Grundig NF-20 Project Scoping

radex7

radex7

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Some time ago I must have needed to take my mind off grad school, so I acquired this hobby, with excess of pieces, and grand aspirations. Here is an example. A triplet of Grundig NF-20, each in various states of assembly. Leftmost - still fully functioning, was actually rebuild by our own Tinkerbelle, maybe that's why it still works? Middle one came with mismatched output transformers, but it has original diodes instead of black can rectifier. Rightmost has not been violated yet, and it sports both types of rectifiers (wafer and a can), but what is different about this one - it has a filter choke!

triple_grund.jpg

So I have been staring at them for quite some time, trying to decide how to tackle this. Here is what I came up with: leave the working one alone - for reference purposes. Strip the middle one to barebones to clean up the chassis, and transfer the guts from the right one onto it. Recap, of course, rectifier replacement, and conversion to stand-alone amp is in order. In the end I'll decide what to do with remaining chassis, transformers, and misc.

Question to you, kind people, what is the story with this filter choke? I have not seen it in schematics that I came across, nor in any images online. Do you see any benefits of using this design over the one without the choke?

Input appreciated! Thanks!

Radek
 
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A choke will provide superior filtering. I like chokes in a power supply when I can have them. Looks like the chassis is drilled for one already, so maybe an option on certain models with this amp?
 
Choke-input supplies are more expensive to build than capacitor-input which is why they are less common even though they confer substantial regulatory benefits.

Chokes add stability and current reserves because they convert current into a magnetic field which can be converted back into current as needed. It's like a shock absorber. The choke also copes with the reactance arising from the capacitive load, adjusting the phase to create a more linear and resistive load. (This reactance issue is called "power factor" and it is why engineers at the power-generating companies have gray hair and digestive problems.) The most substantial benefit of the choke, however, is that it splits the supply at that point, so additional capacitance may be placed on the non-rectifier side of the choke without improperly loading the rectifier. (This can also be done using a resistor for degeneration, but this may change the power supply's impedance, limit current, and waste power as heat.)

Prior to the advent of modern high-current regulators and good capacitors most AC power supplies were choke input. The reason choke-input supplies (CLC) were supplanted by by capacitor-input supplies (CRC) is the expense of raw materials (copper wire and steel laminations), increased manufacturing costs (choke itself plus the associated assembly, installation, and stocking), and the increased size and weight.

I should specifically point out that many power supplies exceed, either through a deliberate decision or ignorance, the tube rectifier's maximum capacitance ratings. (Solid-state rectifiers have such limits, too, as do voltage regulators.) This greatly shortens a tube rectifier's lifespan and can cause voltage regulation problems. Check the ratings for your supply to ensure the first capacitor does not exceed them. You may add considerable more capacitance after the choke. I also suggest bleeder resistors, as I have elsewhere described, for safety of you and your equipment.

You lose nothing—aside from expense, size, and weight—by adding a choke, and gain a great deal in regulation. Since your chassis accommodates one, there is no reason to not use one. When it comes to sizing the choke you'll need to ensure the choke's amperage rating matches your maximum current draw, not your typical draw. Chokes are typically sized by your wallet thickness.
 
gonna guess thats probably not choke input. Doesn't look like big enough iron. If you look at the middle one it appears to have a big honkin power resistor where the choke exists on the right amp. Probably CLC filter setup on the one and CRC on the other. Plus however many other stages of filtering and resistoring may exist.
 
gadget73 said:
Plus however many other stages of filtering and resistoring may exist.
Click to expand...

Yes, good point. Those resistors are used to drop the voltage to form the various supplies, such as for the screen. A capacitor is generally used on each to stabilize the voltage under load, but the filtering of ripple current should have already been done by that point.
 
[QUOTE="radex7, post: 10759668,

Question to you, kind people, what is the story with this filter choke? I have not seen it in schematics that I came across, nor in any images online. Do you see any benefits of using this design over the one without the choke?

Input appreciated! Thanks!

Radek[/QUOTE] The things to worry about the after market choke is if it is the correct current rating and will it supply the correct voltage drop to replace the resistor. If it fulfills those 2 issues then the power supply for the amp will be superior to the original one.
 
I have one of these as well. I'll try and make time to open it up and see how it compares.
 
Thank you all for your replies! I had to read it a couple of times to get the gist of it ;-)

In the mean time - chassis is stripped, and ready to be cleaned.
 
primosounds said:
The things to worry about the after market choke is if it is the correct current rating and will it supply the correct voltage drop to replace the resistor. If it fulfills those 2 issues then the power supply for the amp will be superior to the original one.
Click to expand...

Don't forget that the DCR of the choke can act as a dropping resistor so it may form a part of the voltage regulation.

A low-DCR choke replacement may therefore raise the output voltage.
 
Retrovert said:
Don't forget that the DCR of the choke can act as a dropping resistor so it may form a part of the voltage regulation.

A low-DCR choke replacement may therefore raise the output voltage.
Click to expand...
Right, the replacement choke should be very close or the same value as the resistor it replaced. Or, a little higher to lower the B+ if your wall voltage is more than 117.
 
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