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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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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?
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Topic - Odd Voltage Readings On 27s - FlaCharlie 13:38:33 09/15/16 (3)
- RE: Odd Voltage Readings On 27s - Tre' 15:28:04 09/15/16 (2)
- RE: Odd Voltage Readings On 27s - FlaCharlie 17:45:05 09/15/16 (1)
- RE: Odd Voltage Readings On 27s - Tre' 18:30:25 09/15/16 (0)
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