[AMRadio] 75TH vs. 75TL (was Class C stuff)

[Mike Dorworth]

values of 5:1 primary to 1/2 secondary is typical. An over powered driver is
required to keep from overloading the driver and changing the wave form when
the grids draw heavy current.Ditto very low resistance in windings. The
ratio is set to just have the required voltage. Mike K4XM
----- Original Message -----
From: John Coleman <jec at pctechref.com>
To: <amradio at mailman.qth.net>
Sent: Thursday, March 21, 2002 11:16 AM
Subject: RE: [AMRadio] 75TH vs. 75TL (was Class C stuff)


> It's been a long time since I used a XFMR to drive modulator grids where
> grid current is to be drawn but as I recall a lot of class B grids were
> driven with a dual 500 ohm secondary windings or something with very low
> impedance, and never several thousand ohms.
>
> I must admit, I'm unsure of what your doing so I may be misinterpreting
> here.
>
> John, WA5BXO
>
> -----Original Message-----
> From: amradio-admin at mailman.qth.net
> [mailto:amradio-admin at mailman.qth.net]On Behalf Of Brett Gazdzinski
> Sent: Thursday, March 21, 2002 6:53 AM
> To: amradio at mailman.qth.net
> Subject: RE: [AMRadio] 75TH vs. 75TL (was Class C stuff)
>
>
> Don,
> Well, in my case, I was cheating.
> Since I had/have so many transmitters, for the big ones,
> I use an ls output transformer to drive the grids.
>
> 8 ohms in, 5000 ohms out, driven by a big power amp.
>
> The power amp is a stereo job, so I could have up to 4
> modulator decks hooked up at the same time, as the amp
> had 4 outputs selected by a switch on the front...
> a, b, and a+b.
>
> I had the ppp 100th, and the 4cx250b mod decks for the pp 813
> rig, and the 30k-1 mod deck hooked up.
>
> Since I sold the 30k-1, I now have the pp 812 rig
> and the 4cx250b mod decks hooked up to the 'a' positions,
> with a line level padder in the 8 ohm amp to correct the
> level....the 4cx250b deck takes no driving power (ab1)...
>
> This way, whatever rig I use, I don't have to adjust anything.
>
> I was not using a lot of power out of the 100th/tl tubes
> in the 30k-1, so did not seem to have any problem in
> driving them with this setup.
>
> The sad part of selling the 30k-1 was the ls output/driver transformer on
> the mod deck!
> I was fortunate in finding 4 of these back in the days
> when they went for $10.00.
>
>
> You are correct as far as the stock 30k-1 goes though,
> you can likely swap the 75th tubes to 100th tubes,
> but I doubt 100tl,s would work well there.
>
> Brett
> N2DTS
>
> >
> > Don't overlook the fact that not only is the bias requirement
> >  different,
> > but the audio voltage swing requirement at the grid is also
> > different.  The
> > TL requires a much higher audio voltage at the grid than the
> > TH does, while
> > the grid current will be less.  In other words, the grid
> > impedance of the
> > low mu version is much higher than that of the high-mu
> > version.  For the
> > same class-B driver stage you will need a different turns
> > ratio on the
> > driver transformer.  The 100TH's probably have a step-down driver
> > transformer.  You will likely need a step-up driver xfmr for the TL's.
> >
> > The audio driver transformer is the weak link in most conventional
> > transformer-driven class-B audio amplifiers.  It has the most
> > severe service
> > requirements and a good one is complex to build and expensive to
> > manufacture.
> >
> > Don K4KYV
>
> _______________________________________________
> AMRadio mailing list
> AMRadio at mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/amradio
>
>
>
> _______________________________________________
> AMRadio mailing list
> AMRadio at mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/amradio
>