[AMRadio] Building

Brett Gazdzinski brett.gazdzinski at verizon.net
Sat Dec 1 21:26:28 EST 2007

I have a push pull paralell 100TH mod deck, and its the worst
sounding mod deck I ever built.
You need a lot of driving power, and into an unstable load.

Now I have run rigs with a pair of 100TH tubes as modulators, and it
sounded fine.
I have tried various voltages, mod transformers, resting currents,
and so on, but the push pull paralell never sounds real clean.
Its not foul, just not clean compaired to other mod decks on the same


----- Original Message ----- 
From: "Geoff/W5OMR" <ars.w5omr at gmail.com>
To: "Discussion of AM Radio in the Amateur Service"
<amradio at mailman.qth.net>
Sent: Saturday, December 01, 2007 8:46 PM
Subject: Re: [AMRadio] Building

> jeremy-ca wrote:
> >
> > Old Handbooks, especially the "Radio" version have extensive charts of
> > the PP load of most tubes used a modulators and you already know that
> > the VT-127A's are basically 100TH's so you can easily determine their
> > load impedance. If you loaf them at 500W input using say 1700V @ 300ma
> > for instance (rounded off) the mod xfmr secondary must be close to
> > 5666 Ohms.
> A fine and fitting job for the RCA 1:1 modulation transformer, with
> 5,500 ohms on primary and secondary.
> > Then knowing the PP load of the modulators and the turns ratio of the
> > xfmr you can determine what works.
> If you want to get 'close enough', then, at 1500v, run 4 100TH's in
> push-pull parallel in the modulator for effectively 4400 ohms,
> plate-to-plate impedance, according to the Eimac tube data sheet.
> According to it, a pair of 100TH's in AB(1) has a plate-to-plate load
> impedance of 8800 ohms.  That's kinda high to work into a 5500 ohm
> transformer.  4400 ohms is -much- closer.
> -- 
> Driving your AM Rig without a scope,
> is like driving your car at night, without headlights. (K4KYV)
> --
> 73 = Best Regards,
> -Geoff/W5OMR
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