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Old 4th Feb 2005, 7:33 pm   #1
PaulR
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Default Transistor phono pre-amp problems

I have recently (with lots of help) been restoring a valve tuner amp. The transistor phono pre amp doesnt seem to be working as well as it should as there is a lot of hiss when the volume is turned up and it faintly receives radio stations!

I wonder whether the old transistors could do with replacing to start with. Do they tend to become "hissy" with age? They are shown on the data sheet as 2SB 381 2A and 2SB 381 2B. I have found references to them on the 'net with the 2A and 2B parts of the designation missing, but they seem to be obsolete and not obtainable now.

Firstly, should I try to replace them and secondly if so what should I use?

Thanks

Paul
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Old 4th Feb 2005, 8:09 pm   #2
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Default Re: New Transistors?

This fairly obscure equivalents list suggests a 2SB3812 is a PNP germanium AF transistor and actually gives an OC81D as an equivalent! I suppose you could sub AC126s if you have them, or even OC71s.

http://www.geocities.com/waklobe/lowmed.htm

(the A and B suffixes are probably gain groupings).

These sorts of faults aren't uncommon with old phono preamps and can be caused by capacitor or resistor problems as well as bad trannies. The phono preamp design is probably pretty grotty anyway, so it may be better to build a new phono preamp using a dual op amp and fit it internally:

http://www.geocities.com/rjm003.geo/.../diy_pho5.html

(The power supply design is totally OTT - all you need is to derive +- 5-15V using a resistor divider, stabilized with a couple of zeners if you're really fussy )

HTH, Paul

Last edited by paulsherwin; 4th Feb 2005 at 8:16 pm.
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Old 4th Feb 2005, 9:06 pm   #3
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Default Re: New Transistors?

Paul, your choice of replacement is likely to be determined by what you can get. You can't buy germanium transistors from any of the usual sources like Farnell AFAIK. If the equivalents list referenced above is to be believed, the 2N104 is an equivalent.

An AC126 is likely to be the 'gold standard' in this application - it was Mullard's low noise small signal Ge transistor.

Best regards, Paul

Last edited by Darren-UK; 28th May 2007 at 10:15 pm. Reason: ebay refs removed.
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Old 4th Feb 2005, 10:51 pm   #4
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Default Re: New Transistors?

Hi PaulR


Like Paulsherwin says these transistors are probably not too critical. The AC126 is probably your best bet. These should still be fairly readily available from various sources on the 'net. The AC125 also mentioned is, if I remember correctly, a low-noise transistor designed for pre-amps. I seem to remember that the gain was not too good and the AC126 was the higher gain version but with higher noise. In practice it probably won't make a scrap of difference.


Rich
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Old 4th Feb 2005, 11:39 pm   #5
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Default Re: New Transistors?

Thanks all,

Re AC125s. I hope the gain will be sufficient as it is poor at the moment - maybe due to the existing transistors being on their last legs. Anyway I will fit them if I get them and see what happens. I will also make a list of the other components and have a trip to CPC in due course.

Thanks for the offer Paul. If I don't get the AC125s I will certainly take you up on it.

I can see why having things like this professionally restored is so expensive. I daren't add up what I have spent on it and of course being stereo everything is doubled!

Thanks again

Paul

Last edited by Darren-UK; 28th May 2007 at 10:19 pm. Reason: edited in line with other posts
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Old 5th Feb 2005, 12:38 am   #6
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Default Re: New Transistors?

Quote:
Originally Posted by PaulR
Re AC125s. I hope the gain will be sufficient as it is poor at the moment - maybe due to the existing transistors being on their last legs. Anyway I will fit them if I get them and see what happens. I will also make a list of the other components and have a trip to CPC in due course.
I wouldn't be too concerned about hfe numbers. A typical phono preamp design will use 2 transistors with lots of negative feedback, so quite large differences in current gain won't have much of an impact. If the existing preamp gain is low, that's probably part of the design for whatever reason. If I were you I'd wait for your transistors to arrive before changing other components, especially if you have to order them, though check obvious things like voltages and resistor values.

Best regards, Paul

Last edited by Darren-UK; 28th May 2007 at 10:18 pm. Reason: edited in line with other posts.
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Old 7th Feb 2005, 7:34 pm   #7
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Default Re: New Transistors?

I have found some AC125s at Pete Roberts's site for 67p each so I have ordered some from there.

Now for a really basic question: how do I know which lead is which? The originals are in a plastic case which has a small pip at one side. Will this be the collector, and will the new ones be similarly marked?

Finally, does anyone have any tips on removing the old ones and soldering in the new ones? This is the first time I have got more modern than B9A valves.

Thank you

Paul

Last edited by Darren-UK; 28th May 2007 at 10:21 pm. Reason: edited in line with other posts
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Old 7th Feb 2005, 8:31 pm   #8
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Default Re: New Transistors?

Most transistors from the germanium era have a collector-base-emitter arrangement. The collector will be marked with a coloured spot, or the emitter will be marked by a small lug on the case. The simplest way of confirming this is to stick one in a transistor tester. Even the cheapest DMMs sold on market stalls for a fiver will do basic transistor tests now. Just experiment until you get a reasonable hfe reading (20-500). You can also work out the connections with a standard ohmmeter but it's a lot more fiddly.

If you're lucky the c-b-e connections will be etched on the PCB. If not, the emitter will probably be grounded (think cathodes) but of course these are PNP devices so the ground will be +ve.

Germanium transistors are relatively heat sensitive so you should avoid applying excess heat. An improvised heatsink on the wire being soldered is a good idea - a pair of fine nosed pliers held together with a rubber band will be fine.

HTH, Paul

Last edited by paulsherwin; 7th Feb 2005 at 8:34 pm.
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Old 10th Feb 2005, 8:02 pm   #9
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Default

The transistors have arrived and I have replaced three of them (out of 4). The reason that I havent replaced the 4th is that the channel where I have replaced both is now noisier than the other - emitting a rustling sound at full volume. The channel where I have replaced only one is much the same. I am fairly sure I have connected the leads the right way round as E B C was printed on the circuit board and I was very careful about the amount of heat I supplied.

I noticed that the transistors do seem different as one on each channel has Sony B381 HS2 printed in white on the top and the other has Sony B381 KO2 printed in red on it.

Does this difference have any significance?

( posts merged )

I put back the red topped resistor on the noisy side and it was much improved so I assume there is some difference. Unfortunately a 6.5k resistor broke as I was putting it back into the case so I cant try it out properly until I get another. Ho hum another trip to CPC.

Paul
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Old 11th Feb 2005, 8:51 pm   #10
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Default Re: New Transistors?

Paul, I've been assuming your problems with noise on the phono stage occur with a deck plugged into it. If the phono input is unloaded, you may get very high noise levels because the input impedence is typically quite high. You can simulate a cartridge load by wiring a 47k resistor across the input socket.

HTH, Paul
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Old 12th Feb 2005, 9:42 am   #11
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Default

Thanks Paul,

I probably mislead you there. I never actually removed the amplifier circuit boards I just removed the sliding cover and unscrewed them. The cartridge was connected all the time. I will report what happens when I get the new resistor, but using the external phono amp is becoming very tempting.

( posts merged )

The new resistor has restored the pre amp to life and removed the extra noise from that channel. It remains somewhat noisy at full volume, but better than before fitting the new transistors - maybe it was always like this.

Paul
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