I am rebuilding a Radford STA-15
Schematic:
http://www.freeinfosociety.com/electronics/schemview.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?
Schematic:
http://www.freeinfosociety.com/electronics/schemview.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?
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.
"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.
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!
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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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"!
bigwill said:
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
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.
Agreed.
Avoid also the high dielectric constant types - they tend to be more non-linear at the swing peaks.
Cheers!
(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.
Eli Duttman said:
Agreed.
Avoid also the high dielectric constant types - they tend to be more non-linear at the swing peaks.
Cheers!
I suspect that Radford used - within reason - what ever capacitor type he thought suitable at the time and probably made changes that did not appear on the circuit diagram; I know this is the case with the transistor amplifiers as I did 8 weeks temporary work for him in the mid '70's.
Don't forget that a lot of these old amps may have had unsympathetic repairs over the years so may not have the correct parts. I always look for signs of previous repair before attempting any work - it has saved a lot of time and frustration!
I have repaired/restored STA12's STA15's, all marks of the STA25 and various pre-amps, most of which had capacitors of different dielectric. In the NFB loop I have found both polyester and polystyrene.
Then there comes the question: do you restore back to 1960's spec or make use of modern components?
Don't forget that a lot of these old amps may have had unsympathetic repairs over the years so may not have the correct parts. I always look for signs of previous repair before attempting any work - it has saved a lot of time and frustration!
I have repaired/restored STA12's STA15's, all marks of the STA25 and various pre-amps, most of which had capacitors of different dielectric. In the NFB loop I have found both polyester and polystyrene.
Then there comes the question: do you restore back to 1960's spec or make use of modern components?
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