• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Best operating point for 7044 and 5687

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
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?:eek:

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!
 
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?:eek:

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!

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:eek:

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.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.