Exactness of feedback capacitors? - Page 2

barretter · 4th May 2009, 10:10 PM

The original feedback caps in my STA15 weren't silvered mica. If you look at the circuit diagram you will see pr after the caps' values. That stands for polyester, which the ones in mine were. So if you want the sound of the Sixties, look for some polyesters!

barretter · 4th May 2009, 10:42 PM

Seriously though, LCR do a range of 160V axial 2.5% polystyrene caps from 10pF to 3.3nF. You can easily make 267 and 340 by parallelling a couple of caps and 1000pF is available as a value. Rapid sell them.

theoldtrout · 5th May 2009, 08:23 AM

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?

bigwill · 5th May 2009, 04:45 PM

Oops maybe they have polyester / polystyrene

pointy · 7th May 2009, 11:41 AM

try a few out at the shop with a meter and you should find something that sounds good in the end ................(they never put on schemos
the exact valves only the part number. so there is always some trial and error.)

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
hitachi capacitors? feedback please	jarthel	Parts	0	15th June 2007 12:05 AM
feedback on panasonic pureism and x-pro capacitors	jarthel	Parts	1	3rd May 2007 11:18 AM
how important are feedback capacitors to prevent oscillation in active crossover?	mazurek	Parts	8	22nd January 2006 07:19 PM
'Local Feedback and Nonlinearity of Multistage Feedback Amplifiers'	mikeks	Solid State	62	20th July 2005 08:03 AM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read

4th May 2009, 10:10 PM	#11
barretter diyAudio Member Join Date: Apr 2003 Location: West Yorkshire, UK.	The original feedback caps in my STA15 weren't silvered mica. If you look at the circuit diagram you will see pr after the caps' values. That stands for polyester, which the ones in mine were. So if you want the sound of the Sixties, look for some polyesters!

4th May 2009, 10:42 PM	#12
barretter diyAudio Member Join Date: Apr 2003 Location: West Yorkshire, UK.	Seriously though, LCR do a range of 160V axial 2.5% polystyrene caps from 10pF to 3.3nF. You can easily make 267 and 340 by parallelling a couple of caps and 1000pF is available as a value. Rapid sell them.

5th May 2009, 08:23 AM	#13
theoldtrout diyAudio Member Join Date: Aug 2008	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?

5th May 2009, 04:45 PM	#14
bigwill diyAudio Member Join Date: Dec 2004 Location: UK	Oops maybe they have polyester / polystyrene

7th May 2009, 11:41 AM	#15
pointy diyAudio Member Join Date: Mar 2009	try a few out at the shop with a meter and you should find something that sounds good in the end ................(they never put on schemos the exact valves only the part number. so there is always some trial and error.)

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)