Choke selection?

[leadfootdriver]

My amp doesn't have a choke, but I understand that installing one in place of a droping resistor give increased bandwidth, and a current reserve. Sounds good to me, so I'm in.

Looking at my schematic http://www.schematicheaven.com/newamps/crate_v32_palomino.pdf I see that the filter cap connects in parralell with my droping resistor (R9) to my screen supply.

If my OT's B+ supplys the screen power, why is the filter supply connected to the screens as well?

Do I select a choke with the same resistance as the resistor I'll be replacing, or do I get the least resistance for my app that I can, and change the screen resistors?

 

[jim p]

Choke?

If you added a choke it would not go where R9 is it would be added in the circuit on sheet 2 near C33 and R26. R9 does add to filtering of the power supply and also drops the voltage going to the screen grid resistors and the triodes in the preamp section. This being a push pull amp it does not require as clean a supply as a single ended amplifier because the noise of the plate supply is canceled out in the output transformer.
If you want to add a choke to maintain current regulation you will take a big hit in plate voltage and therefore output power. Take a look at this web page http://www.aikenamps.com/Chokes.html

 

[leadfootdriver]

Thanks for responding!

I thought a choke was supposed to store current, therefore making the amp MORE powerfull on the transients?

I called Torres, and he recommended a Hammond 157G. http://www.hammondmfg.com/153.htm

 

[jim p]

A choke is like a flywheel

A choke is similar to a flywheel it wants to maintain a constant current flow, so in actuality it will not respond quickly to a transit (a capacitor will but the voltage will drop). Also to get the most out of it you would need to change over to a choke input power supply and wind up dropping your plate voltage so you will loose output power. One big reason that chokes were in the power supply of tube amps is when tube rectifiers were used the rectifier could not handle high inrush currents so the value of input capacitance had to be limited with a choke added to prevent the rectifier tube from being fried (check out the data sheets for rectifier tubes). Also the secondary voltage of the power supply was higher to account for the voltage losses of the rectifier tube and filter circuit. If you want headroom to respond to transient current surges what would work best would be to add a solid state regulator to the amplifier with 20 to 30 volts dropped across it. So at the simplest a high voltage mosfet with a zener diode used to set the voltage at the gate that is approx 30 volts less then the voltage at the output of the rectifier that has a capacitive filter on it.
Another thing to keep in mind is your amp is a cathode biased class A amp so the current load on the power supply has less transients relative to a grid biased class AB amp.

As far as the choke you have posted it would not work for the plate supply and looks like overkill for the preamp section. The inductive reactance to 120Hz is 22.6k versus the filter cap that is equal to 13 ohms at 120Hz.

 

[leadfootdriver]

Torres said in this amp, the choke would only filtering the pre's, and screens. Bruce Egnater also said to think of the R9 resistor as a choke.

Thats the only reason I was pointed to it.

 

[leadfootdriver]

So this would be a capacitor input choke where you told me to insert it?

 

[leadfootdriver]

A choke is similar to a flywheel it wants to maintain a constant current flow, so in actuality it will not respond quickly to a transit (a capacitor will but the voltage will drop). Also to get the most out of it you would need to change over to a choke input power supply and wind up dropping your plate voltage so you will loose output power. One big reason that chokes were in the power supply of tube amps is when tube rectifiers were used the rectifier could not handle high inrush currents so the value of input capacitance had to be limited with a choke added to prevent the rectifier tube from being fried (check out the data sheets for rectifier tubes). Also the secondary voltage of the power supply was higher to account for the voltage losses of the rectifier tube and filter circuit. If you want headroom to respond to transient current surges what would work best would be to add a solid state regulator to the amplifier with 20 to 30 volts dropped across it. So at the simplest a high voltage mosfet with a zener diode used to set the voltage at the gate that is approx 30 volts less then the voltage at the output of the rectifier that has a capacitive filter on it.
Another thing to keep in mind is your amp is a cathode biased class A amp so the current load on the power supply has less transients relative to a grid biased class AB amp.

As far as the choke you have posted it would not work for the plate supply and looks like overkill for the preamp section. The inductive reactance to 120Hz is 22.6k versus the filter cap that is equal to 13 ohms at 120Hz.

Okay, I just re-read this. It looks like a choke for this amp is not ideal then.

 

[jim p]

Is it worth the cost?

He told you it would only filter the preamp section because where you wanted to locate it. If the voltage drop is not a problem and you have a Pi filter (capacitor, choke, capacitor looks like symbol for Pi 3.14) you may as well use a resistor it is a lot cheaper. As I said the reason amplifier manufactures used chokes in the past is because the tube rectifier on high voltage high current amplifiers had to have them. Chokes are heavy and expensive so as soon as solid state diodes came about they started to go away. Having a choke input filter on the power supply would cause sag in the plate voltage at peak current demand. Having a Pi filter will give you more plate voltage if you were replacing a resistor in the circuit but at a high cost.

 

[leadfootdriver]

'Still not understanding totally. But if you don think it's a good idea, then I'm with you.