The "Sussex" Homebrew Valve Tester.

[Top Cap]

Hi Mike, re-checked both the Anode and Screen selector switches and all produce correct Gate input voltages. What is strange is that both MOSFET's behave the same in as much as the Source voltages change correctly from 300V down to 225V and then it holds at 220V for the rest of the lower settings. The BUZ80's I am using have an internal Zener (I bet it is 100V) for protection and I am wondering if the Zener conducts when the source drops below input on the Drain (320V) - 100V (Zener) = 220V. I may order some IRF820's but I want to check up on their spec's first. It still bugs me how both circuits are behaving the same even though they are separate from each over, the wiring appears to be correct but will keep looking. Any thoughts on the BUZ80's internal Zener? Could this be the culprit?
Les

[Top Cap]

The IFR830 looks good at 60p each, has an internal diode for protection but it is a normal diode and not a Zener. It might be worth a try me thinks.
Les

[swordholder]

Les,
I didn't know about the zener in the BUZ80, mine were old (but brand new) so may be different.
It would be interesting to know what Neil used in post #342 used in his tester.
I don't think the type is critical as long as the ratings are OK
Regards
Mike

[Top Cap]

Bearing in mind the limited voltage ranges available, I decided to try testing an EL34. The Gm was out of this world , I had set grid drive to 1V rms instead of 100mV. Now reset to the correct figure I duly set up grid voltage as per the AVO 160 Handbook and followed the switch coding as well. The quoted Anode current should be 75mA with that bias and furnish a Gm of 11. I was quite chuffed to get the results on the Sussex as per photo. The internal wiring could have been tidier but I am not a pro-wireman but no doubt it will tidy a little more later.
Les

[M0XNA Neil]

Chaps,

I used the STP5NK80Z which is a To220 800V, 4.3A MOSFET which also has built in clamping zeners. They work fine.

Neil.

[Guitarist28]

Gents,
Are we indicating that the BUZ80's are not suitable?

Regards

Rob

[swordholder]

Rob,
I think the jury's out on that, as I use BUZ80's.
Let's wait and see what Les finds.
Mike

[Andymic]

Originally Posted by Top Cap

The internal wiring could have been tidier but I am not a pro-wireman but no doubt it will tidy a little more later.

Les,

Looks real tidy to me...

[Bero]

Hi all,
I am in process to finalize RAT tester, design by Steve Bench. Seems to me that's very similar to this design.
Things work, but I've stuck with LCD display, mine is only for DC voltage intended.
I've found in another thread on these forum schematics with TL072 and TL071 to convert this.
(https://www.vintage-radio.net/forum/...t=valve+tester)
I can't force them to read zero when input shorted, with no load he read about 400mV.
Can anyone suggest something, here is a schematic diagram, and picture presented what happen with signal across them?
Thanks in advance
Bero

[swordholder]

Bero,
Not totally OK on op amps, but shouldn't the slider of the 10k pot go to -ve, it appears to go to ground on your diagram

Mike

[ppppenguin]

Mike is right. See page 8 of the TL081 data sheet:
http://focus.ti.com/lit/ds/symlink/tl081a.pdf

Also, what sort of LPF is that intended to be? It's not quite right for Sallen Key http://en.wikipedia.org/wiki/Sallen%...93Key_topology

[Bero]

Jeffrey,
In original Steve Bench design, grid 1 of the tested valve is fed with a 1 kHz signal produced by 74HCO4 chip.
Monitoring that signal on anode side, you can read GM/MU of the tested valve using simple external AC millivoltmeter.

I think this arrangement (low pass filter) is because of two reasons,
first is to convert AC millivolt signal to be suitable for DC LCD 200 mV panel meter used here.
And second is to filtering anything else instead 1 kHz signal, to ensure better precision.

This is what I think looking at the arrangement.

But thing don't work properly with this LPF.
Probably something is wrong redrawn on that diagram.
As you can see on the oscilloscope photos above schematics diagram,
Input is 1kHz pure signal, after first amplifier signal is amplified,
until that point seems to look OK.
But I don't know is it OK what's happen after second and third amp?

Mike,
I will try to reconnect slider of the 10k pot to the –ve 12V.
What to do with the pin 4 of the TL071?
Should I leave them unconnected in that case?

Thanks
Bero

[julie_m]

According to this data sheet, the "offset null" pot should be 100KΩ and the slider should be returned to -12V. Which is also where pin 4 (supply -) should be connected.

[ppppenguin]

If you want to make a simple opamp filter look at the Sallen-Key reference I gave before. This is another useful guide: http://focus.ti.com/lit/an/sloa024b/sloa024b.pdf

Also an online SK filter design tool. I cannot guarantee this works but it allows you to design LPF, HPF and BPF with ease.
http://www.changpuak.ch/electronics/calc_08.html

[jimmc101]

Originally Posted by ppppenguin

Also, what sort of LPF is that intended to be? It's not quite right for Sallen Key http://en.wikipedia.org/wiki/Sallen%...93Key_topology

It's a "Multiple Feedback Band Pass Filter".
For an on-line design tool see http://sim.okawa-denshi.jp/en/OPtazyuBakeisan.htm

Plugging in the values shown give a 1.038kHz centre frequency, a gain of 2.35 and a Q of 1.5

Jim

p.s. As has been said, the (100k) balance pot wiper should go to -ve supply, there is a slight chance the OP-AMP may have been damaged by the incorrect connection.

[jimmc101]

Oops, I forgot to address the original question.

The Waveforms look about right.

The second stage has a gain of 2.35 at 1kHz and so is being driven into clipping, reducing the input should restore a sinewave here.

The third stage is a precision rectifier, the voltage on pin 2 is forced to be the same as the input on pin 3 by the feedback through the diodes. The output at pin 6 should be equal to the signal plus a square wave equal to (the forward drop of the diodes plus the DC out).

The effect of this is that the (rectified) current through the 150R is equal the input signal at pin3 divided by Rx where Rx = (330R + resistance of 2k pot to earth).

e.g. If the 2k pot is shorted, then an average signal voltage of 330mV at pin 3 will produce 1mA DC through the 150R giving a reading of 150mV on the DPM.

Jim

[Bero]

Wow...
What a fast help.

And thing works, thanks to all.
Like you suggest, I've connect pot. slider to –ve12V. And now is LCD easy set to zero.

I was considered about pot value (10K in circuit, 100 K in datasheet) but seem that value isn't critical.
First I've try 22K, and 47K. There isn't significant difference on LCD display.

Thanks
Bero

[Bero]

One thing I've forgot to mention.
Neither in Steve Bench schematics isn't said,
that DMM must be connected via blocking cap to get proper readings, even if you use a DMM.

After all this time spending to get LCD readings,
when thing sprang to life (thanks to your advices), I've connect them to output points onto PCB.
Seems to both chips are burned now ... there was about 250V on the anode.

[Bero]

And here you can see progress...
There isn't finished yet, this is roughcast point, but nice to know that thing works.
Regards
Bero

[Bero]

Jim
According to your calculation, I've think about 2K pot value.
If I need reading 200mV across 150R resistor, 200 divided by 150 gives 1.33mA current via 150R resistor.
With 200mV signal at pin 3, divided by 1.33mA current, calculated value of Rx need to be 150R.
But on diagram is Rx =330R+resistance of 2K pot to earth.
Even with 2K pot set to the zero, Rx would be 330R, and this is more than twice of calculated value for Rx.
Is this another wrong redrawn in circuit or calculation mismatch?