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Old 19th October 2012, 01:20 AM   #281
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Are both, grid stairs and plate drive signals clean withouth the DUT connected?
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Old 19th October 2012, 06:08 AM   #282
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Quote:
Originally Posted by avincenty View Post
Are both, grid stairs and plate drive signals clean withouth the DUT connected?
I will check them out and see... Since multiple traces appear even with the blanking signal present, you are probably correct that something is messing up the grid drive and/or the plate voltage.

Jaz
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Old 19th October 2012, 07:29 AM   #283
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I'd suggest first looking at grid and plate signals to confirm they are coming out as expected. Also tracing both signals is key to look at the phase delay and ensure if your phase adjustment circuit is providing the necessary correction to put both signals in perfect sync. That will be the best way to eliminate the retrace.

If you can post your circuit it may be easier to help out
cheers
Ale
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Old 19th October 2012, 09:42 AM   #284
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Ale,

Looking at the grid drive and B+ supply lines, there appears to be some line interference, I can see a slow moving wave moving across the grid drive signal - I think this is causing the retrace. The interference is also clearly observable on the bias supply, even as the output of the 4520 going into Q1 remains pretty clean. So far, I have not been able to isolate the source of the interference, I have tried the following so far: pi ac-line filter, external DC bias supply (i.e., -80V on your schematic), DC filament supply, DC B+ supply.

I used your schematic, other than some minor changes to the no. of steps and volts/step functions, everything is pretty much the same.

Partial schematic:
https://dl.dropbox.com/u/1326040/Grid%20Driver.pdf

Short video showing the line interference:
https://dl.dropbox.com/u/1326040/MVI_1521.AVI

TIA,
Jaz
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Old 19th October 2012, 11:47 AM   #285
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Jaz,
Looking at the video cleary two signs: grid clock is not in phase with anode sweep and also it may look like the PLL is not stable as there is some double tracing which may be the cause of frequency instability.
The grid driver (last mosfet) may need some protection in case you omit the grid resistor in case of a short.
To me, the phase sync issue was the biggest pain in this design. Have a close look at your PLL and phase adjustment circuit first. Everything comes very easy after that
Ale
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Old 19th October 2012, 01:12 PM   #286
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Jaz,

I agree with Ale's comments. I also found that my HV plate drive was much better if I used a simple full wave rectifier rather than the bridge and utilizing the HV transformer secondary center tap to connect the sensing resistor via ground. Also the grid stopper resistor is key for stability and a clear display of curves in some tubes.

Alfredo
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Old 19th October 2012, 01:27 PM   #287
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Thanks for the feedback. I will work on the PLL and the rectifiers this weekend and report back, it's good to have you guys guide me through it.

Thanks,
Jaz
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Old 21st October 2012, 04:57 AM   #288
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I took the cue from Merlin and stripped out the 4046 and 4017, so everything just syncs to the line. The results were much better, the image is steady (as it should) but I still have a small loop for each of the curves... Getting closer.

Jaz
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Old 21st October 2012, 08:57 AM   #289
mogliaa is offline mogliaa  United Kingdom
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Jaz, good progress. I'd suggest you look at both anode and grid signals. Trigger the scope with the grid signal and look at frequency stability and phase. Once you can sync both signals you will then be in a good position to isolate the double tracing. If signals are in sync, it may be either the tube, the wiring or oscillation. Try testing a tube you know is in good shape and is gm is not that high.
Ale
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Old 21st October 2012, 11:00 AM   #290
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Question Trouble-shooting Day 2

Quote:
Originally Posted by avincenty View Post
Jaz,

I agree with Ale's comments. I also found that my HV plate drive was much better if I used a simple full wave rectifier rather than the bridge and utilizing the HV transformer secondary center tap to connect the sensing resistor via ground. Also the grid stopper resistor is key for stability and a clear display of curves in some tubes.

Alfredo
I don't have a center tap xfmr, but I have tried the bridge, half-wave and no rectification at all, they all showed the loop.

Quote:
Originally Posted by mogliaa View Post
Jaz, good progress. I'd suggest you look at both anode and grid signals. Trigger the scope with the grid signal and look at frequency stability and phase. Once you can sync both signals you will then be in a good position to isolate the double tracing. If signals are in sync, it may be either the tube, the wiring or oscillation. Try testing a tube you know is in good shape and is gm is not that high.
Ale
The grid drive and anode are quite clean and sync fine - no drift at all. I tried a few different tubes - same result (perhaps the loop size varied a bit), they were all pretty generic tubes - 12AX7 and 12AV7.

Please take a look at how my sense resistors are placed and see if they could cause the loops, especially the current sense. I think I also tried putting the current sense resistor from the cathode to ground a bit earlier, but had the same result, in any case, I like to have the cathode tied directly to ground, that's how Tektronix does it.

Sense resistors:
Click the image to open in full size.

Grid drive and anode:
Click the image to open in full size.

Curve sample:
https://dl.dropbox.com/u/1326040/IMG_1540_sm.jpg

Thanks,
Jaz
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