I'm helping a friend resurrect an Ampex 440 studio tape deck, and a problem popped up.

After running nicely for hours, one playback channel suddenly got noisy. The prime suspect is the first transistor in the signal chain, a 2N4104. In today's world, what would be a good sub for this "low noise" transistor? The specs:

NPN, non-Darlington
Small PTH package; TO-18, TO-92, etc.
Vce - 60 V minimum
Ic - 50 mA minimum
Gain - 100 minimum

Circuit voltage is 39 V, but my estimate of peak collector voltage is around 24 V.

What is your favorite go-to part for a low-noise audio application?

Separate from that, any insights into what causes a transistor to suddenly get noisier?

Thanks.

ak

 

What is your favorite go-to part for a low-noise audio application?

Depends on the source impedance.

 

You may also try asking this question on the DIYAudio-dot-com forum.

 

For low noise audio, Zetex E-line medium power transistors are known excellent.
They have the large junction, low intrinsic base reistance Rbb.
I use them in 70's reel-reel machines. ZTX851/951 or the similar parts in the lineup.
ZTX851 NPN and ZTX951 mentioned in the Art of Electronics.
The SIP package is not super strong so careful bending the leads with pliers.

The old transistor might have died due to old age, transients or the noise is bias rocks. It depends where it is in the circuit.

 

You may also try asking this question on the DIYAudio-dot-com forum.

Already did. I'm carpet-bombing forums.

ak

 

For low noise audio, Zetex E-line medium power transistors are known excellent.
They have the large junction, low intrinsic base reistance Rbb.
I use them in 70's reel-reel machines. ZTX851/951 or the similar parts in the lineup.
ZTX851 NPN and ZTX951 mentioned in the Art of Electronics.
The SIP package is not super strong so careful bending the leads with pliers.

The old transistor might have died due to old age, transients or the noise is bias rocks. It depends where it is in the circuit.

Those old Ferranti transistors are excellent for low impedance sources. Not so great for high impedance sources.
Another couple of good ones are 2N4401/2N4403, again good for low impedance sources, because of low base spreading resistance

 

Separate from that, any insights into what causes a transistor to suddenly get noisier?

Did it? or was it a nearby resistor or capacitor?

 

Don't know yet. Yesterday I was able to hear the system for the first time (the diagnostics have been remote-controlled until now), and the noise difference between the two channels is not as great as I expected. What is significant is the bandwidth - the suspect channel's noise has a lot more bottom end. To me, this makes that channel's noise seem louder, but if you mentally filter out the extra low-frequency content, the rest of the noise is comparable in both channels.

ak

 

What type of noise? Hum, popcorn, white, other?

Back in the mid 1970s I repaired my dad’s HH Scott amplifier. One channel had white noise. Fortunately, all the transistors were in sockets, so it was a matter of swapping left and right channel transistors, until the noise itself got swapped. Then I consulted the ECG Sylvania transistor replacement book (remember those?), purchased a replacement transistor and job done.

I took the failed transistor to an university professor who was extremely helpful. He took me to the electronics lab’s holiest of holies area, where they had a Tektronix curve tracer. He manipulated the machine while I watched in awe. He found that in a range of collector voltages/ currents the transistor would break down. His theory was that the silicon die may have had some sort of imperfection.