AF125 versus GT322B - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

Oldelectronics · 20th Aug 2017, 5:00 pm

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

flyingtech55 · 26th Jan 2018, 9:33 pm

According to my 1977 ECA transistor comparison book, the AF200 is a valid equivalent to the AF115-AF117. Anyone tried them?

TimR

Herald1360 · 26th Jan 2018, 10:53 pm

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

John_BS · 27th Jan 2018, 6:03 pm

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

Radio Wrangler · 27th Jan 2018, 10:08 pm

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

20th Aug 2017, 5:00 pm	#21
Oldelectronics Heptode Join Date: Jan 2016 Location: Nottinghamshire, UK. Posts: 676	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
flyingtech55 Octode Join Date: Nov 2005 Location: Ayrshire, UK. Posts: 1,096	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 __________________ All those moments will be lost in time, like tears in rain.

26th Jan 2018, 10:53 pm	#23
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	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 __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

27th Jan 2018, 10:08 pm	#25
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,858	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 __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done