Simple audio amp design

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

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).

Really? The AF117 type in particular (ubiquitous in British broadcast recievers of the late 1960s) was *notorious* for shorting internally due to whiskers.

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.

I specialise in old obsolete stuff. I prefer it!

I have about 13 of 'em.

On a sunny day (Sun, 19 Dec 2021 13:21:46 +0000) it happened Cursitor Doom snipped-for-privacy@notformail.com wrote in snipped-for-privacy@4ax.com:

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,

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.

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!

Then make it!

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?

No point. The only parts available are NOS which will have whiskers growing in them anyway, so they'll fail eventually guaranteed.

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.

You'll need to replace the horrible germanium biasing with horrible silicon biasing.

On a sunny day (Sun, 19 Dec 2021 17:30:55 +0000) it happened Cursitor Doom snipped-for-privacy@notformail.com wrote in snipped-for-privacy@4ax.com:

Of course, my Tecsun radio has SSB, PLL, a BFO knob and a tuning knob. Very good allband radio. But my rtl-sdr stick with the xpsa spectrum analyzer software I wrote can use the mouse wheel for tuning to 1 Hz accuracy. Just specify jump size 1:

Amusing what autistics like JL find annoying......

...... Phil

snipped-for-privacy@highlandsniptechnology.com wrote: ====================================

The THD of that tiny amp is 1% at most, ffs it's in a portable radio.

..... Phil

If it's really 1971 vintage, that should work fine, with perhaps a bit of attention to the biasing.

Back in the really old days, germaniums were sometimes leaky enough that their apparent DC betas _changed sign_.

When I interviewed at Tektronix Beaverton back in 1987, one of their crusty old vertical amp wizards (might have been Thor Hallen) gave me a circuit to analyze that made no sense to me whatever--not at DC, not at AC--its biasing was impossible, and I said so.

Turned out to have been an astable that only worked when the DC beta was negative, but didn't much care how negative--it was guaranteed to start up regardless.

Cheers

Phil Hobbs

Germanium doesn't generally have tin-whisker problems (those are particular to a case design, not the transistor inside); it has additional failure modes and aging issues, but so does any technology.

Be aware, though, that Si replacements for Ge transistors does change all the bias currents, I've seen items burned up because of a naiive replace-with-modern-Si strategy.

Just cap couple into the base, no bias needed.

(Electret mikes did that with jfets. I did that in my alligator detector.)

Cheap British dots do fall off the paper. We use macho USA dots.

LM wrote: ========

Next step up was to use AD161/162 output devices. Germanium compliments in TO37 paks.

..... Phil