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Old 18th February 2011, 02:05 AM   #21
TheGimp is online now TheGimp  United States
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Here is a completed first pass.

It still needs a little tweaking.

I think 16 steps is too many to be useful, and should be changed to 8 steps.

I used a faster sweep rate than line frequency to help with persistence on a scope.

No screen drive is included as it is held constant while sweeping the anode and stepping the grid.

The FETs in the simulation are only 250V parts and the bias will possibly shift when changed to 400V or greater parts.

One downside to this design is that it must be adjusted if the anode voltage is changed.
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File Type: png Pass_1_complete.PNG (83.1 KB, 557 views)
File Type: png Pass_1_Simulation.PNG (50.1 KB, 522 views)
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Old 18th February 2011, 08:40 PM   #22
TheGimp is online now TheGimp  United States
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Fixed several problems. The anode sweep has to go each grid step, not once for the complete set.

FETs now IRFI744 good to 450V.

Grid drive in 8 steps now.

One issue is that there is no current limit to protect the tube at low grid voltage. This could be a problem.

Grid voltage is adjustable up to -80V and only goes to 0V. No positive grid drive. A pot can be added to allow the minimum voltage to be set to other than 0 Volts.

There are two pots in the step generation circuit (U13 and U5). U13 allows steps to -80V, U5 only adjusts to -20V, but allows adjustment down to 0-1.75V in 0.25V steps for small signal tubes.
Attached Images
File Type: png Updated Schematic.PNG (81.1 KB, 441 views)
File Type: png 80Vgrid drive.PNG (61.9 KB, 411 views)

Last edited by TheGimp; 18th February 2011 at 08:43 PM.
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Old 19th February 2011, 07:24 PM   #23
mogliaa is offline mogliaa  United Kingdom
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@TheGimp, are u planning to breadboard this?
Cheers,
Ale
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Old 20th February 2011, 07:21 PM   #24
TheGimp is online now TheGimp  United States
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I was back in the ER for chest pains the last few days so I've not been on line (Had a heart attack and then a stent put in about three weeks ago).

I may try to breadboard parts of it, but I doubt I'll build the whole thing. I may just order one of the MODULE LAMPEMETRE ANALYSEUR - VACUUM TUBE ANALYZERs as it would save me a lot of time.

Conceptually I don't see any reason it would not work.

I expect it would take some tweaking to get the ramp reverence points set as the FET Vgs tends to vary a bit, but the adjustments in the circuit should take care of this.

Is there any portion in particular you are interested in/concerned about?
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Old 20th February 2011, 07:55 PM   #25
mogliaa is offline mogliaa  United Kingdom
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I'm glad you are back and ok then :-)
I'm in the process of adapting the Sussex valve tester circuit and was considering in adding a tracer circuit. I bought the module you mentioned as I'm keen to use the PC to record the curves as well. Either way I was interested in building this as well as it looks like a good design to try...
Cheers
Ale
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Old 22nd February 2011, 04:46 PM   #26
TheGimp is online now TheGimp  United States
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This is more of an exercise for me, as I've ordered the MODULE LAMPEMETRE ANALYSEUR - VACUUM TUBE ANALYZER.

I added an SOA (Safe Operating Area) limiter to control the anode supply as I don't see any other practical way to protect the tubes from excessive dissipation at low grid voltages.

The plot shows grid steps in green and anode voltage in blue with the SOA limit in red.

If anyone sees any errors or oversights, please speak up.

Power dissipation dictates all the output FETs have heat sinks.

As is the drivers will need to be on heat sinks as well. However I think the load resistors of the drivers can be increased and made large enough to allow them to run without heat sinks. The changes may require tweaking other components.
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File Type: png SOA_Limit_W_Grid-Sweep.PNG (65.3 KB, 342 views)
File Type: png SCH_SOA_Limit_W_Grid-Sweep.PNG (71.2 KB, 99 views)
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Old 10th March 2011, 05:58 PM   #27
TheGimp is online now TheGimp  United States
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I've started breadboarding some of the circuitry.

I used a CD4060 instead of hte NE555 and CD4020. The 4060 has an oscillator built in. I set it for 16X the desired frequency and used the Q4, Q5, Q6 outputs. I used an OPA2131 for the inverter and a buffer with +/-15 V supplies.

Works fine except some noise from the digital section. I'm going to isolate the digital ground from the analog ground to see if I can reduce the noise.

First look at linearity looks good, but I"m using 0.1% resistors so it should be.
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Old 28th March 2011, 08:04 PM   #28
mogliaa is offline mogliaa  United Kingdom
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I have added some anode curves using Alain's great curve tracer board at my website here:

http://www.bartola.co.uk/valves/

You can find examples of 6CB5A, 6SL7GT, GU-50 and others

Cheers,
Ale
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Old 6th April 2011, 07:47 AM   #29
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Hi Ale.
Are you still going to the audiojumble at Bracknell on the 17 April?
Regards
Henry
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Old 6th April 2011, 06:02 PM   #30
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The cool and very advantageous things about making your own tester are:
You don't have to test every type ever made in the galaxy, and,
You can test at or very near the actual operating point of your circuit, which commercial testers almost never do.

I have a great fascination with old orphan nonaudio tubes, some of which have really killer chars, and can be gotten for nearly free, especially in larger quantities. Here's my tester for small signal miniatures, 6GK5, 6T4, 6AQ5, li'dat.
Click the image to open in full size.
It's built on a breadboard, with the adjustment parts, like dropping resistors out available
Click the image to open in full size.
I can reconfigure it for the victim of choice in a few minutes, and then test and match dozens (or hundreds) of the little buggers so I have better than 1% pairs and quads.

Poinz
AudioTropic

Last edited by Poindexter; 6th April 2011 at 06:07 PM.
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