WHY do these 4 transistors work good in parallel?

[gary350]

Who remembers the DS-501 germanium power amp transistors that was in the 1960s car radios. Datasheet says 15 watts each. I was going to guess 8 ot 10 watts. I still have 1 of these. I remember building a PA system with 1 transistor, mic, battery, speaker, 1 resistor, 1 capacitor. Most car radios had 1 transistor but some radios had 2.

Delco DS-501 exact replacement NTE 105

https://www.nteinc.com/specs/100to199/pdf/nte105.pdf

 

[unclejed613]

after a bunch of searching through old data books, it seems the maker of that part is STC (Silicon Transistor Corp) which is still around (and still in the USA). i kind of doubt they might still have data sheets from the 1970s, but it's possible. it looks like they supply a lot of mil-spec components. their web site is [here]. you can't hurt anything by asking.

 

[gary350]

after a bunch of searching through old data books, it seems the maker of that part is STC (Silicon Transistor Corp) which is still around (and still in the USA). i kind of doubt they might still have data sheets from the 1970s, but it's possible. it looks like they supply a lot of mil-spec components. their web site is [here]. you can't hurt anything by asking.

I did some searching too and found 12 replacements with 12 different numbers. Datasheet for 2N174 replacement says 55 watt audio amplifier amp emitter current 15a continious.

DS-501 = NTE105, 2N174, 2N2491, ID35785, 2N441, ID35785, ECG105, ELM105, 2N3315, 2N2081, HEP231, GE-4. I have only looked at datasheet for a few of these.

I have found Transistors for sale the box says the single transistor is a 55W audio amplifier. The transistor is rated 40v 15a.

The 1960s Corvette car radio has 2 of these transistors.

I once saw a car radio with 4 of these on it.

Some Ebay sellers want $100 to $200 each for a DS-501 I don't think anyone will pay that. LOL

This thread is completely off track but it has gotten me interested in building an audio power amp using a 555 timer to make a 3 KHz HORN for my truck. This has to be louder than the tiny tweeter horn I have. I have a locomotive train horn for my truck with air compressor and tank it too large it needs a 11 gallon tank for 1 good loud 5 second honk. It takes the air compressor 5 minutes to honk again. The train horn is rated 150 dB I'm not sure audio amp horn can do that.

 

[audioguru]

High power in a speaker is produced by having lots of current.
Lots of current is produced by having a high voltage and/or a low impedance speaker (2 ohms).
60 years ago an audio output transformer boosted the output voltage but today a high power amplifier uses a DC to DC boost converter.

 

[gary350]

This could be a fun project if it works. I found this online circuit I have no clue if it really works.

 

[audioguru]

Your DS-501 transistor is PNP that needs a -12V supply. It won't work because your truck has a +12V supply.
The speaker has DC in it that moves its coil to one side instead of back and forth then it works very poorly.
The power is low because 12V/8 ohms makes a current of 1.5A for only half the waveform. Then the power= 12V x 0.5 x 1.5A x 0.5= 4.5W.

 

[unclejed613]

so... if those are replacements for the STA devices, that tells you the STA devices are germanium. so, it seems that commercially, paralleled germanium transistors were a thing, although except for circuit compilations and the occasional amplifier service manual where parallel devices were used, i haven't found any mention of the practice in books like [TI's 1963 transistor design manual]. with silicon, the "current hogging" effect is well known, but it seems that it wasn't as much as a problem when the devices were germanium. i would think it would be the opposite, because transistor characteristics, even within the same lot/date code were all over the place compared to the relatively uniform characteristics that can be had with modern silicon processes.

 

[ronsimpson]

it seems that it wasn't as much as a problem when the devices were germanium

I can't find a good data sheet on germanium transistors or diodes. I thought I would look at Vsat verses temperature but it is not listed. I can't even fine Temperature Coefficient for germanium diodes. I would have to test one. I remember doing this in the 70s and found with a die temp of 70C it all goes to hell.

 

[Nigel Goodwin]

but it seems that it wasn't as much as a problem when the devices were germanium. i would think it would be the opposite .

There's no evidence to suggest that at all, and plenty to suggest that germanium devices were even less thermally stable than silicon.

As Ron mentioned, germanium devices DIE at fairly low temperatures.

 

[ronsimpson]

germanium devices DIE at fairly low temperatures

When I went to high school, I tough the electronics classes. This was a way to get out of "auto repair class". The shop teacher would come by some days to see if things were good. The old text book said we had to connect a heat sink to the leg of a transistor before soldering. I tough that was not necessary. Teacher and I had words in front of every one. I connected a transistor tester to a 2N2222 and put a soldering iron on the metal case. Noting happened. The leakage went up a little, but it was a good part at temperature. (surprise) Then I took a old Germanium transistor and did the same. The gain went to zero and the leakage went up. The teacher made a big speech about the care of transistor and the need for heatsinking. I removed the heat and the gain slowly went back up while the leakage went down. Teacher came by much less often, after that.

 

[unclejed613]

the Vbe temperature coefficient for germanium is less than that of silicon, for germanium it's -1.8mV/degC, while for silicon it's -2.2mV/degC. it's not the change in Vbe that is destructive, but the changes in Icbo (C-B leakage) and the maximum operating temperature. germanium leakage current measured at room temperature is in the milliamp range, but in the nanoamp range for silicon. leakage current doubles for every 9degC increase in temperature. the maximum operating temperature of silicon is 150-200 degC, but for germanium it's 70-80 degC. so, while the change in Vbe is the thermal runaway mechanism for silicon transistors, for germanium, the hazard is from C-B leakage current. as far as parallel Ge transistors, and why it seemed to work, i'm still looking, but so far, it's conspicuous by it's absence in the texbooks and design guides. current hogging seems to be mentioned in reference to silicon transistors, but not germanium. if i were to make a guess why, it would be that germanium transistors also exhibited current hogging, but the leakage current problem was much larger in magnitude, and the current hogging problem went unnoticed until silicon came along and current hogging was more visible.

one thing i found interesting is that germanium is in use again as a semiconductor, but this time mixed with silicon. it seems that the mixture of silicon is much less noisy than either material by themselves, and SiGe devices are being used for very low noise UHF and microwave amplifiers.

 

[Nigel Goodwin]

the Vbe temperature coefficient for germanium is less than that of silicon, for germanium it's -1.8mV/degC, while for silicon it's -2.2mV/degC. it's not the change in Vbe that is destructive, but the changes in Icbo (C-B leakage) and the maximum operating temperature. germanium leakage current measured at room temperature is in the milliamp range, but in the nanoamp range for silicon. leakage current doubles for every 9degC increase in temperature. the maximum operating temperature of silicon is 150-200 degC, but for germanium it's 70-80 degC. so, while the change in Vbe is the thermal runaway mechanism for silicon transistors, for germanium, the hazard is from C-B leakage current. as far as parallel Ge transistors, and why it seemed to work, i'm still looking, but so far, it's conspicuous by it's absence in the texbooks and design guides. current hogging seems to be mentioned in reference to silicon transistors, but not germanium. if i were to make a guess why, it would be that germanium transistors also exhibited current hogging, but the leakage current problem was much larger in magnitude, and the current hogging problem went unnoticed until silicon came along and current hogging was more visible.

You're seriously overthinking this - and where did you even get the idea that Germanium transistors don't require current sharing resistors?.

one thing i found interesting is that germanium is in use again as a semiconductor, but this time mixed with silicon. it seems that the mixture of silicon is much less noisy than either material by themselves, and SiGe devices are being used for very low noise UHF and microwave amplifiers.

I didn't think it had ever gone away?, it's always been used for specific niche applications.

 

[ronsimpson]

WHY do these 4 transistors work good in parallel?

Maybe they don't. Just because some one found 4 transistors in the junk pile does not indicate they worked good. I have a huge pile of prototypes that .....

 

[throbscottle]

I think the STA7018 is the same as the 2SC1413A (according to this page: https://www.ceitron.com/semi/ceisanken.html )
It has quite a high VCEsat 10v@5A so maybe that's why the paralleling works.

 

[unclejed613]

I think the STA7018 is the same as the 2SC1413A (according to this page: https://www.ceitron.com/semi/ceisanken.html )
It has quite a high VCEsat 10v@5A so maybe that's why the paralleling works.

those weren't made by Sanken, but by STC (Silicon Transistor Corp) that's the logo on the transistors. i figured out kind of quickly that whatever search engine had linked STA7018 to Sanken, and was not correct. the logo on the devices in TO-3 cans on the heat sink have an STC logo, not a Sanken logo (Sanken transistors are marked with a SK connected by a top line and bottom line, and their "long form" logo is SanKen with the S and K connected as in the short logo, and the rest of the letters in between the top and bottom lines, like this:

while continuing to look for info about germanium output stages, i ran into a blog where this "audiophile" was saying that germanium transistors were superior to silicon for single-ended class A audio power amplifiers, and that all the published information including the data sheets for germanium transistors have been somehow altered to make silicon transistors look better. he mentioned the "high mobility of electrons" in germanium as being a desirable characteristic in audio power amp outputs (he's talking about the C-B leakage current which is the primary cause of failure for germanium devices).

 

[throbscottle]

I tried looking for STC first but could only find a page about the now defunct UK company "Standard Telephones and Cables Ltd". The only other STC's I could find don't make transistors. I hadn't realised that host.web-print-design.com/stc was them. Pretty low key site. Catalogue is here but I can't read the file: http://host.web-print-design.com/stc/products.htm it's a .doc and Libre Office doesn't like it.

 

[gary350]

 

[gary350]

What is a real class A amplifier. +10v and -10v or +10v to 0 volts to 8 ohm speaker.

I think it is plus and minus 10v?

I think +10v to 0 might be class C but I remember is college we were told class C sine wave is cut off at 70%.

 

[Nigel Goodwin]

What is a real class A amplifier. +10v and -10v or +10v to 0.

I think it is plus and minus 10v?

Neither, power supply has nothing to do with the class, it's simply a matter of biasing.

 

[audioguru]

A class-A amplifier usually uses a single output transistor that is conducting and heating all the time. Your previous post shows a DS-501 transistor in class-A but instead of using a DC load resistor then a capacitor coupling the AC to the speaker you wrongly use the speaker as the AC and DC load.

A class-C transistor conducts only part of the total time which is never used for audio but is used for RF with a tuned LC to produce the entire voltage swing.

You showed a sinewave with AC having plus and minus 10V peaks but no DC bias, then you show another sinewave with a +5V DC bias and a swing from 0V to +10V.