[AMRadio] Subj change - HB designs

Geoff/W5OMR ars.w5omr at gmail.com
Mon Jul 30 10:19:13 EDT 2007

Brett gazdzinski wrote:
> Think that circuit is better then a hifi transformer
> (say 2500 ohms center tapped) to 8 ohms?
> 8 ohm side goes to the hifi amp, the 2500 ohm
> side goes to the grids.
> Swamping resistors go across the 2500 ohm side
> (1250 ohms to center tap)
>> http://www.qsl.net/wa5bxo/driver1.html

In this case, there's nothing connected to the low-Z side of the 
transformer.  I only acts as a center-taped High-Z choke.  The audio 
then goes to the bases of a pair of vertical output transistors in 
emitter-follower configuration.  The emitter outputs of the transistors 
then directly drive the grids of the Class B modulator tubes.

The only variation from that circuit, to what I'm using now, is the 6k 
Ohm 1:1 isolation transformer. 

Before, the audio was pulled from the plates, through some .6uF @ 1000v 
capacitors, for DC blocking.
Now, the 1:1 transformer isolates the DC from the secondary, which then 
still acts as a high-Z choke.

Transformerless audio-driving a Class B modulator.  This way, you can 
impose whatever audio you can get from your speech-amp (source) onto the 
grids of the modulator... and, with ~600v peak-to-peak, you could drive 
even a pair of 450TL's with this setup.  It's certainly over-kill for 
pentodes, tetrodes, or smaller triodes like 811's, or 812's.


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