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What's Up With These Voltage Readings?
As I mentioned below, I decided to eliminate the volume pot on my SE 27 - 6A5G amp that uses battery grid bias. Got the 27s set up with the 100k resistor (actual value 98.7k each) between the battery negative and the grid.
With no power to the amp, the battery terminal (measured from ground) reads -9.69v with -9.59v on the grid side of each resistor. When I power it up one 27's grid (tube A) only drops slightly to -9.53. The other one (tube B) slowly drops and eventually stabilizes at -7.14v after several minutes.
Why is there such a large voltage drop - over 2v difference? I'm guessing there's a problem with Tube B. Can anyone explain what it is?
Both tubes test strong and the amp sounded fine. Wouldn't using this tube (B) cause the battery to drain much faster?
Each tube also has a 150 ohm (actual 148.2) cathode resistor. Tube A measures .859v (5.8 mA) while Tube B is at .955v (6.44 mA). Plate voltages (measured from cathode) are 160.7 and 146.1 so plate dissipation is essentially the same: .93 w and .94 w.
Update: I pulled and replaced Tube B and now all the voltages on both are very close. Also retested the tube. It tests 950 on my Hickok (1000 is new) with no shorts, gas and plenty of life. On my Eico, which has a good inter-element leakage test, it's at 96% with no shorts or leakage indicated.
So what's going on?
Follow Ups:
All tubes have some grid leakage current.
The higher the grid leakage current the lower the bias voltage from the battery will be because current is flowing through the 100k grid resistor.
With no current at all flowing through the 100k resistor, there will be no voltage drop.
More current, more voltage drop across the 100k grid resistor.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
One thing that confuses me is that I had the same tube installed previously and it didn't seem to be drawing grid leakage current. Voltage on grids of both 27s were stable and close the the voltage of the battery itself.
What would cause a tube to develop this condition? Is it related to circuit design or operating points?
Is there anything that can be done to reduce or eliminate the problem, other than just using a different tube? Or is it even a problem?
Will adding a grid stopper help?
I don't know why today would be different than yesterday.
I don't know how to answer any of your questions except,
"Will adding a grid stopper help?"
If the tube is oscillating for lack of a grid stop resistor then maybe that oscillation is increasing the grid leakage current. But I don't know, that's just a guess.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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