Power resistor inductance?

[d95err]

I'm experimenting with some SPICE simulations and some tube models available at www.duncanamps.com. I thought it would be cool to try to simulate the Trainwreck power supply (or possibly the whole amp).

One intriguing thing about the TW is the 25W resistor used for the screen supply filter. Does it really work like a small choke? How much inductance would such a resistor really have? Would this inductance be enough to significantly reduce power supply ripple (compared to e.g. a metal film resistor of the same resistance)?

Has anyone made such measurements? (I don't have the tools necessary, unless there is some simple way to measure it with a common multimeter).

[Bob-I]

I measured a 620ohm 25 watt resistor using my cheep capacitance/inductance meter. It measured .04mH. Since the ones we typically use are closer to 6mH I'd say that the inductance isn't a factor.

Interesting thought.

[raiken]

The inductance of the resistor is negligible at the frequencies of interest in the power supply.

For example, using the value measured by the previous poster, 0.04mH, you would get an inductive reactance of 0.03 ohms at 120hz. This, in conjunction with the usual 50uF filter cap, which has a reactance of 26.53 ohms at 120Hz, would give an attenuation of around -0.01dB at 120Hz (the power supply ripple is twice the 60Hz mains frequency because it is full-wave rectified).

By contrast, a 1K resistor, which has a reactance of 1K at 120Hz, gives an attenuation of around -32dB at 120Hz in conjunction with the 50uF capacitor, which would reduce the residual 120Hz hum by a factor of around 39 times.

Okay, you might ask, why use a choke? Well, a 6H choke would have a typical resistance of of only 100 ohms or so, so it would drop less voltage than the 1K resistor, for DC. However, for the 120Hz AC ripple riding on the DC voltage, the inductive reactance would make it look like a whopping 4.5k resistor. This 4.5K, along with the 50uF cap, would result in an attenuation of -44dB at 120Hz (ignoring the effect of the 100 ohm choke resistance, which would be negligible compared to the 4.5K reactance of the inductor, but it does help to dampen the overshoot at the corner frequency of 9.2Hz). So, by using a 6H choke, you get 12dB more attenuation of hum (a factor of 4 times less ripple) along with less voltage drop to the screens (more power output) and less preamp hum.

The only thing a resistor buys you is screen supply sag, which you may or may not like. You can always do both - use a choke and add a largish series resistor for better ripple rejection and screen supply sag.

Randall Aiken

[d95err]

Thanks for a very good explanation!

[raiken]

Thanks for a very good explanation!

You're welcome!

RA

[John C]

I too want to say thanks, raiken! You mentioned a 6H choke as an example. Would you like to recommend any specific choke for a Trainwreck?

Here's a nice explanation, including guidance for selecting a choke:

http://www.aikenamps.com/Chokes.html

Thanks again.

[John C]

Here's another link:

http://www.hammondmfg.com/193.htm

I ordered the one at the top of the list, "193B", from Antique Electronics. I'm guessing it's overkill, but I don't mind.

[ODwan]

Hello,
could it be that the small inductance in the powerresistor can act as a HF choke filtering out line related and diode switching noise? Just a thought...
Timo