|[AMRadio] Subj change - HB designs|
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)
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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