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

[John Coleman]

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

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