[AMRadio] Legal limit AM amplifier, homebrew

[ne1s at securespeed.us]

> Good afternoon from a newbie to the AM Radio reflector.

Welcome, Rick!

> I'm putting together an AM/CW station consisting of a Heath DX-100B
> and a Hammarlund HQ-129X.  At some point I would like to add a
> legal-limit amplifier to the station.

Excellent.

> I know a lot of people feed their 100-watt-class AM transmitters to
> class B or AB linear amplifiers but it seems like that is terribly
> inefficient... seems like a legal limit Class C amp with its own
> high-level modulator would be a lot more efficient, and that's what
> I'd like to consider building.

Great idea. I have several stations here, and have done it both ways. Both
methods work, although high-level plate modulation is less finicky to
adjust, as others have mentioned in the replies.

-snip -

> I want this to be completely homebrew (I don't want to use some
> high-power hi-fi stereo amplifier or guitar amp for the audio stage),
> and to the extent practical I'd like it to be something that a ham
> might have built in 1947, the year of my birth (the Hammarlund
> HQ-129X is also from that year).  The ultimate goal is that the whole
> station be typical of that year... I know the DX-100 is newer than
> that but it's what I have right now and can be replaced later with
> something older.
>

You can use the RF section of the DX-100 to drive a class-C RFPA, and the
modulator section as a speech amp/driver for push-pull class B modulators.
The secondary of the modulation transformer in the DX-100 appears to be
center-tapped and brought out to the accessory socket for that purpose,
i.e. to drive class-B modulator grids directly.

> So... what do you all think I'll need for components for all of
> this?  Did they make 4-400's and 4-1000's in that year?  I don't
> think so... so what tube(s) should I be considering for the RF final
> amp and the modulator output stage?
>

I see Eimac advertising the 4-250A in the July 1947 issue of QST, and may
have been available earlier. It's a tough call whether the 4-400 or 4-1000
were available in 1947 - but I'm fairly sure if they weren't available
then, they were soon thereafter.

I don't think you could go wrong with an 813 (or a pair) modulated by
811s. Still plenty of good tubes around for not a lot of money. (You'll
need to cut the RF output from the DX-100 way back - the best way to do
this, I think, is to run the DX-100 PA stage at a lower plate-supply
voltage - maybe use the LV B+ supply (if it can supply the added load), or
an adjustable external supply.)
Or, if you want to build a class "C" push-pull amp, use push-pull 812s or
811s there too, with "criss-cross" neutralization. As Bernie said, this
will probably mean plug-in coils in the grid and plate circuits. B&W,
National, Bud all made suitable coils, and they can be frequently found at
NEARFEST (I assume the 1 call means you are in New England? I'm in Maine).
Also, BC-610 coils work well in the plate tank of such an amplifier.

> Any ideas where I can find a suitable power transformer and
> modulation transformer for this?
>

This stuff is around - get on with your DX-100/HQ-129 station and ask
around. Several AM operators hoard this stuff so that it will be available
when someone who needs and will use it comes along.

> Also, keeping it to something a ham might have built in 1947 means no
> solid state rectifiers in the power supply.  I really don't know much
> worth knowing about the tube-type rectifiers of the day for that
> power level... any suggestions?

A pair of 866A MV rectifiers in a full-wave configuration are the classic
solution. I'm using a pair in a common supply for the class C RFPA and
class B modulators in a 300W output rig and they have served me well in
that service. You may want to use separate supplies or larger MV tubes if
you run more power than that. Check the tube specs for your situation.

73 and GL,
-Larry/NE1S