It's probably useful, even if it can't be SPICE'd*
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Fred Nachbaur
 
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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 |
+--------------------------------------------+

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John Byrns
 
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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/
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Brian
 
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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


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Fred Nachbaur
 
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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 |
+--------------------------------------------+

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Brian
 
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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
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Fred Nachbaur
 
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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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