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Author Topic: Replacing Power Supply Components  (Read 2209 times)
WA7URV
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Posts: 28




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« on: February 14, 2011, 09:23:26 PM »

If a circuit specifies an 8mH, 200 ohm filter choke, what is the impact of replacing it with a 8mH, 259 ohm choke.  I would assume that if voltage is constant between the two, the 259 ohm choke would provide less current.  Am I correct on that; what other possible effects might come from such a replacement?

I also could replace the 8mH, 200 ohm choke with a 8mH, 150 ohm choke.  Using my same thinking as above, will I deliver more current (again derived from the same voltage.)

With electrolytics, I am considering replacement of a 30mfd/400v part with a 33mfd/450v part.  I understand that the voltage rating would have no impact, and in this case, I'm assuming the capacitance value will also have minimal if no impact.  Am I correct on that?

How about replacings a 15mfd/400v electrolytic with a 22mfd/450v part? 

I would love to hear knowledgeable opinions on the general effects of these types of replacements.

73,
Phil, WA7URV
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KE3WD
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« Reply #1 on: February 15, 2011, 07:21:56 AM »

With electrolytics, the differences in capacitance you cite are going to prove small indeed, I wouldn't sweat that. 

The chokes may be a different story, though, depending upon circuit placement and use. 


73
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KE4JOY
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« Reply #2 on: February 15, 2011, 07:59:43 AM »

If it is just a 'filter choke' it probaly will be fine.

The caps will work fine most important is their voltage rating.
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AC5UP
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« Reply #3 on: February 15, 2011, 08:40:57 AM »

If a circuit specifies an 8mH, 200 ohm filter choke, what is the impact of replacing it with a 8mH, 259 ohm choke.  I would assume that if voltage is constant between the two, the 259 ohm choke would provide less current.  Am I correct on that; what other possible effects might come from such a replacement?

You are correct... A little less current, a little less voltage as well since the higher resistance choke will 'sag' more on peaks than the original. Assuming the old choke is bad, I'd use the 259 Ohm choke - but - if I found the voltage drop was more than I'm comfortable with I might parallel the choke with a ~ 1,000 Ohm 2 watt resistor to reduce the voltage drop.

Quote
I also could replace the 8mH, 200 ohm choke with a 8mH, 150 ohm choke.  Using my same thinking as above, will I deliver more current (again derived from the same voltage.)

Same concept as above, and if you wanted to reduce the voltage increase you would add a 50 Ohm resistor in series.

Quote
With electrolytics, I am considering replacement of a 30mfd/400v part with a 33mfd/450v part.  I understand that the voltage rating would have no impact, and in this case, I'm assuming the capacitance value will also have minimal if no impact.  Am I correct on that?

How about replacings a 15mfd/400v electrolytic with a 22mfd/450v part?

Larger value electrolytics are often rated at +100% / -50% tolerance so don't sweat a few uf's one way or the other. I usually try to increase the capacitance as a hedge against the loss of capacitance as the part ages so that I might get a few more years out of the repair. However..................... On tube gear with a choke input to the filter it was SOP to "tune" the L/C ratio for best suppression of 120 Hz ripple. Engineering books published nomograms for easy parts selection, which is amusing since those parts tended to have a wide tolerance. Yes, you can increase the capacitance a few clicks but I wouldn't go too far as you may end up with more hum. A new 22 uf condenser might be closer to 40 uf...
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N2EY
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« Reply #4 on: February 15, 2011, 04:08:31 PM »

If a circuit specifies an 8mH, 200 ohm filter choke, what is the impact of replacing it with a 8mH, 259 ohm choke. 

I think you mean an 8 H choke (8 henries). 8 mH is 8 millihenries, which is an RF choke, not a power supply choke, in hollow-state.

In a typical hollow-state power supply, the effect of using a slightly-higher-resistance choke is that there will be somewhat more voltage drop across it, resulting in a slightly lower B+ at the output of the supply. The actual lost voltage depends on how much current the choke handles. The voltage regulation will be a tiny bit worse too.

For example, if the current is 100 mA, the difference between a 200 ohm choke and a 259 ohm choke is 5.9 volts. Not enough to be concerned about at voltages above 100 or so.

If the current is 300 mA, the difference is 17.7 volts - still hardly worthy of consideration in most cases.

What is the application?

I also could replace the 8mH, 200 ohm choke with a 8mH, 150 ohm choke.  Using my same thinking as above, will I deliver more current (again derived from the same voltage.)

Nope.

The available current depends on the components' current rating - power transformer, rectifier, and filter choke. The one with the lowest current rating determines the maximum supply current. Such components can be run at less than full current, but not more.
 
With electrolytics, I am considering replacement of a 30mfd/400v part with a 33mfd/450v part.  I understand that the voltage rating would have no impact, and in this case, I'm assuming the capacitance value will also have minimal if no impact.  Am I correct on that?

How about replacings a 15mfd/400v electrolytic with a 22mfd/450v part? 

Both are close enough that the replacements will work fine.

What is the exact application?

73 de Jim, N2EY
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WA7URV
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Posts: 28




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« Reply #5 on: February 15, 2011, 09:50:24 PM »

Thanks AC5UP and Jim, N2EY.  I appreciate the help.

Jim, I am considering building an alternative to the Collins 516F-2 Power Supply for a KWM-2.  I'm using this as a learning experience; I'm in no rush.  In fact, a friend of mine is helping me find appropriate transformers, since the original transformer is complex, pretty much unavailable (although I'm still looking) and even if I find one, it will be very expensive (an NOS one on Ebay just sold for over $280!)

By the way, absolutely you are correct that we're talking about Henries here, not "milli-henries."  Thanks for the catch.

There's all sorts of possibilities with this project, including substituting solid state rectifiers in place of the tubes.  I'm in exploration mode right now.

Again, thanks for you help on this!

73,
Phil, WA7URV
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N2EY
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Posts: 3879




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« Reply #6 on: February 16, 2011, 03:23:59 AM »

The Collins power supply sounds like a neat project. IIRC, it needs two levels of B+, heater power and bias.

The first step is to determine the actual requirements (voltage, current, ripple, regulation) of each.

A good resource is power supply simulation software:

http://www.duncanamps.com/software.html

has an example for free (scroll down)

As for solid-stating, there are some factors to consider.

A solid-state bias supply is fine because the bias requirements are low and bias should come on immediately. In fact the bias supply may already be SS in the 516-F2.

But the B+ is more problematic:

1) If vacuum rectifiers are simply replaced by SS in the B+ supply, you get more voltage because there's less drop with SS. That's not a good thing if it pushes the B+ higher than the set is designed for. Adding resistors is possible, but if you're going to do that you might as well stick with hollow-state rectifiers. The correct solution is a redesign of the supply which usually involves a slightly-lower transformer voltage.

2) With vacuum rectifiers the B+ comes up slowly and gradually, and the tubes in the set get a chance to heat up before being socked with plate and screen voltage. With SS, the B+ comes on almost instantly, when the tubes are ice-cold. To add to the problem, the cold tubes aren't drawing much current so that initial B+ can be considerably higher than the operating value. Not good for tube life.

This problem can be fixed by a "soft-start" circuit which gives the tubes a chance to warm up before the B+ comes one. Usually requires separate transformers but you'll probably wind up with that anyway.

3) SS power supplies with capacitor-input filters can have a tremendous turn on "thump" as the capacitors try to charge during the first half-cycle. This too can be fixed by a soft-start circuit.

The point is that it's not just a matter of using SS diodes instead of vacuum ones.

Old ARRL Handbooks, and the RCA tube manuals, have good chapters on power supply design. They're available free for the download.

A very good resource for both:

http://www.tubebooks.org/technical_books_online.htm

(WARNING! Highly habit-forming and hard-drive-filling!)

73 de Jim, N2EY
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