ILoveHiFi wrote:
Becasue cathode to heater voltage and biasing is no secret.
The ideal cathode to heater votlage would be 0.
By changing the resistor divider ratio to set the heater bias votlage you can get close to zero.
But if one valve has hv on the cathode arround 1/2vcc, the other is close to ground.
Then you wanna bias half way in between, which is 1/4vcc
Yes agreed
But its interesting in the links that others trying to copy the M7 are using 40V difference between cathode and heater, that makes me wonder of they have the heater bias set from the other M7 which has a cathode follower output.
The interesting thing is that I would expect this with SRPP but this circuit isn't SRPP.
So whats going on for so many people to "get it wrong", or is it correct to the original M7?
Here is the same heater bias with the cathode follower:
https://www.google.co.uk/search?q=audio ... vtM:&vet=1And this from a different redraw:
https://www.google.co.uk/search?q=audio ... vIM:&vet=1If you look at the circuits here on an M77 its the reverse situation high voltage on the cathode but standard heater CT to ground?
http://www.dms-audio.com/audio-note-m77-diy-cloneIts very strange for so many people who must have researched this circuit to all get it wrong
Am I missing something obvious here??
It looks to me like the cathode follower needs the heater lift but the "original M7" I expect to see about 5.5V on the cathode.
Which would make the others wrong, but both should work with a heater to cathode max allowed at 90V (spec sheet).
LMAO I could use a couple of LEDs in place of the bottom half of the voltage divider and get regulated 5V and bias the heater to 5.5V
Or a 5V regulator with a diode on the ground leg. Its all a bit nuts, I'll just measure the cathode and look again.
Is there some reason they don't connect the heaters to Ground via the usual couple of resistors
I wonder if it has a sonic impact on the circuit...I wonder if you can get some sort of modulation from ground noise.
Maybe the 5V reference isn't such a mad idea.
In the past I have floated heaters with a high voltage polypropylene capacitor that just charges from the heater leakage.
Regards
M. Gregg