Homebrew AM Rig

[K9ACT]

I can't find the thread that started this so I am starting a new one.

After spending some more quality time with my 811 rig, I have come to the conclusion that the 811A might just be the ideal tube to base a medium power AM rig around.

Many people have asked me why I use an 811 for RF and the knee jerk answer was that it was left over from my 8000 project.  In retrospect, there are a few other very good reasons:

1.  As a triode, it needs no screen supply
2.  As a zero bias tube, it needs no bias supply
3.  It is readily available and if also used in the modulator, one spare covers all
4.  It will operate on 800 to 1500 VDC
5.  Puts out 100W all day or 200W with two in parallel
6.  Requires only 5 W RF to drive and about 10W audio

I built mine from left over junk, wood, Masonite and PVC.  It’s not pretty but works very well.  I uploaded a new schematic which now includes the modulator and values for the tank circuits for 40 and 80.

Schematic:   http://schmidling.com/811_png.emf

It can be viewed with Paint or if you have TinyCad, I can send a .dsn file.

There are pics at http://schmidling.com/radio.htm

You can drive it with a Ranger or any ricebox for RF and any of the suggested solid state audio amp kits.

Questions and comments welcome,

Jack k9act@schmidling.com

[w8khk]

Hello Jack,

Cool design, simple and straightforward.  One question about the schematic....  The 100k resistor between the power supply and the mod tranny- 100k would seem to be the value for a bleeder, connected from ps to ground rather than in series with the load?  other than that, looks great!

73,
Rick

[W9GT]

Hi Jack,

Yes the 811A (or 812) is ubiquitous.  Great tubes to use for medium power rigs.  The classic design that is most often seen is a pair of 812s modulated by a pair of 811As.  Used by Millen and others.  The RF deck is commonly push-pull to make use of commonly available tank circuit components and, of course, there are some advantages to a push-pull output for harmonic rejection, etc.  Paralleled tubes in a single ended configuration work fine as well, just be careful about those harmonics.  Following these finals with a good antenna tuner is a good idea to get the needed selectivity and harmonic rejection...again ya got to watch how you set up the tuner also.

It seems that the biggest problem is getting mod iron for plate modulators.  It would be fun to experiment with some alternative set-ups using cathode or other forms of modulation to eliminate the mod xfmr.  Anyway...great to see someone building and experimenting....that's what its all about!!!

73,  Jack, W9GT

[Pete, WA2CWA]

I can't find the thread that started this so I am starting a new one.
Jack k9act@schmidling.com

FYI:
Can't seem to find your old posts or someone else's old post: click on the person's call, i.e. K9ACT, go to left side of screen under Profile Info, click on Show Posts.

[K9ACT]

I fixed the bleeder resistor and changed the file format to .png.

It has come to my attention that this is easier for more people than .emf

I would love to do a push pull but the caps and coils get harder to find and it requires two NC's which are also geting hard to find.
 
Mod iron is an issue but somehow I ended up with 3 of them for my 8000 rig so if one works at it, they area available.  It also helps to know K7YOO.

And on old posts, I didn't know the call, forum or topic so it's not that easy. 

js

[N3DRB The Derb]

jack,

do you ever hang out on the cloudy nights astronomy forum? the scope in yer avatar is some serious glass. 

[KB2WIG]

Maybee the cloudy nights are best utilized for the brew reduction-conversion modes??   .... ..   klc

[K9ACT]

jack,

do you ever hang out on the cloudy nights astronomy forum? the scope in yer avatar is some serious glass. 

I used to haunt mainly usenet astro groups but I have been to Cloudy Nights and I think they have some links to my astro pages.

The "serious glass" is now, unfortunately, packed up in a nice box for some aspiring astronomer.  After ten years, I concluded that I used up the sky, at least around here and decommissioned the observatory and sold what could be sold and carefully packed up what could not.

That left not only cloudy nights but all nights to pursue other hobbies and guess what popped into my brain?

js

[N3DRB The Derb]

I started buying 60 mm Japanese scopes for fun a while back. cheap hobby. Some of them are really good within their range.

I hang out over there some. They have some real optical snobs though.

[W9GT]

I would love to do a push pull but the caps and coils get harder to find and it requires two NC's which are also geting hard to find.
 

That stuff shows up on eBay all the time Jack.  Also hamfests are a great place to pick up old coils , neut caps, variables, etc.  There are also a few guys that even restore/rebuild  the old coils as a hobby/business.  You can find the parts fairly easy if you ask around.  Ask on this site and you might be surprised at what turns up.

73, Jack, W9GT

[K9ACT]

That stuff shows up on eBay all the time Jack.  Also hamfests are a great place to pick up old coils , neut caps, variables, etc. 

Well, the last few fests I have been two were very week on big caps and for PP you need dual caps.

However, another advantage of PP that I forgot is that you don't have to have a balanced grid tank.

So, one goes with whatever dual cap is available.

js