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Germanium PNP transistor based amplifier (OC36)
Germanium PNP transistor based amplifier (OC36)
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Old 14th December 2014, 02:53 PM   #1
TheAmmoniacal is offline TheAmmoniacal  Norway
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Default Germanium PNP transistor based amplifier (OC36)

I've just recently been interested in building some amplifiers, which I've done with modern transistors just fine. But I have this box of much older transistors, all in TO-3 packages, and was curious if I could use of these in a project.

Among them is a handful of OC36 germanium transistors, I've seen some 'vintage' schematics use the OC71 for example. The most interesting is that the OC36 is very similar to the 2N278 in specifications, and the 2N278 is a power transistor made for class A and AB stage amplifiers.

Anyone know if the OC36 could be used in an audio amplifier? Anyone know of schematics based on germanium PNP transistors?

Something along the lines of
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Old 14th December 2014, 07:22 PM   #2
sbrads is offline sbrads  United Kingdom
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When I was 13 in the 60's I made my 1st amp, a 4 watt class A valve kit and it sounded nice considering the cheap speakers and record decks in common use then. I then started learning about semiconductors which was just about all PNP germanium stuff then and I made an amp using a pair of (similar to OC36) OC35's. It did not sound good or go anywhere near as loud as the 4 watt valve amp despite being higher wattage, 10 watt I believe. That experience got me very interested in Hi-Fi and led to a long path of upgrades before I was happy with the sound quality. So, build one for interest's sake but don't expect much, and they blow up really easily! That AD140 class A circuit looks a bit weird and wastes a load of power in R22, I wouldn't use that.

Last edited by sbrads; 14th December 2014 at 07:31 PM.
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Old 14th December 2014, 07:52 PM   #3
TheAmmoniacal is offline TheAmmoniacal  Norway
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Yeah, I don't expect any high quality audio from it, it's just for the learning part of it. Another schematic I've found using the 2N278 is this one: http://www.radiomuseum.org/images/tu...278_umgeb1.png

I can't find a larger picture of that unfortunately.
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Old 14th December 2014, 08:13 PM   #4
Elvee is offline Elvee  Belgium
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If you search in my previous messages, you'll find large numbers of vintage/Ge amplifiers examples I have posted.
Some of them did sound very good actually.

That said, I agree with sbrads on the inefficiency issues of your circuit.
Even if you constrain yourself to PNP's throughout, there are better solutions.
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Old 16th December 2014, 12:43 PM   #5
tubewade is offline tubewade  United States
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I used to build lots of PNP Ge amplifiers as experiments because they were cheap and easy to find. They are not tolerant of any overvoltage situation and I got the best result from them using autotransformer coupled outputs. Some of them could make decent power from low B+ voltage and sounded pretty good as booster amplifiers for portable radios, etc. Fun, fun tinkering on the cheap.

I still have a few amplifiers around with Ge output devices. One of them is a Sylvania Exponent 4/40 portable stereo phono system. It sounds very good for a portable system from 1966.

Kind regards.
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Old 26th January 2019, 12:18 PM   #6
Phaddy is offline Phaddy
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Back in 1970, I had the first Philips Solid State stereo recorder. It used Philips Germanium PNP transistors, 10 Watts per channel. Thru external Rola 8 inch C8MX Speakers, it was quality sound, impressed quite a few listeners.

There were also hybrid car radios, using a single 2N301 or equivalent Germanium PNP output transistor, which ran a little warm. The rest of the radio had valves (tubes). The radios sounded OK.

Just have to get the biassing correct for the transistor, not too little, not too much. Get it wrong and it clips the dips, or arrests the crests.
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Old 11th February 2019, 05:44 AM   #7
Ian Greenhalgh is offline Ian Greenhalgh  United Kingdom
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I've been given a bag full of unused old transistors, most of them are Germaniums. The most interesting to my untrained eye are half a dozen Motorola OC35 in TO-3 metal cases.

All I have been able to learn about them is that they are a PNP power transistor.

Can I build a simple class A amp using them, just a low power device, such as a headphone amp?
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Old 11th February 2019, 06:30 AM   #8
Phaddy is offline Phaddy
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The 0C35 has low current amplification. Not ideal for your application.
If you have 0C70, 0C71, 0C72, 0C45, AF117, AF118, AF127, AF128 etc, better choice.
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Old 11th February 2019, 08:44 AM   #9
JMFahey is offline JMFahey  Argentina
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Originally Posted by Phaddy View Post
The 0C35 has low current amplification. Not ideal for your application.
If you have 0C70, 0C71, 0C72, 0C45, AF117, AF118, AF127, AF128 etc, better choice.
It is a *power* transistor, it will have less Hfe than a *preamp* type one ..... different functions
That´s not the main parameter here, but voltage/current/power handling.

And it won´t be used "alone" but as part of a Power Amplifier, which will at least use 2 transistors.

Search for old Philips/Siemens/Telefunken manuals from the 60´s, you´ll find some design examples using them.

Mind you, if single ended Class A you will need output transformers; a few designs might be some kind of OTL (output transformer less).

Or you might build a variation of the old and famous John Linsley Hood 10W Class A amplifier but with 2 simple but important modifications:

Click the image to open in full size.

a) invert polarity of all transistors.
This will let you use your PNP Germanium there.
Also invert any electrolytics.

b) invert power supply polarity.

given the headphone use, I would build the nominally "3 ohm" version suggested above, simply because it uses the lowest supply voltage: 17V DC which is well within ratings.

No, you won´t get 10W RMS out of it into regular headphones ... nor you need it unless you want to blow them and get deaf in the process

The actual power out into headphones will be more than enough anyway.

Being an excellent design with a small but devoted cult following, I bet you will find lots of info around ... maybe even some PCB design ... maybe even ready made boards.
Such as: http://www.angelfire.com/sd/paulkemble/sound3b.html

Just remember to invert all polarities ... that of electrolytic caps also.

Oh, also remember to heatsink them well.
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Last edited by JMFahey; 11th February 2019 at 08:47 AM.
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Old 11th February 2019, 09:04 AM   #10
jan.didden is offline jan.didden  Europe
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In my early life I have used OC35 and OC29 TO3 power transistors. Memories!
Make sure you have enough drive, the Hfe can easily drop below 10 for these devices. I think the min spec was 5....

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