Exactness of feedback capacitors? - diyAudio
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Old 3rd May 2009, 02:40 PM   #1
bigwill is offline bigwill  United Kingdom
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Default Exactness of feedback capacitors?

I am rebuilding a Radford STA-15

Schematic:

http://www.freeinfosociety.com/elect...ew.php?id=2361


Now, the the feedback compensation caps in the original are silver mica and while you can get these, they are hard to find and pretty expensive. Other capacitor types don't have such a tight tolerance so you can't get these values. Does it matter if I can't find the EXACT value, and is it preferable to get the closest higher or lower valued caps?

I'm guessing higher, so that the amp is "slower" and less likely to oscillate?
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Old 3rd May 2009, 05:54 PM   #2
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Will,

"Perfection" of the phase compensation is directly related to the specific O/P trafo you use. Tweaking for the best looking waveform on the o'scope screen is the method employed.

I have no notion of what sort of parts are easily obtained in the UK. Silvered mica is not especially problematic in North America. Given the tweaking requirement, it might be better to install trimmer parts of the sort shown here. You could construct assemblies of a NPO ceramic part paralleled by a trimmer.
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Old 3rd May 2009, 06:09 PM   #3
bigwill is offline bigwill  United Kingdom
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TYVM for the reply, I'm back with another question

Are ceramics OK to use in the feedback network?
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Old 3rd May 2009, 06:35 PM   #4
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I've restored a few Radfords and used polystyrene caps in the NFB loop and in the compensation across the EF86's anode load. I've put a trim capacitor - as already suggested - across these caps to adjust for best square wave. Be careful if you decide to use a trim cap on the EF86 anode load compensation, it's at high voltage!
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Old 3rd May 2009, 07:12 PM   #5
bigwill is offline bigwill  United Kingdom
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Quote:
Originally posted by theoldtrout
I've restored a few Radfords and used polystyrene caps in the NFB loop and in the compensation across the EF86's anode load. I've put a trim capacitor - as already suggested - across these caps to adjust for best square wave. Be careful if you decide to use a trim cap on the EF86 anode load compensation, it's at high voltage!

One thing I'm not sure about - how do you know your cap + trimmer is within the window of adjustment range to get the optimal square wave response? Do you just try different fixed caps until you find you can 'nail' it?
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Old 3rd May 2009, 08:16 PM   #6
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Quote:
Be careful if you decide to use a trim cap on the EF86 anode load compensation, it's at high voltage!

While costly, trimmer caps. rated up to 500 WVDC are in current production. Look here. Perhaps something NOS originally intended for transmitter service could also be sourced.


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Quote:
Are ceramics OK to use in the feedback network?

Caution is very much in order about using ceramic dielectric in the signal path. However, NPO ceramic should be OK in the pF. values associated with this application. Don't buy "Brand X"!
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Old 3rd May 2009, 10:32 PM   #7
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Quote:
Originally posted by bigwill



One thing I'm not sure about - how do you know your cap + trimmer is within the window of adjustment range to get the optimal square wave response? Do you just try different fixed caps until you find you can 'nail' it?
You could change fixed caps but it takes a while. Have a look at the STA15 circuit diagram and start with the cap values shown; you'll probably have to parallel two or three to get the value required but if the transformers are original you should be OK at those values.

Or, for example, looking at the capacitior C17 [1000pF] in the 8ohm NFB loop, make up a value of say 800pF and connect a variable capacitor of say 400pF across it, adjust for a tidy square wave. Disconnect the capacitor[s] and measure, then substitute with fixed caps. I use a couple of old tuner capacitors from old am radios and several potentiometers to get the best possible square wave, it does save time. I'm in south Somerset if I can help.
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Old 3rd May 2009, 10:55 PM   #8
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How I determine the FB caps is by using a cap sub box, while watching the scope with a 1 or 5 KHz squarewave.
(5KHz is better for really high BW OPT's)

But you have to use the OPT that you will use in the final design (as mentioned).

If you plan on rolling OPT's, have that cap in an easily accessible place for swapping... like on a terminal strip not buried under wires.


Quote:
Originally posted by Eli Duttman
Caution is very much in order about using ceramic dielectric in the signal path. However, NPO ceramic should be OK in the pF. values associated with this application. Don't buy "Brand X"!
Agreed.

Avoid also the high dielectric constant types - they tend to be more non-linear at the swing peaks.

Cheers!
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Old 4th May 2009, 01:44 PM   #9
bigwill is offline bigwill  United Kingdom
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How inconsiderate of the designer to include global negative feedback
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Old 4th May 2009, 09:47 PM   #10
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Quote:
Originally posted by bigwill
How inconsiderate of the designer to include global negative feedback
PP pentode outputs - gNFB is pretty much mandnatory.
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