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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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20th Aug 2017, 5:00 pm | #21 |
Heptode
Join Date: Jan 2016
Location: Nottinghamshire, UK.
Posts: 676
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Re: AF125 versus GT322B
Ref#14 and OC169's, I tried 4 of these despite reservations about price in my none working (just a faint hiss) Hacker LW/MW Herald and they, despite being a tad expensive, are worth it even coming with each one tested and marked up with its gain etc. The set is now fine and as this set is a keeper, worth the money .Being a bit paranoid I tend to order 5 when I need 4 "just in case"! So out of interest can you mix and match the AF117 equivalents or is it best to stick to one type on a board?
Kev |
26th Jan 2018, 9:33 pm | #22 |
Octode
Join Date: Nov 2005
Location: Ayrshire, UK.
Posts: 1,096
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Re: AF125 versus GT322B
According to my 1977 ECA transistor comparison book, the AF200 is a valid equivalent to the AF115-AF117. Anyone tried them?
TimR
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26th Jan 2018, 10:53 pm | #23 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,535
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Re: AF125 versus GT322B
According to Alltransistors : https://alltransistors.com/transisto...ansistor=21786
AF200 only has Ft of 10MHz so it'll work in a MW/LW set and maybe as a VHF IF (10.7MHz) but it's unlikely to work as an FM front end device. Radiomuseum has it as a TV IF device which suggests around 35MHz but unhelpfully doesn't quote Ft. So, a bit of a maybe, really. Edit: The AF200U has Ft 100MHz to add to the uncertainty: https://alltransistors.com/transisto...ansistor=21787
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27th Jan 2018, 6:03 pm | #24 |
Octode
Join Date: Dec 2008
Location: Wincanton, Somerset, UK.
Posts: 1,782
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Re: AF125 versus GT322B
I have some AF200's and they appear to have more gain than an AF117 at 10.7MHz.
I'm sure I've posted this before! John |
27th Jan 2018, 10:08 pm | #25 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,896
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Re: AF125 versus GT322B
Ft is the point on the gain/frequency plot where the current gain has fallen to unity, so a transistor with, say, 20MHz Ft will only have a gain of 2 at 20MHz, which could be somewhat disappointing. Because an FM IF strip is supposed to be a limiter, you do want several times the operating frequency as the minimum Ft.
There are oodles of types of silicon transistors around which can do these tasks with ease, with high reliability and with a total lack of tin whiskers. The drawback is that the bias-setting resistors may need fiddling with. The only cases where silicon substitution isn't feasible is in circuits running on very low battery voltages. Often, though, people will pay a fair bit of money to avoid doing a few calculations. David
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