Hello,
I have tried to build a simple voltage amplifier using one half of 7044 biased at about 18ma (plate resistor 7k5; cathode resistor 300R bypassed with 470µf; supply 290V)
Sound is ok but the tube is very microphonic. I have tried 5 other GE 7044, same thing, have I done something wron?
I have plugged in a 5687, since it's almost similar as the 7044: less microphonic
So what's the best operating point for both tubes RC loaded?
Thanks!
I have tried to build a simple voltage amplifier using one half of 7044 biased at about 18ma (plate resistor 7k5; cathode resistor 300R bypassed with 470µf; supply 290V)
Sound is ok but the tube is very microphonic. I have tried 5 other GE 7044, same thing, have I done something wron?
I have plugged in a 5687, since it's almost similar as the 7044: less microphonic
So what's the best operating point for both tubes RC loaded?
Thanks!
kooltubes said:Hello,
I have tried to build a simple voltage amplifier using one half of 7044 biased at about 18ma (plate resistor 7k5; cathode resistor 300R bypassed with 470µf; supply 290V)
Sound is ok but the tube is very microphonic. I have tried 5 other GE 7044, same thing, have I done something wron?
I have plugged in a 5687, since it's almost similar as the 7044: less microphonic
So what's the best operating point for both tubes RC loaded?
Thanks!
Hi kooltubes,
I don't think you did something wrong. I think the problem is the 7044's since they are all microphonic.
The operating point looks o.k., maybe a little high plate/anode dissipation. General rule is about half the Ub on the anode/plate.
Greetings,
Ray.
kooltubes said:Hi Ray,
I have changed the cathode resistor to 470R, which gives me about -7v on the grid and 184V on the plate; current is about 14ma, the microphonics are less and the sound better.
What do you mean by "Ub on the anode/plate"? I am still a newbie
Hi kooltubes,
That means that there should be approx. half the supply-voltage on the plate/anode when idle. The idea is that the anode-voltage can then swing as much up as down. So, in your case, the 184V is a bit on the high side (but will work o.k. with small signals). You can enlarge the anode/plate resistor to achieve about 140...150V voltage drop at 14mA (10k). Take a look at the datasheet graphs if you want to pick a nice operating point.
Greetings,
Ray.
You're welcome!
If you'd like to know more about tubes: there's a thread here http://www.diyaudio.com/forums/showthread.php?s=&threadid=38278
with a lot of info on tubes, since you say you're a newbie
Greetings,
Ray.
If you'd like to know more about tubes: there's a thread here http://www.diyaudio.com/forums/showthread.php?s=&threadid=38278
with a lot of info on tubes, since you say you're a newbie
Greetings,
Ray.
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