Negative voltage on heaters of 12AX7?

[C Moore]

Was this common for stereo amps? There is 50 volts from the PT- SS diodes- 100/75 can cap- some 18 ohm resistors- then it is on to V102 and V2, V101 and V1.
Thank You
Just wanted to ask this one more time. Was this done for noise reasons?

[IMG:160:105]http://i111.photobucket.com/albums/n143/zzmoore/th_LK48B_C1.jpg[/img]

[KT66]

I do not know how common this was, but what is going on is that the heaters are being run on DC. Pin 9 is the center tap of the 12 volt filament winding, if only pins 4 and 5 are used, then 12V is the appropriate voltage. They have four 12V filaments connected in series, so this would require 48V, which is why the 50V. As for the negative voltage I don't know why they did that, hopefully someone will shed some light on the subject.

[Andy Le Blanc]

You have to ask , negative with respect to what?

[C Moore]

You have to ask , negative with respect to what?

I only have a "basic" understanding of electronics, so pardon me if your question is over my head. I would assume it is with respect to chassis ground. Is that correct?

[drz400]

You have to ask , negative with respect to what?

Exactly, the heater doesnt have a direction

[C Moore]

You have to ask , negative with respect to what?

Like I said before, this statement must be over my head..... with respect to ground, just like all voltage specs on the schematic. Am I wrong?
I am still wondering, was it run at negative DC for noise reasons? Isn't that always the reason for DC heaters?
Thank You

[Merlinb]

It does not much matter whether heaters are run from positive or negative DC. In this case the negative voltage is also used for the power valves' bias; the designers obviously decided to kill two birds with one stone and use the same supply for heaters and bias.

[Andy Le Blanc]

It most likely to keep the Heater- Cathode voltage from exceeding it max ratings
negative or positive.

[FUCHSAUDIO]

It's something that was done commonly in HH Scott and Fisher hi-fi stuff, from which Lafayette (I'm guessing Panasonic, who made most of their stuff), likely got the idea.

The reason it is a negative voltage is because that supply is used for both the negative bias to the power tubes, and to provide quiet DC for the preamp tubes, at the same time. The 48 volts is the series total of the 4 filaments, and is easily filtered down through the multiple caps and 18 ohm resistors, to be quiet, from the raw bias. I'm guessing the raw bias to be maybe 50 or 60 volts, so there's enough filtering to keep it quiet, and not enough drop to make things run hot...cute idea. The tube filaments like DC, but don't care what polarity it is.

carry on....

[drz400]

delete is there no delete post button?