|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
24.17.48.137
In Reply to: RE: Quick question: How do you filament bias a tube that does not have a filament? posted by Nickel Core on October 16, 2014 at 07:02:39
I think you need to understand why people use filament bias before you look at an indirectly heated tube and pronounce that it's a good idea.
In your 437A phono preamp, you have the advantage that noise in the heater supply has some isolation from the cathode. If you try to integrate the heater supply into the bias supply, you will introduce a lot of noise.
You cannot filament bias an indirectly heated tube because it has no filament, period.
The arrangement you are describing is more like tapping the heater supply for a fixed bias voltage.
Follow Ups:
I dont' agree...
See my response below.
I filament bias because
1) I don't want to use cathode bypass caps and
2) I don't want to raise the Zout of the stage. A cathode resistor at this place would be much larger, adding to more ohms to the output Z. I cannot have that because its driving a 10K LCR directly, so I want Zout under 1K. The current through the heater is much larger then through the tube, so the resistor value can be lower, and so Zout will be lower.
As said again and again, the noise is not a problem... I can hear it only when he volume pot is over 80% and its not related to the signal.
The Schottky diode option is new for me. I might try this once as said below. I'm sure it will work of course, but I doubt it will sound better. But again, I will try.
Its quitter then several commercial phone stages I've heard.
NC
It simply isn't filament bias, but rather a fixed bias supply.
If you have capacitors in the DC supply feeding the tube/resistor, then you haven't exactly accomplished your goal of not having a cathode bypass cap.
This is sort of like saying that you have a regulated screen grid on a triode. A triode doesn't have a screen grid, just as an indirectly heated tube has no filament.
(Your plate voltage shouldn't be that far off IMO by-the-way)
Its not a fixed bias supply.
My method is the same as the method called filament bias, I only use the heater instead of the filament (which I'm sure is also called a filament in some books). The 'old' reference is Ef (also for indirectly heated tubes).
However, you can call it anything you want, I'm fine with that.
The heater voltage is supplied by a VCCS.
NC
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: