2N2907A instead of BC179

soapboxsam · 8th August 2011, 08:55 PM

I want to build a small amplifier which has a BC179 in the schematic. I would like to use a more up to date PNP transistor such as the 2N2907A. Will this transistor sub for the BC179?

john_ellis · 8th August 2011, 10:11 PM

???

2N2907A is about the same age as a BC179.

It may sub for a BC179, but the BC179 is low noise.
It depends on the circuit, but the 2N2907A has similar gains at 1 mA so probably yes.

John

soapboxsam · 9th August 2011, 04:15 PM

Thanks for the quick reply. I guess I should have said a more readily available transistor since I already have plenty of 2N2222A and 2N2907A transistors. When looking on a transistor data sheet, what do I look for to determine the noise factor of a given transistor or FET? Or is there anything?

john_ellis · 9th August 2011, 09:53 PM

Hi
The noise figure is given for low noise transistors but not necessarily for other types. So if it isn't there it isn't. If it is, it will have something like N or Nf and should be specified in dB for a given frequency or range, input impedance and bias current.
More to the point is this transistor in a preamp stage or power amp - if not a preamp, it probably won't matter too much.
John

john curl · 9th August 2011, 10:10 PM

The 2N2907 will work in many circuits, but the 2N4403 is better.

kenpeter · 10th August 2011, 03:00 AM

I was able to find cheap surplus FPQ2907, FPQ2222, and also 3906 in quad DIPs.
In my app I don't care too much about the beta match, only VFWD BE matching.
I'm assuming 3904 must be common as quads too, but I didn't see any...

I have baggies of 2n4403, but only singles. A little too small to clip pairs together
with a TO5 heatsink as the clip. Dunno why they are smaller than normal TO92?

soapboxsam · 11th August 2011, 11:51 AM

Thanks for the replies. Actually, what I am building is a low level preamp to overdrive the front end of a SS guitar amplifier. I posted here because I figured you folks would have more info on SS components. The original circuit uses a J201 FET input and a BC179 on the output. I have a bunch of 2N2907A transistors and was just wondering if it would be a good sub for the BC179 but from further research, it appears the 2N2907A will produce more noise than the BC179 according to folks on some blogs. I found some data on the BC179 which shows it having a typical noise reading of 1.2dB and a maximum of 4dB. I also find a data sheet for the 2N2907A that showed a maximum of 4dB but no minimum or typical. I think I'll just go ahead and order a few BC179 PNP's and build it. If you're interested I have attached the schematic. Also available at:

JFET_preamp_schematic_s.gif picture by Soapboxsam - Photobucket

kenpeter · 12:58 PM

Thats a darn good circuit topology.
Might even be a decent tube emulator for reasons of voltage feedback to source.
I'd be tempted to put a diode drop in series with R5, but probably just fine without.
Recommend reading: OHSchade, RCA Beam power tubes 1939, especially Fig 35.
Observe how Triode curve set is constructed from Pentode curves. Same theory.

Your PNP is located where the component noises you speak of are not a problem.
I'd be far more worried about the guitar's cable.

soapboxsam · 11th August 2011, 03:09 PM

Thanks for the info Ken. I found a listing of 10 BC179s on eBay and bought them. Cheaper than buying just one plus the shipping. I got the J201 FETs in yesterday. I already have a small project board wired so I'm gonna go ahead and install the 2N2907A and see how she works. I'll replace it when the 179s come in to compare. It's small enough to fit into my bass guitar as the pots have an access plate on the back. Another thing I'm thinking about doing is installing a 50K pot in place of the 47K - R9 resistor, so I can have some control over the amount of signal presented to the input of the amplifier.

I am using what I know are very good cables. I built them over 20 years ago. I can't remember who I got the cable from but I do know the cable is Belden low capacitance and the connectors are Switchcraft. They have never failed and have never been noisy.

I'll post here when I get it finished and working. Thanks again to all for your help in clearing up my concerns on this project.

kenpeter · 11th August 2011, 06:51 PM

If thats going inside your bass, no problem.
Can't pickup cable noise where none exists.
Got plenty low output impedance to drive long cables after the fact.

8th August 2011, 08:55 PM	#1
soapboxsam diyAudio Member Join Date: Jan 2009	2N2907A instead of BC179 I want to build a small amplifier which has a BC179 in the schematic. I would like to use a more up to date PNP transistor such as the 2N2907A. Will this transistor sub for the BC179?

11th August 2011, 12:48 PM	#8
kenpeter diyAudio Member Join Date: Nov 2007 Location: Dallas	Thats a darn good circuit topology. Might even be a decent tube emulator for reasons of voltage feedback to source. I'd be tempted to put a diode drop in series with R5, but probably just fine without. Recommend reading: OHSchade, RCA Beam power tubes 1939, especially Fig 35. Observe how Triode curve set is constructed from Pentode curves. Same theory. Your PNP is located where the component noises you speak of are not a problem. I'd be far more worried about the guitar's cable. Last edited by kenpeter; 11th August 2011 at 12:58 PM.

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8th August 2011, 10:11 PM	#2
john_ellis diyAudio Member Join Date: Sep 2006	??? 2N2907A is about the same age as a BC179. It may sub for a BC179, but the BC179 is low noise. It depends on the circuit, but the 2N2907A has similar gains at 1 mA so probably yes. John

9th August 2011, 04:15 PM	#3
soapboxsam diyAudio Member Join Date: Jan 2009	Thanks for the quick reply. I guess I should have said a more readily available transistor since I already have plenty of 2N2222A and 2N2907A transistors. When looking on a transistor data sheet, what do I look for to determine the noise factor of a given transistor or FET? Or is there anything?

9th August 2011, 09:53 PM	#4
john_ellis diyAudio Member Join Date: Sep 2006	Hi The noise figure is given for low noise transistors but not necessarily for other types. So if it isn't there it isn't. If it is, it will have something like N or Nf and should be specified in dB for a given frequency or range, input impedance and bias current. More to the point is this transistor in a preamp stage or power amp - if not a preamp, it probably won't matter too much. John

9th August 2011, 10:10 PM	#5
john curl diyAudio Member Join Date: Jul 2003 Location: berkeley ca	The 2N2907 will work in many circuits, but the 2N4403 is better.

10th August 2011, 03:00 AM	#6
kenpeter diyAudio Member Join Date: Nov 2007 Location: Dallas	I was able to find cheap surplus FPQ2907, FPQ2222, and also 3906 in quad DIPs. In my app I don't care too much about the beta match, only VFWD BE matching. I'm assuming 3904 must be common as quads too, but I didn't see any... I have baggies of 2n4403, but only singles. A little too small to clip pairs together with a TO5 heatsink as the clip. Dunno why they are smaller than normal TO92?

11th August 2011, 03:09 PM	#9
soapboxsam diyAudio Member Join Date: Jan 2009	Thanks for the info Ken. I found a listing of 10 BC179s on eBay and bought them. Cheaper than buying just one plus the shipping. I got the J201 FETs in yesterday. I already have a small project board wired so I'm gonna go ahead and install the 2N2907A and see how she works. I'll replace it when the 179s come in to compare. It's small enough to fit into my bass guitar as the pots have an access plate on the back. Another thing I'm thinking about doing is installing a 50K pot in place of the 47K - R9 resistor, so I can have some control over the amount of signal presented to the input of the amplifier. I am using what I know are very good cables. I built them over 20 years ago. I can't remember who I got the cable from but I do know the cable is Belden low capacitance and the connectors are Switchcraft. They have never failed and have never been noisy. I'll post here when I get it finished and working. Thanks again to all for your help in clearing up my concerns on this project.

11th August 2011, 06:51 PM	#10
kenpeter diyAudio Member Join Date: Nov 2007 Location: Dallas	If thats going inside your bass, no problem. Can't pickup cable noise where none exists. Got plenty low output impedance to drive long cables after the fact.

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