[AMRadio] Subj change - HB designs

[Brett gazdzinski]

But is there any advantage over the 8 ohm to grid
setup?

The disadvantage is a lot of complexity, additional power
supplies, transistors that can fail, heat sinks, etc....

A lump of iron is a lot simpler...

Brett
N2DTS 

> -----Original Message-----
> From: amradio-bounces at mailman.qth.net 
> [mailto:amradio-bounces at mailman.qth.net] On Behalf Of Geoff/W5OMR
> Sent: Monday, July 30, 2007 10:19 AM
> To: Discussion of AM Radio in the Amateur Service
> Subject: Re: [AMRadio] Subj change - HB designs
> 
> 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.
> 
> --
> 73
> W5OMR
> 
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