Forums

Simple audio amp design

Started by Cursitor Doom December 19, 2021
Greetings, gentlemen,

I have a vintage MW/LW/SW 12V battery-powered radio I picked up recently 
at a yard sale. Being 50 years old, however, it has a few faults and 
needs realignment and whatnot. Before I can proceed with that, I've 
discovered that the transistors it uses (mostly obsolete germanium 
devices) suffer from tin whiskers so I will need to re-jig the board so 
they can be replaced with silicon ones instead. The design is very 
simple so it *shouldn't* be a big deal. The bias requirements are of 
course different between germaniums and silicons so perhaps it would be 
better to start from scratch? I want to remain faithful to the original, 
simple design ethos with its five discrete transistors whatever I do. 
Any suggestions?

Here's the schematic:

https://disk.yandex.com/i/a2xX6_IadjbgIw

CD.
On a sunny day (Sun, 19 Dec 2021 12:40:16 +0000) it happened Cursitor Doom
<cd@notformail.com> wrote in <spn97h$rou$1@dont-email.me>:

>Greetings, gentlemen, > >I have a vintage MW/LW/SW 12V battery-powered radio I picked up recently >at a yard sale. Being 50 years old, however, it has a few faults and >needs realignment and whatnot. Before I can proceed with that, I've >discovered that the transistors it uses (mostly obsolete germanium >devices) suffer from tin whiskers so I will need to re-jig the board so >they can be replaced with silicon ones instead. The design is very >simple so it *shouldn't* be a big deal. The bias requirements are of >course different between germaniums and silicons so perhaps it would be >better to start from scratch? I want to remain faithful to the original, >simple design ethos with its five discrete transistors whatever I do. >Any suggestions? > >Here's the schematic: > >https://disk.yandex.com/i/a2xX6_IadjbgIw
That is a classical audio amp. Apart from I have never had a transistor fail due to 'tin whiskers', if you replace ALL of those transistors with Si then T2 R7/R8 will also change the bias to the output stage to 2 x Vbe, then 2 x .7V. It should simply work. But maybe you should increase R3 by 2 or more times or so to get enough bias for T3. Or simply replace it with a LM380 or LM386! Better even: Buy a nice Tecsun PL600 all band radio, but... I bought one in 2014 for 60$, now I see them for 266$ on ebay! Or the TECSUN PL660, perhaps, about 100$ on ebay. Seems the PL600 is no longer made, and the 660 is the new model. Do not hang on too much to old stuff. (that said I hang on to my old analog scope).
On Sun, 19 Dec 2021 13:05:36 GMT, Jan Panteltje
<pNaOnStPeAlMtje@yahoo.com> wrote:

>On a sunny day (Sun, 19 Dec 2021 12:40:16 +0000) it happened Cursitor Doom ><cd@notformail.com> wrote in <spn97h$rou$1@dont-email.me>: > >>Greetings, gentlemen, >> >>I have a vintage MW/LW/SW 12V battery-powered radio I picked up recently >>at a yard sale. Being 50 years old, however, it has a few faults and >>needs realignment and whatnot. Before I can proceed with that, I've >>discovered that the transistors it uses (mostly obsolete germanium >>devices) suffer from tin whiskers so I will need to re-jig the board so >>they can be replaced with silicon ones instead. The design is very >>simple so it *shouldn't* be a big deal. The bias requirements are of >>course different between germaniums and silicons so perhaps it would be >>better to start from scratch? I want to remain faithful to the original, >>simple design ethos with its five discrete transistors whatever I do. >>Any suggestions? >> >>Here's the schematic: >> >>https://disk.yandex.com/i/a2xX6_IadjbgIw > >That is a classical audio amp. >Apart from I have never had a transistor fail due to 'tin whiskers',
Really? The AF117 type in particular (ubiquitous in British broadcast recievers of the late 1960s) was *notorious* for shorting internally due to whiskers.
>if you replace ALL of those transistors with Si >then T2 R7/R8 will also change the bias to the output stage to 2 x Vbe, then 2 x .7V. >It should simply work. >But maybe you should increase R3 by 2 or more times or so to get enough bias for T3. > >Or simply replace it with a LM380 or LM386!
No! No! NO!! No ICs thanks very much! I may even try potting the new silicon replacements into hollowed- out casings of the Ge devices to keep it original looking.
> >Better even: Buy a nice Tecsun PL600 all band radio, >but... I bought one in 2014 for 60$, now I see them for 266$ on ebay! > >Or the TECSUN PL660, perhaps, about 100$ on ebay. >Seems the PL600 is no longer made, and the 660 is the new model. > >Do not hang on too much to old stuff.
I specialise in old obsolete stuff. I prefer it!
>(that said I hang on to my old analog scope).
I have about 13 of 'em.
On a sunny day (Sun, 19 Dec 2021 13:21:46 +0000) it happened Cursitor Doom
<cd@notformail.com> wrote in <63curgddjaitboe4872itb788d5i9n1fdg@4ax.com>:

>On Sun, 19 Dec 2021 13:05:36 GMT, Jan Panteltje ><pNaOnStPeAlMtje@yahoo.com> wrote: > >>On a sunny day (Sun, 19 Dec 2021 12:40:16 +0000) it happened Cursitor Doom >><cd@notformail.com> wrote in <spn97h$rou$1@dont-email.me>: >> >>>Greetings, gentlemen, >>> >>>I have a vintage MW/LW/SW 12V battery-powered radio I picked up recently >>>at a yard sale. Being 50 years old, however, it has a few faults and >>>needs realignment and whatnot. Before I can proceed with that, I've >>>discovered that the transistors it uses (mostly obsolete germanium >>>devices) suffer from tin whiskers so I will need to re-jig the board so >>>they can be replaced with silicon ones instead. The design is very >>>simple so it *shouldn't* be a big deal. The bias requirements are of >>>course different between germaniums and silicons so perhaps it would be >>>better to start from scratch? I want to remain faithful to the original, >>>simple design ethos with its five discrete transistors whatever I do. >>>Any suggestions? >>> >>>Here's the schematic: >>> >>>https://disk.yandex.com/i/a2xX6_IadjbgIw >> >>That is a classical audio amp. >>Apart from I have never had a transistor fail due to 'tin whiskers', > >Really? The AF117 type in particular (ubiquitous in British broadcast >recievers of the late 1960s) was *notorious* for shorting internally >due to whiskers. > >>if you replace ALL of those transistors with Si >>then T2 R7/R8 will also change the bias to the output stage to 2 x Vbe, then 2 x .7V. >>It should simply work. >>But maybe you should increase R3 by 2 or more times or so to get enough bias for T3. >> >>Or simply replace it with a LM380 or LM386! > >No! No! NO!! No ICs thanks very much! I may even try potting the new >silicon replacements into hollowed- out casings of the Ge devices to >keep it original looking. > >> >>Better even: Buy a nice Tecsun PL600 all band radio, >>but... I bought one in 2014 for 60$, now I see them for 266$ on ebay! >> >>Or the TECSUN PL660, perhaps, about 100$ on ebay. >>Seems the PL600 is no longer made, and the 660 is the new model. >> >>Do not hang on too much to old stuff. > >I specialise in old obsolete stuff. I prefer it! > >>(that said I hang on to my old analog scope). > >I have about 13 of 'em.
I was so happy when I bought my first PLL radio, just type in the frequency! I have always used scopes for fault finding, even worked at Tek. build my own ones at home. But as collectors item? No way! Things are moving so fast, a good Rigol will get you value for money. And I do not have space for all those boat anchors anyways,
On Sun, 19 Dec 2021 12:40:16 +0000, Cursitor Doom <cd@notformail.com>
wrote:

>Greetings, gentlemen, > >I have a vintage MW/LW/SW 12V battery-powered radio I picked up recently >at a yard sale. Being 50 years old, however, it has a few faults and >needs realignment and whatnot. Before I can proceed with that, I've >discovered that the transistors it uses (mostly obsolete germanium >devices) suffer from tin whiskers so I will need to re-jig the board so >they can be replaced with silicon ones instead. The design is very >simple so it *shouldn't* be a big deal. The bias requirements are of >course different between germaniums and silicons so perhaps it would be >better to start from scratch? I want to remain faithful to the original, >simple design ethos with its five discrete transistors whatever I do. >Any suggestions? > >Here's the schematic: > >https://disk.yandex.com/i/a2xX6_IadjbgIw > >CD.
Why are some people so adverse to drawing a simple straight 4-wire connection? It's not as if dots slide off the page or anything. -- I yam what I yam - Popeye
On Sun, 19 Dec 2021 15:07:33 GMT, Jan Panteltje
<pNaOnStPeAlMtje@yahoo.com> wrote:

>On a sunny day (Sun, 19 Dec 2021 13:21:46 +0000) it happened Cursitor Doom ><cd@notformail.com> wrote in <63curgddjaitboe4872itb788d5i9n1fdg@4ax.com>: > >>On Sun, 19 Dec 2021 13:05:36 GMT, Jan Panteltje >><pNaOnStPeAlMtje@yahoo.com> wrote: >> >>>On a sunny day (Sun, 19 Dec 2021 12:40:16 +0000) it happened Cursitor Doom >>><cd@notformail.com> wrote in <spn97h$rou$1@dont-email.me>: >>> >>>>Greetings, gentlemen, >>>> >>>>I have a vintage MW/LW/SW 12V battery-powered radio I picked up recently >>>>at a yard sale. Being 50 years old, however, it has a few faults and >>>>needs realignment and whatnot. Before I can proceed with that, I've >>>>discovered that the transistors it uses (mostly obsolete germanium >>>>devices) suffer from tin whiskers so I will need to re-jig the board so >>>>they can be replaced with silicon ones instead. The design is very >>>>simple so it *shouldn't* be a big deal. The bias requirements are of >>>>course different between germaniums and silicons so perhaps it would be >>>>better to start from scratch? I want to remain faithful to the original, >>>>simple design ethos with its five discrete transistors whatever I do. >>>>Any suggestions? >>>> >>>>Here's the schematic: >>>> >>>>https://disk.yandex.com/i/a2xX6_IadjbgIw >>> >>>That is a classical audio amp. >>>Apart from I have never had a transistor fail due to 'tin whiskers', >> >>Really? The AF117 type in particular (ubiquitous in British broadcast >>recievers of the late 1960s) was *notorious* for shorting internally >>due to whiskers. >> >>>if you replace ALL of those transistors with Si >>>then T2 R7/R8 will also change the bias to the output stage to 2 x Vbe, then 2 x .7V. >>>It should simply work. >>>But maybe you should increase R3 by 2 or more times or so to get enough bias for T3. >>> >>>Or simply replace it with a LM380 or LM386! >> >>No! No! NO!! No ICs thanks very much! I may even try potting the new >>silicon replacements into hollowed- out casings of the Ge devices to >>keep it original looking. >> >>> >>>Better even: Buy a nice Tecsun PL600 all band radio, >>>but... I bought one in 2014 for 60$, now I see them for 266$ on ebay! >>> >>>Or the TECSUN PL660, perhaps, about 100$ on ebay. >>>Seems the PL600 is no longer made, and the 660 is the new model. >>> >>>Do not hang on too much to old stuff. >> >>I specialise in old obsolete stuff. I prefer it! >> >>>(that said I hang on to my old analog scope). >> >>I have about 13 of 'em. > >I was so happy when I bought my first PLL radio, >just type in the frequency!
Stability-wise they are of course far superior. However, if you want to have a good, thorough snoop around the bands for what's on, then analogue tuning still rules IMO. No serious SWL should be without a tuning knob!
>I have always used scopes for fault finding, even worked at Tek. >build my own ones at home. >But as collectors item? No way! >Things are moving so fast, a good Rigol will get you value for money. >And I do not have space for all those boat anchors anyways,
Then make it!
On Sun, 19 Dec 2021 08:17:12 -0800, jlarkin@highlandsniptechnology.com
wrote:

>On Sun, 19 Dec 2021 12:40:16 +0000, Cursitor Doom <cd@notformail.com> >wrote: > >>Greetings, gentlemen, >> >>I have a vintage MW/LW/SW 12V battery-powered radio I picked up recently >>at a yard sale. Being 50 years old, however, it has a few faults and >>needs realignment and whatnot. Before I can proceed with that, I've >>discovered that the transistors it uses (mostly obsolete germanium >>devices) suffer from tin whiskers so I will need to re-jig the board so >>they can be replaced with silicon ones instead. The design is very >>simple so it *shouldn't* be a big deal. The bias requirements are of >>course different between germaniums and silicons so perhaps it would be >>better to start from scratch? I want to remain faithful to the original, >>simple design ethos with its five discrete transistors whatever I do. >>Any suggestions? >> >>Here's the schematic: >> >>https://disk.yandex.com/i/a2xX6_IadjbgIw >> >>CD. > >Why are some people so adverse to drawing a simple straight 4-wire >connection? It's not as if dots slide off the page or anything.
I know! Stupid things like that really hinder my understanding of a circuit's function as I'm focusing on that rather than anything else. So anyways. Are you going to redesign this damn thing for me or what? ;-)
On Sun, 19 Dec 2021 17:32:52 +0000, Cursitor Doom <cd@notformail.com>
wrote:

>On Sun, 19 Dec 2021 08:17:12 -0800, jlarkin@highlandsniptechnology.com >wrote: > >>On Sun, 19 Dec 2021 12:40:16 +0000, Cursitor Doom <cd@notformail.com> >>wrote: >> >>>Greetings, gentlemen, >>> >>>I have a vintage MW/LW/SW 12V battery-powered radio I picked up recently >>>at a yard sale. Being 50 years old, however, it has a few faults and >>>needs realignment and whatnot. Before I can proceed with that, I've >>>discovered that the transistors it uses (mostly obsolete germanium >>>devices) suffer from tin whiskers so I will need to re-jig the board so >>>they can be replaced with silicon ones instead. The design is very >>>simple so it *shouldn't* be a big deal. The bias requirements are of >>>course different between germaniums and silicons so perhaps it would be >>>better to start from scratch? I want to remain faithful to the original, >>>simple design ethos with its five discrete transistors whatever I do. >>>Any suggestions? >>> >>>Here's the schematic: >>> >>>https://disk.yandex.com/i/a2xX6_IadjbgIw >>> >>>CD. >> >>Why are some people so adverse to drawing a simple straight 4-wire >>connection? It's not as if dots slide off the page or anything. > >I know! Stupid things like that really hinder my understanding of a >circuit's function as I'm focusing on that rather than anything else. > >So anyways. Are you going to redesign this damn thing for me or what? >;-)
I dislike audio. Why not buy some germanium transistors from Ebay? Or a decent modern amp? -- I yam what I yam - Popeye
On Sun, 19 Dec 2021 09:44:17 -0800, jlarkin@highlandsniptechnology.com
wrote:

>I dislike audio. Why not buy some germanium transistors from Ebay?
No point. The only parts available are NOS which will have whiskers growing in them anyway, so they'll fail eventually guaranteed.
>Or a decent modern amp?
Naw. Gotta be discretes to keep it original. Fortunately, this is a hybrid radio which uses both Si and Ge trannies, so I can get away with replacing the Ge with Si devices whilst remainiing faithful to the original concept. Of course I could stick an audio IC in there, but it would be a cop-out and the kind of solution only a cad, a bounder or possibly even a blackguard would consider. So get your slide rule out. :-)
In article <hisurgda6flk6vrr51fiehes54s0cpe6g8@4ax.com>, 
cd@notformail.com says...
> > Naw. Gotta be discretes to keep it original. Fortunately, this is a > hybrid radio which uses both Si and Ge trannies, so I can get away > with replacing the Ge with Si devices whilst remainiing faithful to > the original concept. Of course I could stick an audio IC in there, > but it would be a cop-out and the kind of solution only a cad, a > bounder or possibly even a blackguard would consider. > So get your slide rule out. :-) > > >
Have you tried this place, they list germanium transistors but at a steep for some of them. https://ntepartsdirect.com/