"Old-style" transistor audio amplifiers.

[dominicbeesley]

If you want to use Germanium and want some consistency, cheapness and availability there are plenty of ex-soviet transistors around. They can be had cheaply from the Ukraine (I've never had any bother with customs charges etc). I quite enjoy finding the datasheets then translating them from the Russian

[ionburn]

Originally Posted by G6Tanuki

One thing that I've thought about over the years is whether it would be possible (or sane) to "super-alpha" the output transistors to get extra gain. Sticking an OC71 on the front of each OC81?

And using another OC71 or similar as a 'bias regulator' .

I think this is where Silicon has the advantage as, with a DC coupled pair, the leakage current of the first transistor would be amplified by the second, which with Germanium is a problem and the temperature stability would not be good.

I think I have seen circuits in which a transistor is used for bias regulation, although have none to hand. I do have some BA156 diodes which were designed for bias regulation although I have had them so long that I do not know their exact use.

[G6Tanuki]

Originally Posted by Argus25

What a terrific idea. I'm equally fond of this type of amplifier too.

One good option for the output transistors, if you did want to stay germanium, is the AC188. At one stage I saw many of these for sale, but I can't recall the supplier.

Another excellent transistor, if you could get some, is the OC74. It out does the OC72.

The Original RCA 2N3053 is a lovely silicon transistor, and also would look great with the type of heatsinks you are planning. The 2N4036 is another TO-5 option.

I think this idea you have is like wanting to build something from "favorite things" (like the brown paper packages tied up with strings analogy)..it is great.

I once decided to build an all germanium radio with all my favorite transistors in it, these included the AF178, which is actually the better replacement for AF11x transistors, an audio amp with an AC188 driver, and two OC16's as the output transistors, you can see them on the hand crafted copper heat sink:

http://worldphaco.com/uploads/THE_AF...OC16_RADIO.pdf

I've got some genuine Siemens/Intermetall AC153 here but they're the type that are in a case that's a bit smaller than the old TO-1 envelope that OC81s and AC128s come in - meaning they won't fit the classic copper-clip-type heatsinks unless I squash them a bit....

I also have a couple of AC107 from the same stable but they're really low-noise preamp transistors rather than finding a natural home as a driver-stage.

I've got no underlying visceral desire to go with Ge devices though, what with all the issues of variable leakage current, gain-matching and thermal runaway they bring. Silicon's a lot easier to tame.

Whatever, I think I'll be able to bash together something that will work as well as if not better than the original Ge designs. I'm not after hifi here after all [hearing damage means I've got major roll-off of response below about 200Hz... I like to describe it as my 'organic hum-filter'].

[Argus25]

Originally Posted by G6Tanuki

I also have a couple of AC107 from the same stable but they're really low-noise preamp transistors rather than finding a natural home as a driver-stage.

AC107's are an interesting transistor, I have experimented with these. There is both a metal and a glass case version. They were originally intended for low noise pre-amps in the front ends of tape recorders in the mid 1960's. The Americans really only had one equivalent of these, the RCA 2N2613. There was a paucity of low noise germanium transistors in that era, from the UK and USA at least, these are the only two. The music industry got on to the notion of these for treble boost amplifiers in electric guitars. The Gretsch company in the USA put the 2N2613 into some mid 1960's guitars, here is a story about these:

http://www.worldphaco.com/uploads/Gretschdoc.pdf

On the other hand in the UK, the transistors used for this treble boost application were the OC44 and OC71. Brian May for example had one with an OC44 and the Rangemaster unit that T.Rex used had an OC44 or OC71. Why the AC107 wasn't selected I don't know, it was either overlooked or too expensive.

On the other hand, Toshiba in Japan were making a range of low noise germanium transistors in that era for Japanese tape recorders. The Japanese excelled in battery operated tape recorders. There were at least six type of low noise transistor. The common ones are the 2SB439 and 2SB440, which were used together in the front ends of many tape recorders and 8 track tape players.

[G6Tanuki]

Fascinating story on the AC107 and equivalents: I've only ever come across them 'in real-life' being used as preamp for a ribbon-microphone.

Wonder if anyone ever tried wiring two in parallel? Multiple paralleled transistors allegedly reduce intrinsic device-generated noise by 1/n where n is the number of paralleled devices.

[TonyDuell]

Originally Posted by Argus25

AC107's are an interesting transistor, I have experimented with these. There is both a metal and a glass case version. They were originally intended for low noise pre-amps in the front ends of tape recorders in the mid 1960's.

I thought I recognised that number. The AC107 was used as the first stage in the otherwise-valved Philips EL3514 'upright' tape recorder. Now I know a little more about them... Thanks!

[vidjoman]

AC107's were used in the 1st Leak transistor amplifier in the front end of the pre-amp.

[DangerMan]

There was a Wireless World article by AR Bailey (November-December '66) which described a (then) pretty decent 20W amplifier using a home made driver transformer and DC coupling to the loudspeaker.
It's available on the Americanradiohistory website.

[Argus25]

Originally Posted by TonyDuell

The AC107 was used as the first stage in the otherwise-valved Philips EL3514 'upright' tape recorder. Now I know a little more about them... Thanks!

Funny you should mention a device which used a transistor in the front end, with valves. I have attached a circuit that nobody on this forum will have ever seen before. This is because it was designed by my brother in 1973 and only deployed in NZ as far as I know and not published elsewhere. It boosts the gain of an audio amplifier input (typically using an EF86, EF37A, or 5879) where the gain was not enough overall for a magnetic phono cartridge. This transistor (npn germanium) was added to the front end of Juke box amplifiers to get the gain up for this application, without having to add another valve stage. I hope it surprises and delights, see attached.

[kalee20]

No I've not seen that circuit, though I have come across the idea of driving the transistor into the cathode of an earthed-grid valve, cascade-style.

Interesting! I like the feedback from anode to base, to define the gain.

I'm a bit worried about the DC conditions, as it is totally dependent on Vbe for the OC139, which is smaller than for silicon hence proportionally more temperature dependent!

[Argus25]

Yes, with the emitter grounded the and the bias sourced from the valve's anode, the negative feedback stabilizes it, despite the temperature dependence of the Vbe, not dissimilar to the scenario you see when a transistor's base is biased from its collector with its emitter grounded. But it would be less thermally stable than a silicon device I agree. It might be better with a small value emitter resistor added, which could be bypassed to AC too. Apparently though it worked fine as it was. The idea behind it is that the valve is driven both at its cathode & grid.

[Ed_Dinning]

Hi Danger Man, yes the Bailey amp was built en-mass by the engineers at Reyrolle on Tyneside. A pretty good performer it was too.

Mine is still about and still functioning. We had the advantage of being able to get Radiometal lams and trifilar wire for the driver transformers.

Ed