[Amps] HF power amp input impedance

[Steve Thompson]

> I have an older solid state amp based on a pair of MRF 454s. The amp is 
> missing three common design features found in most similar HF power 
> amps, a bias circuit for the base, negative feedback, and a phasing 
> transformer on the DC input.
> 
> 1. What is the normal bias set for this type of amp. Setting bias to the 
> current turn on point about .62 V results in a clean signal between 
> about 14 MHz and 30 MHz.
> 
Whether it's bipolars of FETs, set the bias point by monitoring 
the current drawn and adjusting the bias voltage to get the 
current you want. Applying some nominal bias voltage to devices 
when you don't know the characteristics risks damaging them.

If the amp appears overdriven, have you tried reducing the drive? 
Gain rises significantly at lower frequencies, exploring the 
Motorola app notes will cover the use of feedback to level it.

Steve