Wednesday, August 7, 2019

vintage vs new components - the good, bad and ugly

My components are group B. With a Beta of 200 - 450 a couple of them should may a high gain amp.

That's a good gain alright with some negative feed back it should be stable. This is with a 2k headphone.

The circuit would drive an earbud too.

Look at the high frequency response up to 30 Mhz.

I tried some different transistors and the HF band is still high gain. Why does this matter?

In the early days of radio we had some component limitations working to our benefit. The transistor high frequency response being 1Mhz or less limited the set to TRF but it simplified the filtering requirements. With all the transistors I've shown so far the frequency response is into the shortwave range. If RF comes into the input it will be amplified and radiated by the earbud leads. When I hear a squeal I can move the earbud leads and "find a station". "Hey I built this cool regenerative receiver." Well not really but you will find this type set being built. In a radio we use filters to keep RF away from the headset to prevent this. This is one reason I like the old Germanium audio transistors. They are low gain and have a low frequency response but they don't pass RF like the later transistors. Let's look at the circuit with a germanium transistor.
It is not a high gain amp but the response drops off below the short wave.

This is my circuit as built. It will drive the earbud with my AF generator set to minimum. It will also squeal with a test lead connected to the input. The question is , "how to keep RF out of it?".


  1. A #43 bead on the collectors. #76 if you are near am AM blowtorch.

  2. Thank you! I have some beads put up if I can remember where up is.

  3. The beads did calm the beast. I made the prototype on a piece of press board. It would be good to put it on a ground plane, a metal box would be even better.