The AM Forum

AM and closet CW fans.

Well I did it again. I got out the soldering iron before looking at the schematic.

Here was my problem. The ART-13 sounds like a hammer when it is keyed in CW mode. It scares XYLs, cats and small children. To fix this, I have been keying my ART-13 on a circuit suggested by the good old CQ Surplus Conversion Handbook. It basically adds grid blocked keying to the final. Now grid block keying an 813 takes some fairly high negative voltage. They suggested -400 VDC. Anyway, the circuit used some caps resistors and a choke to shape the waveform and I used a small relay to do the HV keying.
By the way, the other contacts of the relay key the sidetone oscillator. The regular keying relay (big kerplunk) gets keyed by a timer circuit which keeps it activated all the time you are keying. That had a retriggering effect as you key and it can be set from just under 1 second to 5 seconds for the release.
It keys like a Marauder or any modern radio.

In this way the oscillator is always running as you are keying just the final. This circuit has been running for upwards of 15 years without a hitch.

Here is the problem. Backwave. The 813 is not neutralized and killing just the final does not eliminate the signal completely. I figure it is putting out somewhere around 200 mW key up! This is a little bothersome to the listener.

Neutralization would help and I will try that, but I thought I would try keying the stage ahead of the final, the last 1625 along with the final. This should be good for another 30 dB or so of isolation.

I was able to get in and lift the 100K grid resistor off ground, add a bypass and strap over the keyed -400 VDC no problem. Actually it was a B**** but I did it. It worked fine on 20M and on 40 M, killing the backwave but had no effect on 160M or 80M's backwave problem.

What stupid detail had I missed?

Mike WU2D

Without looking at the skizmatic, the 1625's are part of the osc. multiplyer(S)..   the 1st 1625 is the 2X,3X or 4X freq mult, the 2nd mult.  is a 3X....  which multiplier did you werk on? ... any freq greater than  about 6mc(?) both  the 1st and 2nd are utlized........  hope this helps.....  today a had my plate meter crap out compleetly....  that little 4k meter resistor turns black when the smoke goes byby....   klc

KLC,

Exactly!  I was grid block keying only the second multiplier stage. On lower bands, this stage was not even in the circuit! :(

So my great idea stopped working below the 6 MHz band. I suppose I could pop the lid again and do the same hookup to the first multiplier and see what happens keying all three tubes! I do fear some pulling on the oscillator keying the first multiplier though. There is no buffer and I do not regulate anything. I suppose that I could see how it sounds and If I run into trouble, I could try regulating the screen of the 837 PTO stage.

I suppose in hindsight that I should have tried differential keying the oscillator along with the final and not touched the multipliers. Now that would have been a classic approach. It would require adding a triode and a regulator tube I think.

Mike WU2D

Mike,
 In the June 65 issue of 73, it is suggested that a 50 ~ 100 uF 'litic cap be placed from the test key switch to gnd.. this sounds like a clip lead mod... I havn't tryed it with my ATC's, im just getting these squared away after thay sat in a barn for?? years.....   gud luck  klc

Yup,

That quiets down the relays when you are keying it stock.

Mike WU2D

Pump it up,

For the final trick, I have to call your attention to CQ Magazine from June 1965 where there is a great ART-13 article. High Power.

This is a great surplus issue by the way and it includes such classics as the R-19/R-508 for 2 Meter AM, Converting the RT-19/ARC-4 to 2M AM and the Collins ARR-15/R105 autotuning Receiver.

Anyway, the name of the article is High Power with the ART-13. Now most people run the ART-13 at around 200Watts input power with 28 VDC, 450 VDC medium V and 1500 VDC on the HV.

This author, Roger Smith K4PFK, refers the reader to an article in QST October 1963 which converts the ART-13 to SSB - Oh My! The article most importantly shows how to beef up the tank for higher power operation. Autotune still works as normal.

Rogers idea was to automatically switch the HV from 1500 VDC to 2500 VDC when you go from voice to CW and to add a bit of negative bias on the 813. Oh darn. CW ONLY

Anyway 500 Watts input at 75% efficiency or 380 Watts output was the result on 80M. Don't forget the fan, a simple muffin fan blowing out right in back of the PA cage is a standard for the ART-13, whatever the power level.

Mike WU2D

 I thought the final mod was to add 10 meters.......  If ur interested, ive got the mod,: I havent tried that yet, as I
 Fried parts of BOTH my 13s' and  am going to have to pull the autotune to get to them (cathode resistors in the hf osc) which I dont want to do without  the famous "crank" to re sync the autotune...
 I run my HV with a 1250v. .25 amp  xfermer and one 23uF 5kv  oil can.   the low voltage uses  a 23uF cap also.... ( See the yeasu stick threat....   no mistakes) so the voltages are up there, but within limits....   as far as the real high v opps, the limiting seems to be the 811s; they dont like reel high volt.....    812s??  theres a thought.....  off the top o the head needs more drive than the 811, but lots of power available to mudulate a higer voltaged 813???    hummmm         klc

My solution was to key the ART-13 400 volt supply using a 1600 volt-rated horizontal sweep transistor for a TV set in a series pass configuration to the transmitter. It all fit on a little perf board that I added to the supply.

That kills the multipliers, drive and the screen supply to the 813. 100% silent electronic keying.

You can find the article up in a back issue of ER.

Or, you could just use a relay to key the 400 volt DC supply. Piece of cake. Any relay with contacts rated for 240 VAC will work fine, unless you can work better than 30 WPM...Then we start getting into vacuum relay territory...

Bill

Bill,

High side keying sounds like a winner and it sure is simpler to install than Grid Block. Do you run the VFO 100% of the time during keying? Does it include keying the onboard sidetone circuit? I did a TCS up like that one time but used a 555 for sidetone with a HV transistor keying to cathodes to ground while a relay did the HV on the OSC keeping it on 100% during keying - imagine doing that to an innocent TCS.

What was the ER issue with your transistor mod?

KB2WIG - Yes,I forgot the 10M modification! Classic-  I have not tried that one.

Mike WU2D

Actually the article was in 73 rather than CQ magazine.  I Know..I wrote it back when I was K4PFK in high school.  As I recall Wayne Green paid me $40 for it, which was actually a fair amount of money back in those days for a high school kid!  BTW..I still have the old rig although it's been stored in the shack attic for many years now.

Roger Smith, AK4RS  (ex-K4PFK)

You can run.........but you cannot hide......  GOOD STUFF ROGER!

You can replace the 811's with 811A's for a bit more dissipation or 572B's if you want to see what it takes to blow the mod xfmr.

I use the ER keying circuit and the ATC spends most of its time on 17M AM and CW except this past weekend when I was chasing the big ships on other bands. Normally use a hopped up BC-312 but put the RBB/RBC on line to make it all Navy.

Carl
KM1H

I use the 1600v TV sweep transistor to cathode key all my rigs.  It cuts the voltage across the key back to about 6 volts and a few milliamps.  My Gates would be especially hard on my Vibroplex bug, since the keyed stage (pair of 807's) runs about 200 ma.

In my HF-300 rig I had to neutralise the keyed stage, the 802 buffer/pre-driver, to eliminate the back wave.  I tried using an 807 in that stage, but had the same back wave problem. Plus the 807 was somewhat squirrelly, while the 802 is nice and stable with velvet-smooth tuning.