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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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13th Jul 2017, 12:21 pm | #1 |
Heptode
Join Date: Jan 2016
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AF125 versus GT322B
I recently bought a couple of Hacker Heralds A VHF set in red and a MW/LW set in black (I have run out of room for the 50s valve sets I like!)I was totally amazed by the VHF sets performance. The MW/LW set just hisses a bit!I can see that the screen lead on at least one AF117 has been cut so I think the problem is with the AF117s? I have some AF125s and some Russian GT322Bs. Does anyone have a recommendation as to which is the best replacement for this job Any other hints would also be much appreciated.
Regards Kev |
13th Jul 2017, 12:47 pm | #2 |
Dekatron
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Re: AF125 versus GT322B
The AF125 is a direct equivalent in a different package. The GT322B looks slightly different on paper but works fine in practice. Each has a different pinout from the AF117 and each other.
https://www.vintage-radio.net/forum/...5&postcount=36 The MW/LW set will likely take any old RF/IF Ge transistor or even Si but the VHF one may be a bit more fussy.
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13th Jul 2017, 1:04 pm | #3 |
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Re: AF125 versus GT322B
Chris has said it all.
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13th Jul 2017, 1:44 pm | #4 |
Dekatron
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Re: AF125 versus GT322B
Luckily, the only Ge in the VHF Herald is in the audio amplifier.
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13th Jul 2017, 2:05 pm | #5 |
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Re: AF125 versus GT322B
The only critical possition I have found is the frequency changer, just about most general purpose RF transistors Ge and Si seem to work in the other possitions.
Sam. |
13th Jul 2017, 4:09 pm | #6 |
Nonode
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Re: AF125 versus GT322B
The AF125 is a direct equivalent in a different package.
I thought the AF127 is the direct equivalent to the AF117
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13th Jul 2017, 4:16 pm | #7 |
Dekatron
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Re: AF125 versus GT322B
They're all the same bit of Ge.
It's possible that they were selected and "binned" according to some aspect of their performance (standard practice in the semiconductor industry), but it's also possible that they were simply labelled up according to what Sales wanted. Certainly, no data sheet I've ever seen lists any material difference between them. |
13th Jul 2017, 7:48 pm | #8 |
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Re: AF125 versus GT322B
The Mullard data books quote different parameters for each transistor to imply that they are optimised for different functions, but a quick look in Towers reveals the truth.
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13th Jul 2017, 8:11 pm | #9 |
Dekatron
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Re: AF125 versus GT322B
Yes, looking in my Mullard 1971/2 data book, the only difference between them is Cob. It's 2.5pF for the AF114 and AF115, 3.5pF for the AF116 and 4pF for the AF117. All other parameters are identical.
Meanwhile, Towers lists 3pF for all of them. I wonder where they got that from? Either way, 2.5pF to 4pF - if we believe Mullard - is a pretty small change in reality. And there will be a tolerance associated with that. So as I say, I've yet to discover a material change. Was this just Marketing, or do any insiders remember anything? |
13th Jul 2017, 8:50 pm | #10 |
Heptode
Join Date: Jan 2016
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Re: AF125 versus GT322B
Thank you for the input guys very much appreciated.
regards Kev |
13th Jul 2017, 10:40 pm | #11 |
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Re: AF125 versus GT322B
The electrical properties of the AF124-7 are fine to replace the AF11X series or OC169 series transistors, which all need replacing because of the tin whiskers...But they are what I regard as a poor physical replacement. Part of the charm of radios of this era, like the Hackers and the Eddystone EC10 was the large cased transistors, and the AF 125's just don't look good and are sometimes difficult to wire in without cumbersome lead wire extensions. They just don't have "That look"
In the past I researched the options and found three types that are suitable (Photo attached) The AF178 is nearly ideal with better specs, the AF185 is good too, but unlike the AF178 it has become more difficult to get. The American 2N2084 is excellent and comes with long lead wires which can be very helpful. The third option is the resin filled Tesla OC169 & OC170, these are the bees knees as the linear pin out array matches the AF115 series. |
13th Jul 2017, 11:03 pm | #12 |
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Re: AF125 versus GT322B
I've not come across the Tesla OC169/OC170, but I'd guess they're less likely to suffer from the 'Tin Whisker' problem due to their being resin filled. Presumably any available are NOS by now, given that they use the OC numbering system, which predates the Pro-Electron (AF) system. Regarding the use of AF12x/178, etc., transistors, I've sometimes mounted them upside down to enable the leads to reach the appropriate holes in a PCB designed for AF11x types, though I agree they can be a pain to fit. The long leads of the GT322B are the main reason, apart from cost, why I prefer to use those nowadays.
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13th Jul 2017, 11:14 pm | #13 |
Heptode
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Re: AF125 versus GT322B
Just a quick look ones that look NOS are about £7 each.
Last edited by Oldelectronics; 13th Jul 2017 at 11:19 pm. |
13th Jul 2017, 11:27 pm | #14 |
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Re: AF125 versus GT322B
I know there are rules for links to ebay addresses etc, but the fact is that this seller in Slovakia, not only has these Tesla transistors but they are all pre-tested before he sends them. Yes the Tesla OC170's come up on uk ebay.
My advice would be stock up on these, while you can, something like this is unlikely to last. They are about 7 GBP, but they are worth it. |
13th Jul 2017, 11:37 pm | #15 |
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Re: AF125 versus GT322B
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13th Jul 2017, 11:48 pm | #16 |
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Re: AF125 versus GT322B
Hmm, £7 a pop is quite a lot to replace an AF11x. These 1960s domestic RF/IF circuits are very tolerant of transistor types, perhaps because there was so much production variation anyway. It's possible to mount a replacement transistor inside an old AF11x can if internal appearance is important, though I do take the point about the leadouts.
The last set of AF11xs I replaced were in a TR82C and I used ex equipment 2N1307s, which have a TO5 case and didn't look out of place. These wouldn't be suitable for an FM set though. |
14th Jul 2017, 1:05 am | #17 |
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Re: AF125 versus GT322B
That is true and I guess if this was for a radio service job where the customer had no interest in the physical appearance of the transistors and pcb, it would be totally out of the question. But for your own prized and loved radios, it then becomes worth it I think.
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14th Jul 2017, 10:33 pm | #18 |
Heptode
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Re: AF125 versus GT322B
Slightly off topic I am cleaning etc a Roberts R200 which - touchwood - sounds great but has the AF117s fitted (Mullard). Is failure inevitable or do some go on and on? While it's working the way it is, my instinct is to leave well alone.
Kev |
15th Jul 2017, 12:29 am | #19 |
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Re: AF125 versus GT322B
Failure is effectively inevitable, but the existing transistors may soldier on for decades. It's really a personal choice to replace working AF11x transistors. The R200 isn't very fussy about transistor types and I would change them, but I have a number of suitable replacements to hand. It's a perfectly valid choice to leave the originals in place.
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15th Jul 2017, 10:40 am | #20 |
Heptode
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Re: AF125 versus GT322B
Thanks Paul.
Kev |