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Best operating point for 7044 and 5687

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kooltubes

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Joined 2004
#1
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 :att'n:

So what's the best operating point for both tubes RC loaded?

Thanks!
 
#3
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 :att'n:

So what's the best operating point for both tubes RC loaded?

Thanks!
Click to expand...

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.
 
#4
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😱
 
#5
I don't think that the problem is the operating point.
Some people like it at approx. 90V @ 10-15mA or 150V @ 10-15mA. Others (like ANJ) use it even lower at 2.5mA only !
Also 15K plate resistor is very popular as a loadline.
 
#6
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😱
Click to expand...

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.
 
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