The AM Forum

From what I have read, both here and in the ARRL hadbooks, amplitude modulation is best if accomplished in the plate circuit, not in the grid circuit of the final.  So, the first question - is this a "worth doing" project?  If not, I'll leave well enough alone.

Second - the HX-50 uses a single 6DQ5 final.  Obviously I will need a little more FIRE to make any splash on 75M.  I was considering building an amp with either a couple of 811's or an 813.  Ameritron amps seem to need a bit more drive than this rig can supply.
Any suggestions from the gurus here?

There is a design in, I believe in the ARRL 1981 handbook, that uses a single 833a as a grid driven amplifier. I am currently using it with a rig that uses the 6DQ5 as a final.  I get about 250 watts carrier out on 75m AM by plate modulating the 833a with a pair of 811a's.  Something to consider.  Hope this helps. :)

I drive a 4-811 G-G amplifier with a GRC-9 and a Command Set which are only 20 Watt Class rigs. The key is a tuning network on the amplifier's grid inputs to get a good match. Otherwise (untuned) you need more drive 50-75W.

I think a pair of 811's would drive fine with a PI network on the input and would be a simple amp to build.

The other approach is conventional grid input AB which does not take much drive. You could use anything that way - pair of 813's?  4-1000...

Mike WU2D

Mike, how do you couple the GRC-9 to the grounded grid amp? Link, or do you just hook coax (or twinlead?) from the doublet connector on the grc-9 to the amp input network?

Dontcha mean "amplifier's CATHODE input?" It is a grounded grid amp, right?

Also, you've got some antenna relay switching going on, right, or do you use a separate rx? How are you "keying" the antenna relays? Manually, I assume?

Once I get my GRC-9 running, I wanna stun the local broadcast transmitter guys with the amazing t-17 carbon mic audio. ;)

Thanks,
Jason kf6pqt

Don't forget http://www.angrynine.nl/GN-58.jpg ...

Hi Jason,

First I grounded the bottom terminal to the ground post on the RX. The hot of the coax was attached to the top terminal and the briad to the grounded bottom terminal. I tuned for maximum output. If one was to insert an SWR bridge inline, it would not look pretty - but it worked.

I have not perfected a match, but I have found that putting a 200 pF cap in series with the hot lead right at the terminal greatly increases my drive to the amplifier. I hope to build a "matchit" in a small box.

Yes I meant the cathode on the G-G and the grid on the conventional amplifier. Yes I switch manually which is crude but an RF detector and relay would be easy to build into the matchit box!

Here is a good article that I found:  

http://www.mines.uidaho.edu/~glowbugs/tx/9Tuber.pdf

Mike WU2D

you should build a 4X1 amp first, and strap them with the T17.

If I cant get my hands on another 833a soon, then I'll do an 813 x 2 811s!

Mike, interesting (to me at least) that the built in tuner in the GRC-9 wont match up to the amp. Thanks for the info, have done much reading on that site in the past!

-Jason kf6pqt

My 2 cents worth is to not modify the rig in this way.

The HX-50 is probably screen modulated anyhow.
Since this is a relatively rare rig, and one that is low power, unless it is pretty well near junque box condition, I'd opt to keep it fairly stock. Modify the audio circuit for better audio if needed and think of it as an "exciter" for something larger?



Once you start to build an amp from scratch, ur pretty close to building an AM transmitter too. A number of local AMers here have done very well building a version of the K1JJ transmitter that can be found on this site. It's 2 x 813 modulating 2 x 813s.

Nothing wrong with a linear, but you'll get more "fi-yah" if the finals are modulated. Eitherway is ok fine.

Otoh, why not just buy a basic plate modulated rig? Johnson, Heath, B&W, anything with a pair of 6146 or similar in the output, and mod that? That's generally considered the baseline for AM phone in terms of output power.

And, fyi there are a fair number of great sounding AM rigs on the air on the east coast that are various mod schemes all "low level" (not plate) including modified Yaseu FT-101s, Kenwoody twins, Drake T-4s, and even one SB-401 that I can remember!! These folks are using a linear usually, but the sound is ok fine after the mods.

           _-_-WBear2GCR

I have an HX-50. It is waiting in line for a going through and hi-fi AM mods. It is definately not grid modulated. It uses a balanced modulator and a crapstal filter as the sideband slicer. the factory skizmatic is a little hard to follow the audio path through, but it appears to bypass the crapstal filter in the AM mode. With a little audio tayloring it should quite easily make good Hi-Fi AM.

Upon initial tests, it seems to make around 12-15w of AM carrier. That should be sufficient to drive just about any modern leanyour amplifryer with out any problems.

I should be getting to that project later towards winter. I will post the results of my findings here when I'm done. If I could make an Invader 2000 work well, this thing should be a walk in the park!
                                                                                     The Slab Bacon

So your suggestion is to plate modulate the linear amplifier....and just use the HX-50 to drive it....huh....never thought of it that way.

I think that is a great idea.

By the way....I have an FT-101B and an FT-401B (aka FTdx-401).  They both need some bench time when I have some free time.  The 101 is going to get a digital display from aada.com and I have been working on a scheme to open the ssb filter and use some varicap diodes to allow me to make it a variable bandpass filter.  Under 3Khz on xmit and rx is just too narrow to have good audio on AM.  I don't relish the thought of replacing the cw filter with an AM filter and modifying the mode switch.  I've been told my modification can't be done - just makes me that much more determined to do it.    ;)

The laws of areodynamics say the bumblebee can't fly, but since he doesn't know that, he just flits along and gets on with life.

The HX-50 is in transit from an eBayer that already told me it doesn't work.  Lights up when power is appied but that's about it.  It will be a good companion to my HQ-160.

Thanks for the suggestions.......

I'll tell you a rig that is dirt cheap, screen modulated, and got the ef johnson thang. The Challenger. With a very few mods and a few "fix the cost driven price point" bad spots in the rig, it makes as good a driver for a amp as possible. Some of you might remember when I was using mine years ago. 100 watt on cw, about 15 watt carrier on AM.

No, just use the HX-50 as the low power exciter to drive the amplifier. Dont plate mod anything. It should be a nice exciter as it sits, just fatten up the audio.

Ah Yes, the good old YELLENGER!! ;D ;D

There are all kinds of cools ways to modulate low power stages.

I once tried to SCREEEM! modulate a command set using the original modulator - no go. I never figured out why it would not work right. Let's just say the audio was a bit thin.

W1VZR Pete plate modulates his command set with an MD-7 and it works great.
Good old cathode modulation with a low mu triode works good on low power rigs and it is Hi-Fi. Pete does this with a transistor on the cathode of the 6AQ5 final in his famous Cakepan design.

The HP-606 generator uses a 12B4 to cathode modulate a pair of 12BY7's in push pull with clever diode feedback on a diff pair. It's good enough for HP.

Mike WU2D

Sounds like a neat lash up for a PW rig. The 12B4 also makes a good driver for Class B modulators because of its low plate resistance.

my 1939 Stancor 110 C was a pair of 6f6's moddin a single 812. I wonder why more dont build a Cathode Modded rig. The transformers are dirt cheap, nobody wants em. 100 watts will mod a 4X1 at full strap.

Re:


A few years back I used a 606A to check into our Sunday morning AM net on 1815.  Amazing what 50 or 100 mw can do when the band is quiet. The 606A and B have the ability to modulate up to 100% unlike many other signal generators, and the modulation quality is quite good.  The only drawback I note in the 606A is a bit of FM warble when under modulation which gets worse as the carrier frequency goes up.  The 606B has an extra buffer stage that serves to eliminate this issue.

Mark K9TR

I don't know how many times in years past I pondered plate modulating various SSB xmtrs, (and have heard guys talking about plate modulating linears, etc.) There are 2 main reasons this is doomed to failure
1) Power supply capacity and duty cycle
2) Tank circuit--If you use an external modulator to modulate any SSB xmtr at 100% in the CW full output condition, the plate tuning cap, bandswitch,  blocking cap, and various other tank components will promptly zorch
forget about it
Skip

Yah Right, I was going to try and externally plate modulate B+ on a Heath Chippewa amplifier, pair 4-400's. However, I realized that the plate tuning cap would never stand off the peaks. That was such a nicely built big amplifier, before Heath realized they could build the Warrior for a lot less and hams would still buy that.