811 mod probs

[K9ACT]

I need some help interpreting the scope view of my modulation.

From mic to grids, the audio chain is the same as the 8000 rig which has great audio.  To get things going and to eliminate lots of variables, I just switch a pair of cables from the audio output trans to the 8000 grids or to the 811 grids.

The 8000 sounds good and the o'scope pics look like the book.  The pics below are of the 811 output and a 1kc sine wave into the speech amp.

The pics show no modulation and  with the grids leads swapped.  The center is what people sounds best but not good and the right is a bit worse.

What is the problem and what is the fix?

js

[Bacon, WA3WDR]

If the difference between picture two and picture three is the 811 grid leads being swapped, and the same driver is OK on the 8000 modulator, then I think that one of the grid lines from the 811 grids to the driver amp is miswired somewhere.  I am thinking that the center tap and one grid connection are reversed somewhere in your link to the 811 modulator.  That modulation pattern is so bad that it has to be something simple like that.

Then look into the rounded positive modulation peaks.

If I understand, you have an 811 RF final, the 8000 modulator and the 811 modulator.  If so, try swapping the 811s into the RF final, and see which one gives the best positive modulation peaks, using the 8000 modulator.  Then use the other two 811s in the 811 modulator, and try swapping them and see which gives the best positive modulation.

[K9ACT]

If I understand, you have an 811 RF final, the 8000 modulator and the 811 modulator.  If so, try swapping the 811s into the RF final, and see which one gives the best positive modulation peaks, using the 8000 modulator.  Then use the other two 811s in the 811 modulator, and try swapping them and see which gives the best positive modulation.

I don't follow all that but I have an 8000 modulated by a pair of 813's and an 811 modulated by a pair of 811's.  the only thing they have in common is the audio chain up the grids of the mod tubes, either/or of course.

js

js

[Bacon, WA3WDR]

I have an 8000 modulated by a pair of 813's and an 811 modulated by a pair of 811's.  The only thing they have in common is the audio chain up the grids of the mod tubes, either/or of course.

Ah.  OK.  Then the first thing to do is to fix the audio issue, which I think is a grid connection problem to the 811 grids.  Make sure both grids are connected, and that they both have audio on them, of the right polarity, and make sure the CT of the driver transformer sees the ground of the 811 modulator, etc.  You could have one 811 grid to the driver transformer CT, and one driver transformer grid lead to the other 811 grid, and the other driver transformer grid lead to ground.  So the 811s would be fighting each other on one half of the audio waveform, and completely dead on the other half of the audio waveform.  This would give about the right plate current, but really one-sided audio.  Or you could have the transformer connected to the grids correctly, but somehow one grid is grounded, and the CT of the driver transformer is not connected to the 811 modulator ground.  This would give less modulator plate current than expected, and really one-sided audio.

What does the 811 modulator grid current look like?

When that is fixed, I think you will see fairly decent modulation, but a soft rounding of the positive peaks.  Try swapping the 811s around for the best positive modulation, if swapping them around make any difference.  Also while trying the different combinations, try a little more grid drive to the 811 RF final and see if it helps the positive modulation peaks.

[K9ACT]

Bingo!

It was the grid cables.  They were wired correctly but the connector at the speech amp end had an intermittent on the female side and a bit of a squeeze with a pliers made it all better.  Looks just like the big rig now on the scope.

Now if I could just find someone to talk to on 40 to confirm things.

One thing troubles me though, the idling mod current is around 45 ma which is about twice was it was when not working.  This makes sense if I was only using one tube then but it seems rather high and I can't find any info on what it should be.

I don't have any way of measuring the grid current separately but the plate current meter reads about 5 ma with no HV on and I can talk it up to 50 or more.  With 100% modulation, it is about 150ma.

Thanks for the tip,

js

[Bacon, WA3WDR]

Very cool!  Now you have another AM transmitter going.  Keep this up, and there will be no room in your house for you!!!

[w5omr]

Bingo!

It was the grid cables.  They were wired correctly but the connector at the speech amp end had an intermittent on the female side and a bit of a squeeze with a pliers made it all better.  Looks just like the big rig now on the scope.

Now if I could just find someone to talk to on 40 to confirm things.

One thing troubles me though, the idling mod current is around 45 ma which is about twice was it was when not working.  This makes sense if I was only using one tube then but it seems rather high and I can't find any info on what it should be.

The 811A tube spec sheet says that with
1000v on the plate, for two tubes in Class B, the resting current is 44mA and Zero Volts of bias.

54ma resting, at 1250v and no bias. 

32mA at 1500v and -4.5v of bias.

[K9ACT]

The 811A tube spec sheet says that with
1000v on the plate, for two tubes in Class B, the resting current is 44mA and Zero Volts of bias.

Close enough.... thanks.

js

[Bacon, WA3WDR]

I think that the disconnected grid collected extra electrons from the thermionic emission of the filament (Edison Effect), making it negative with respect to the filament, and cutting the tube off completely.

I'm not sure whether this would cut off a tube needing more negative bias, such as an 812, but it did the trick on an 811 that wasn't gassy.

[K9ACT]

Well, it is working pretty well right now so I had to go mess it up.  I pick up some cool stuff at the fest today put it back of 40 meters with a brand new shiny store bought tank coil and a Dow Key so I am now push to talk.

I heard W9AD check in while I was in the middle of things  but heard no one else on 40 tonight.  After I got it back on the air I yelled for half an hour and gave up.

I also got a new 2 gang grid cap but the best I could do was 100 pF.  I was hoping to switch from 40 to 80 without switching but don't think I can make it with this cap.

Switching a balance coil seems like a real pain.

js