The AM Forum

A friend of mine wanted to have a QSO on 40m and so I decided to see how the ART-13 would do up there as usually it is set for 75m. Once I had the HF oscillator set for 40m and keyed down I noticed that the grid drive was significantly lower than it is on 75m. To be specific, positions 1-6 on the "A" control result in excellent grid drive (slightly above half way up the white coloured scale for grid drive) but as soon as I switch to position 7 the grid drive drops to barely into the white. At first I suspected a weak V103 as V103 (a 1625) is switched in at position 7 but substituting a good 1625 did not solve matters.

I read through the section of the manual dealing with aligning the high frequency multiplier. One thing that I noticed is that, as I rotated control "A" from 7-12 the grid drive would vary very little. Each setting is connected to a capacitor and so if one of the capacitors was out of alignment I would expect to see good grid drive on all of the settings except the one connected to the capacitor that was out of adjustment. However, my problem shows itself as insufficient grid drive across all "A" settings with little variation in the grid drive. This would lead me to believe that the capacitors are all in good adjustment but that something else is bothering it.

I noticed in the manual that one of the first steps one does when adjusting the capacitors for the high frequency multiplier settings 7-12 is to adjust a trimmer capacitor, C136, for maximum grid drive. Could it be that this trimmer capacitor is out of adjustment thus causing insufficient but consistent levels of grid drive across the 7-12 settings range of "A"?

Any ideas would be greatly appreciated.

If anyone has any ideas/tips before I get into this they would be much appreciated. For the folks who have ART-13's, what sort of grid current are you seeing?

Bottom line starting point: what is the power output on 75 and 40?

Carl

Hello Carl,

At 1200vdc 75m output is 125w and on 40m the output is around 85-90w.

My ATC version runs the same voltage and I get 125-130W thru 20M and falls to 100W on 17M where I mostly use it on AM.

I wonder about a "known good" tube at RF unless its been proven in a comparison with a new one. You also have the 837 to consider as well as the 813 but I doubt the latter will fall off that fast in frequency but it is possible.

Carl

I spent about 5 hours this evening working on my ART-13. Carl, thank you for the tip about the 837. I had a spare new one that I decided to substitute in and this helped bring up the grid current by a noticeable amount. I went through and did an alignment on the frequency multiplier capacitors used for V102 which is the second multiplier tube. Both the padder capacitors and the trimmer capacitor C136 were quite stiff and so I had to be very ginger in applying only enough force to get them moving again. It turned out all of the padder capacitors were off a bit and I was able to bring up grid current across the bands. C136 seemed to be set pretty much at its optimum point.

I then decided to swap in some new 1625's I had into both multiplier sockets. This, in combination with a subsequent touch up alignment to the frequency multiplier capacitors, brought grid current up to a healthy level for positions 7-12 on control A. Output power on 40m has increased from 85w to 100+w with 1200vdc.

I recently picked up a ME-165/G dummy load and it certainly came in handy during all of this testing. When I was moving the transmitter I accidentally touched the antenna current meter glass which broke the admittedly weak bond that the roughly 68 year old glue was making in keeping the meter glass affixed to the meter housing cover. Tomorrow I'll lightly apply some glue to the glass. It's too late and I'm too tired to be doing any detail work right now.

I'm happy that I was able to getting everything sorted in regards to the grid current. I promised a friend of mine a QSO on 40m and I didn't want to have it until I had the ART-13 fully sorted above 75m. Now we can finally have that long awaited chat.

Next up I need to pull the power supply rack out and replace the HV capacitors with 10 new 470mfd 450v Nippon Chemi-con's and fabricate mounting brackets to attach to the side rails of the supply so that I can have a protective sheet of thick plexiglass over the top of it for the sake of safety.

Glad to hear it wasnt a painful fix. That rig is usually pretty immune to anything serious which is amazing since the expected life was 18 months!

Carl

Great news on getting the ART13 happy. The grid current should at least be in the paint. By the way, I have been bit by those stacked trimmers - they are hot!

Mike

Mike,

If I remember correctly the grid current when in position 7 for A was just about touching the bottom of the paint. When I had finished it was sitting just at the "A" of the "P.A GRID" marking on the meter. I was bitten by those trimmers as well. A slip of my alignment tool certainly woke me up a bit..


Mike

I have the same problem. Low grid current in position 7-13. In my case , the power is just 45w in 40 meters, because my power supply brings only 800v to the plate of the 813.
I have swapped the 1625 tubes from another art-13 that brings 90w in 40 meters. I am wondering what can i do to increase grid current a little to have a decent 70-80 w of power in 40 meters. Any suggestions?