plate resistor, input lead

[vlljpior]

Hi, i found when i place my plate resistor wire close to my grid resistor my hiss problem come off. But i have a little squeal problem. What can cause this problem ? If anybody have a idea, i be happy...

 

[SY]

Oscillation from stray coupling, more than likely.

 

[Sch3mat1c]

Ya. And hiss stops because the amplifier is overcome by the stronger high frequency oscillation.

Hiss is fixed by NOT using carbon composition resistors, or switching to lower noise figure tubes (may require redesign of the circuit).

Tim

 

[vlljpior]

Yes, i'm using carbon composition resistors, so i have to switch to what type...

 

[Sch3mat1c]

Anything. Carbon or metal film are popular.

Tim

 

[SY]

For the plate resistor, wirewound. Grid stopper should be carbon comp (you ARE using a grid stopper, aren't you?). Grid leak can be metal or carbon film.

 

[Sch3mat1c]

For the plate resistor, wirewound.

If it needs to handle a few watts. Metal oxide is also possible, though may give more noise. Most of the time, 1/2W will suffice. Ohm's law and P = V*I if in doubt.

Grid stopper should be carbon comp (you ARE using a grid stopper, aren't you?).

Resistance is resistance AFAIK. At least at the typical <100MHz parasitic frequencies. Whatever's on hand or cheapest or easiest to get will work...

Tim

 

[SY]

Carbon comps as grid stoppers are more effective at RF suppression, though this is admittedly gilding the lily. Resistance IS resistance, but reactance and Q are reactance, too. Real resistors do have reactive elements.

Wirewound plate resistors are much quieter than metal films and that's no gilding, it's the whole flower. Additionally, they have a lower deltaR/deltaV than other types, significant at the large deltaV seen at plates.