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High power Germanium power amplifier (any thoughts??)
High power Germanium power amplifier (any thoughts??)
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Old 30th June 2020, 08:37 PM   #1
CaterpilarSK is offline CaterpilarSK  Slovakia
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Default High power Germanium power amplifier (any thoughts??)

I dont think anyone has been in my situation more of because these very high power germanium transistors are kind of a rarity and there is not many of them at all..

I got a hold of a factory matched pair TESLA 7NU74. TO3 style 50W PNP transistors. Uce 70V with a Rbe of 30 ohms at least thats what the datasheet states (yep thats right none of that low residual current silicon rubbish).

I couldnt find ANY high power germanium amplifier schematics. What I plan to do is salvage a well known silicon based amplifier schematic and adapt the power output section for the 7NU74s (PNPs) unlike for the bootstrapped NPNs.
More likely I will avoid any feedback.

I will use silicon semiconductors for doing the preamplification and the drive and the bias (transistor diode bias). Ill be using BD139,BD140 to do the driving of the finals (7NU74) and probably do some easy preamplifying stage with a BC547 and a current source so I have stron signal going to the drivers aswell. At least thats what is my plan

If anyone has a actual schematic that would be a high power germanium amplifier (30W and so on) I would appreciate if you would show it to me and let me know your thoughts and ideas on this. I have not come up with a schematic yet, but its on the list of things to do.

Specs of the granny tranny:
-Ucb: 90V
-Uce: 70V (with the 30ohm Rbe)
-Ueb: 15V
-Ic: 15A
-Icbo max: 1000uA
-Ucb: 6V
Hfe: 50-130
Ie: 10A
-Uces max: 1V
Ft: 150kHz

Got a little tired of looking at the transistors in the box so I want to do some adventure stuff with them .

Edit: I have no intentions in using any kind of iron in my build. No transformers beside the power supply.

Last edited by CaterpilarSK; 30th June 2020 at 08:42 PM.
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Old 30th June 2020, 08:58 PM   #2
indianajo is online now indianajo  United States
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Look for a geloso G-253-hf amplificatore stereofonico. Schematic has been on here, no use posting it again. Uses ac128 ac129 as drivers and ac139 quasicomp pnp outputs.
Better use for these is as repair parts for some old person's blown up hifi. They didn't last very long.
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Old 30th June 2020, 09:09 PM   #3
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They work poorly on HF (through-current increases above 5 kHz).
YouTube
YouTube
YouTube
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YouTube
https://www.youtube.com/watch?v=uUiPht_44GU
https://www.youtube.com/watch?v=_CooM4b93fY
https://www.youtube.com/watch?v=I-R2FoSEcuQ
https://www.youtube.com/watch?v=jeziUCw9aeM
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Old 30th June 2020, 09:14 PM   #4
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Initially, the amplifier Leak was on germanium transistors.
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Old 30th June 2020, 09:24 PM   #5
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It really works!
Espacenet – search results

Самодельный УНЧ на германиевых транзисторах
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Old 30th June 2020, 09:49 PM   #6
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Old School
Bias and co. in a 1960s SS amp Hohner Electra Piano

http://bee.mif.pg.gda.pl/ciasteczkow...emier_ss30.pdf
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Old 30th June 2020, 09:58 PM   #7
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Possible scheme. It is required to replace NPN Si with PNP Ge and reduce the voltage bias
Retro Amp 50W Single Supply
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Old 30th June 2020, 10:29 PM   #8
CaterpilarSK is offline CaterpilarSK  Slovakia
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Well wow. Even symmetrically powered ones! Not bad. I'll look into it tomorrow in my crude simulator. Unfortunately I don't have many GE transistors besides this pair of pretty big ones.

I can't really do the preamp and driver out of germaniums and I'll try my best to avoid that. Thank you kind sir.

A note: I'll be likely to use BD139 and BD140 as the drivers. Majority of amplifiers use the "variable diode" technique of biasing the output stage. Germanium transistors as far I know have rather low junction voltage drops unlike the semicoductor counterparts. I wonder if I can still stick to the variable diode or I if I should go with the potentiometer that would be connecting the two bases together of the drivers (BD139,140). I still have the junction drops of the drivers there to deal with so I think it should be okay to bias the thing with the variable diode method (or amplified diode... The transistor with the pot).

Thank you in advance
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Old 30th June 2020, 11:02 PM   #9
indianajo is online now indianajo  United States
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Quote:
Originally Posted by CaterpilarSK View Post
A note: I'll be likely to use BD139 and BD140 as the drivers. Majority of amplifiers use the "variable diode" technique of biasing the output stage. Germanium transistors as far I know have rather low junction voltage drops unlike the semicoductor counterparts. I wonder if I can still stick to the variable diode or I if I should go with the potentiometer that would be connecting the two bases together of the drivers (BD139,140). I still have the junction drops of the drivers there to deal with so I think it should be okay to bias the thing with the variable diode method (or amplified diode... The transistor with the pot).
The geloso G-243 used a 500 ohm resistor between the bases of the germanium drivers. The pull up driver had 68 ohms to centerline, the pull down driver had 10 ohms. The pull down driver had 68 ohms to negative rail.
Class B was popular in germanium power push pull amps. People were still buying radios with 10% HD in those days.
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Last edited by indianajo; 30th June 2020 at 11:06 PM.
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Old 1st July 2020, 12:15 AM   #10
indianajo is online now indianajo  United States
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As is usual, diyaudio can't find it's own data. I'm not burning up my upload limit to post something that is here already. RDS.it says the G243 is a preamp, the associated power amp is the g244.
electrotanya has the g-244-hf schematic, they say. download it yourself. GELOSO G244 HF ST AUDIO AMPLIFIER SCH Service Manual download, schematics, eeprom, repair info for electronics experts
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