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Bell transistors for phono stage..

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dr.partha

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Hi friends!
Can anybody please tell me whether AC128/176 or AC187/188 are still available (NOS )? I've substituted these bell transistors by modern variety. But I'm not getting the exact sound which I'm habituated to listen from my Philips 533.I don't know what is the exact problem but I'm getting a sound having almost no bass and high treble! Please help me to solve the problem.
Regards,
 
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IndianEars

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The AC series of Transistors were Germanium Transistors. I have no idea if they are still available as retail in India. They are available ( at Absurd Prices ) on the net.

All Germanium transistors have a switch on threshold of 0.3 Volts compared to modern day Silicon transistors which have a threshold of 0.7 Volts. Hence in most circuits where there is minimal emitter degeneration ( small emitter resistor, in plain speak) a Silikon Transistor cannot be substituted for a Germanium.

What are the exact substitutes that you havce used ?
Also can you share the circuit diagram ?

Anyway, your symptoms for Low Bass and excess treble are either due to :

1. Incorrect RIAA Equalisation. Have you bypassed the RIAA Equalisation stage by mistake ?

2. The Philips 533 uses ( I think) a Ceramic cartridge, which MUST have a High Input impedance stage ( 1 Meg Ohms, preferred, and 500K Ohms absolute minimum) immediately after it.

If the input impedance is not high enough, you will get exactly the symptoms that you have mentioned.

If you are comfortable, I would suggest that you build and substitute a High Impedance Ceramic Phono stage ( ideally with jFET input) for your 533.
 

reubensm

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Hi friends!
Can anybody please tell me whether AC128/176 or AC187/188 are still available (NOS )? I've substituted these bell transistors by modern variety. But I'm not getting the exact sound which I'm habituated to listen from my Philips 533.I don't know what is the exact problem but I'm getting a sound having almost no bass and high treble! Please help me to solve the problem.
Regards,
The title of your thread says "Bell Transistors for Phonostage" and you've mentioned AC128/176 and AC187/188 which are transistors used in output stages. For phonostage, generally one would find transistors like AC125 at the front end, and AC126 at the rear end. If you have used power transistors, you are definitely on the wrong track.

AC127/AC128 and AC187/AC188 pairs are usually hard to find these days but older electronics spares shops will surely have them as they were the most common types used in radios and tape recorders in the 1970s and 1980s (before the advent ICs for regular use in India). I am not sure you will find AC 125 or AC 126 transistors unless you salvage them or their corresponding OC series transistors from old transistor radios. I think the Mullard equivalent was OC71.

In the past, I do recollect replacing these transistors in audio stages of old transistor radios with BC 556 or equivalents as these are PNP types as well. I had made some minor mods to the power rail and bias settings while making these substitutions. They used to work well. With something like a transistor radio, audiophile quality is not expected anyways :)

 
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dr.partha

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I'm sorry for my mistake in the thread heading. Yes, these were not used in phono stage. I've found AC187/188 pair but those don't have any heat shink and their body is not at all like bell transistors. Will this affect sound quality?
And I didn't change any other parts yet the sound is lacking bass. I've just replaced old pair (bell) by new pair. I'm trying to post a diagram asap.
Thanks for your kind reply.
Regards,
 

IndianEars

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A different encapsulation should make no difference to the sound.

Most probably the Heatsink is a slip on, like a cap. You can take it off the old transistors and slip it on the new ones, if it fits...
 

reubensm

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I'm sorry for my mistake in the thread heading. Yes, these were not used in phono stage. I've found AC187/188 pair but those don't have any heat shink and their body is not at all like bell transistors. Will this affect sound quality?
And I didn't change any other parts yet the sound is lacking bass. I've just replaced old pair (bell) by new pair. I'm trying to post a diagram asap.
Thanks for your kind reply.
Regards,
From my experience, AC187/188 can be used as a direct replacement for AC127/128. The AC187/188 pair are much better transistors and I've noticed that they sound better. My very first amplifier which I built as a kid was a AC127/128 design but I used AC187/188 for it. The reason was as I could take my power supply from 9 volts to 15 volts and get more power out of it. You can bravely settle for the 187/188 pair.

Regarding the heatsink, these transistors came in 2 varieties, with a built-on heatsink and without heatsink.



You can easily buy these little slip-on heatsinks in electronic spare shops or you can even make your very own. Take a thickish metal plate (no rocket science and lengthy forum discussion required, to decide on the material, its simple, aluminium works well), fold one end of it to encapsulate the transistor completely (use heatsink cement for good thermal conduct between the transistor and your heatsink) and you can bolt the other end of the metal plate to a larger metal surface or a larger heatsink. A lot of ingenuity can be used here.

Here's a picture of my very first little amplifier which I built in 1981. The output transistors are AC187/AC188 and slip-on heatsinks are used. I have bolted the thermistor to the heatsink as well. I used some ingenuity to ensure that stress was not put by the weight of the heatsinks, on the transistor legs.

 
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dr.partha

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Update:
Recently I met a guy who was claiming to have sound knowledge in restoring old tts and record players. When I told him the problem in details he advised me to put a. 047pf capacitor parallel to the base-emitter Junction of AC 187.I did it and surprisingly the problem was solved!! But I couldn't understand why this happened!
I've almost forgotten Physics (as I only deal with medicines) and so I am not able to find any logic of it.
 

IndianEars

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The Capacitor has Bypassed (lowered the amplification for ) the mid and high frequencies.

Hence bring up the (relative) level of Low Frequencies.

Hence you now have more bass.
 
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