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I'm in the US and fixing a vintage German integrated amp which has a crackle static issue I've isolated to a transistor in the preamp section. The transistor is specified in the schematic (below) as a BC147A (datasheet specs) and the actual part in the amp appears to be a BC147B (datasheet specs) These are both vintage European parts, not generally available here and now. (I'm trying to avoid navigating international postage during the pandemic on an NOS part.)

I'm not an electrical engineer and frankly don't know a whole lot about transistors outside their basic use in a circuit, but I'm willing to learn. So I'm trying to take the opportunity to understand what guidelines I should follow when trying to source a replacement transistor for a part that I can't source. (I spend a lot of time trying to fix up old audio stuff so this comes up from time to time.)

The very bottom of this web page offers this handy list:

  • Use a device with equal or greater breakdown voltage
  • Use a device with equal or greater operating current
  • Use a device with equal or greater power dissipation
  • Use a device with equal or greater gain bandwidth product
  • Use a device with equal or lower switching time
  • Use a device with equal or lower trigger current
  • Use a device with equal or lower reverse current

I'm trying to understand if this set of guidelines makes sense and how important each of these variables might be (or how to determine given the device how important). I'm a dummy on most of this stuff, so my apologies for looking for something more rote, but I'm happy to learn if anyone wants to point me off somewhere that could help contextualize parts for audio circuits

I had someone advise me that RCA used to do a bunch of "universal" replacement transistors, and that in my case their SK3444 part (datasheet) would be appropriate. It doesn't follow quite all the rules in the list above though, for example the original part seems to have somewhat greater (45V) breakdown voltage collector-to-emitter than the replacement (40V). Not all the above specs seem easily findable for both parts.

Thanks for any insight.

Braun Audio 310 schematic (detail)

BZo
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    I'd like some details about how exactly you "isolated [it] to a transistor in the preamp section." In particular, the one you are indicating. What indications lead you to this conclusion? You write, "[I] frankly don't know a whole lot about transistors outside their basic use." How did you isolate it to this particular BJT? (I'm not doubting you, just curious about your process.) – jonk May 05 '20 at 03:46
  • @jonk Carefully directed component freeze spray, in this case. "Isolate" was a bit of a hand wave-- I'm not (say) using a scope, and obviously there could be nearby issues-- bad solder joints?-- or I could be barking up the wrong tree entirely. But I didn't know much about substituting transistors in this kind of context, and this was an opportunity to find out more (and leave the below trail of useful tips for others, too). – BZo May 05 '20 at 14:55

2 Answers2

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IN low frequency (audio) circuits the most important characteristics generally are:-

  1. Polarity: (NPN or PNP)

  2. Collector-Emitter breakdown voltage: should be at least 20% higher than the supply voltage.

  3. Current gain (hFE): should be similar.

  4. Maximum current: Should be much higher than the operating current.

  5. Maximum power: This is usually similar for the same package size.

The BC147B is NPN, 45 V breakdown, hFE 200 minimum, and max current 200 mA.

The popular 2N3904 is 40 V breakdown, hFE 100-300 at 10 mA, max current 200 mA. These specs are similar enough that it will probably work fine, provided the supply voltage is not over 33 V.

Bruce Abbott
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  • Thanks Bruce. Super helpful. This may be a stupid question but if the breakdown voltage is 40V why should the supply be constrained to 33V? Is that a standard margin of safety? I added the schematic detail showing the transistor marked. It implies to me, a dummy, that the input voltage is +25V. Does that look right to you? Thanks – BZo May 05 '20 at 03:33
  • Margin of safety (to account for supply voltage variations, spikes etc.) and also some characteristics may change close to the breakdown voltage. Without knowing the exact circuit you want a good safety margin. 40V is the **absolute maximum** rating (with Base open), above which "the serviceability of the device may be impaired" - IOW no warranty! Your circuit shows ~19V across the transistor in normal operation. Theoretically it could go as high as 50V, but if it did break down then R535 would limit current so it shouldn't be harmed. – Bruce Abbott May 05 '20 at 07:12
  • Thanks again. Am I right that you're saying ~19V by looking at the difference between the +25 marked above the collector and the +5.8 marked below the emitter? (Re: the 50V max, given that the original spec (BC147A/B) has a breakdown voltage of 45V I guess it's reasonable to assume that the designers of the circuit took that into consideration?) – BZo May 05 '20 at 14:44
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    Yes. The designers must have taken it into consideration, though you have to wonder why the transistor became noisy. Did it get damaged by a voltage spike? This part of the circuit has low ac level so the Collector voltage should normally stay around 25V +- a few volts. – Bruce Abbott May 05 '20 at 19:46
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Switching time doesn't really enter into an application like audio, and trigger current doesn't apply to BJTs at all. Leakage current is seldom important except in specialized applications.

hFE is important (in fact the difference between A and B suffix BC147s is the hFE).

ft can be important, depending on the application, but too high ft can cause worse problems (oscillation) than too low.

A part that is sold as "general purpose", "output amplifier", "switching transistor" etc. may be better suited than one that is aimed at a different set of applications.

Start by seeing if a common jellybean transistor like a 2N4401/4403, C8050/C8550 will work.

Spehro Pefhany
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    Thanks for this Spehro. I wish I could flag this as an additional accepted answer since there's equally good info here. I appreciated your taking the time to respond. – BZo May 05 '20 at 21:48