To use or not to use a choke in an old amp?

[smackoj]

A couple semi-rookie questions regarding inductors - chokes. Is there a particular set of circumstances that a tube amp designer faces that demands the use of a choke in the power supply? I would guess it adds more cost to the amp design? Also, if I am restoring an old 50s-60s-70s vintage amp and there is no choke used in original form, is there a benefit to adding a choke if I have one laying around not being used? Lastly, does a choke always take the first position in the power rail after the rectifier?

Thanks gents.   

[jjasilli]

Operationally, it's possible to use chokes (inductors) in place of resistors, and vice versa.  E.g., inductors could be used in place of resistors in the B+ rail.  This would reduce ripple, but not drop voltage as much.  An inductor could be used in place of a grid leak resistor.  Issues are cost and weight as you point out.  Another issue is that ea inductor generates a magnetic field, so noise could get induced into the signal. 


Re immediately after the rectifier -- either a cap or inductor comes first (though, again, a resistor could come first in place of an inductor).  An inductor-1st circuit drops B+ voltage more and has better regulation.  But the inductor must be huge, to handle the current draw of the entire amp, including the plates.  This is a hi-fi thing, I think.


In guitar amps, we typically see a choke after the plate supply, between the plate & screen filter stages.  For vintage style tone, I love chokes and am prone to add them to amps even if not original to the circuit.  I added a choke to my SF Princeton; Silvertone 1482; and VibroChamp which required moving the OT to a supplemental chassis.

[HotBluePlates]

... does a choke always take the first position in the power rail after the rectifier? ...

Expanding what Jjasili said:
 -  Look hard at the rectifier output.  Are you sure you don't see a cap-to-ground at the rectifier?

 -  If you do, this is "cap input" and the cap is in the "first position" after the rectifier.  Rectified voltage tends to be PT volts RMS * 1.414, minus the effects of PT winding resistances, rectifier voltage drop, and influence of filter cap size vs current draw of the circuit.

 -  If there is no cap-to-ground at the rectifier cathode, the circuit is choke-input.  Rectified voltage tends to be PT volts RMS * 0.9, minus the effects of PT winding resistances, rectifier voltage drop, and influence of filter cap size vs current draw of the circuit.  Voltage output is more-stable ("better regulation"), but sensible design also suggests the amp circuit should be Class A for near-constant current draw from the power supply.  Class A results in stable supply voltages anyway, because there's little current variation from zero- to max-output, and less voltage variation due to changing voltage drops across PT winding resistance & rectifier.

 -  Most guitar amps have a choke between the 1st & 2nd filter caps;  only the screen and preamp current passes through the choke.

 -  A designer would choose a choke when a lot of filtering is desired (choke's inductance presents ~3kΩ or more of reactance to the circuit), but where little change of d.c. volts is desired even when current draw changes a lot.  The DCR of this example choke might only be 90-150Ω, and so causes very little voltage drop with changing current compared to using a 3-4kΩ resistor that would give the same filtering.

 -  Chokes always add to weight, take up space, and cost more than a resistor.  Manufacturers often saved them for bigger, more expensive, Class AB amps that need a stable screen supply even with highly-varying screen current, to support maximum output power.

[smackoj]

Excellent information gents, thanks much! One more question. I understand that a rating of more Henrys means a bigger unit with more wire and more ability to sufficiently carry bigger, more dynamic loads. So I would imagine there is a math equation that predicts the size of the choke needed? Can I assume that a larger, say 10-15 H choke would be a better choice always over a smaller, less H value choke? I realize that if the amp orig. came with a choke in the power supply that the schematic will show a part number or H value when doing a replacement. So I'm concentrating on adding a choke to an amp that did not originally call for one.

[jjasilli]

For chokes there are 3 factors:  Henries, Current Rating, and DC Resistance.  I guess more H requires more windings which would increase the size of the choke.  Higher current handling requires thicker wires which fattens the choke geometrically for the same # of windings.  I've come across ways to calculate this on the web -- tutorials for winding your own inductors.


Unless you want to roll your own, calculations aren't needed.  Just spec out your choke; find find the manufactured product and checkout its dimensions.


How to spec it out:   see how much current will be drawn through it.  But, generally 50mA for small bottle PP amps and 90mA for big bottles PP.  Operationally the most critical factor is current handling capacity, so as not to fry the choke.


Typically you'll want low DC Resistance, say 150 Ohms or less.  But higher DC resistance could eliminate the need for screen resistors. 

Henries boils down to taste IMHO.  E.g., in similar amp designs Fender uses about 1H; Marshall about 20H if I remember right. 


Ted Weber sells, or use to sell, a choke substitution box!

[smackoj]

great information. thanks