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Sunday, January 24, 2021

VBE Multiplier Makes KLH Receiver "Cool Running"

Yesterday I turned to the SolderSmoke wizards for advice on how to fix my KLH Model Twenty-one II FM receiver.   I had finally gotten the thing working -- it wasn't the speaker, it was the AF amp, probably one of the final transistors was blown.  I replaced the finals and the driver. For the finals I used a TIP29C and a TIP30C.  For the driver a 2N3906.    With this fix the receiver was sounding good, but the heat sink on the AF amplifiers was way too hot. 

If you look at the comments in yesterday's post, you will see some great suggestions on how to fix this problem.  The comments and Google led me to Alan W2AEW's YouTube channel and his video on a circuit called the VBE multiplier.  Voltage Base-Emitter multiplier.  I'd never used this circuit before.  It allows you to adjust the bias on the bases of the two transistors in a push-pull amplifier.  

This morning I built the circuit on a small piece of PC board.  There were just two components: a 10k trimmer pot and a 2N3904 transistor.  

With the little board installed, I adjusted the pot for a 1.2 volt difference between the bases of Q6 and Q7. I ended up with base voltage values almost identical to those called for in the KLH schematic. 

The receiver sounds very nice now, and is no longer on the verge of bursting into flames.  I even made up my own version of the pillow that KLH claimed was necessary for proper acoustic suspension.  

Sometimes it is nice to be able to listen to something other than the chatter on the ham bands. And it is fun to do so with a receiver that you have worked on.  

I even used some Desitin as a substitute for heat sink compound. 

Thanks to Rogier for the receiver, to ZL2DEX, K0EET, W2AEW and David McNeill for the good advice. And to Dale K9NN who sent me a box of parts from which emerged the 10k pot I used in this project.  Thanks guys. 73  


  1. Yay! So glad to hear that my video helped you out with the Vbe multiplier!

  2. Vbe multipliers have been in use for >50 years. Having a pot with this range will cause unlimited current Q6 an Q7. How about limiting the range to eliminate the smoke.


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