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Old 11th Oct 2013, 11:45 am   #961
Dekatron
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Default Re: The "Sussex" Homebrew Valve Tester.

A grid stopper resistor and/or decoupling capacitors close to the anode and grid works well in many testers to stop oscillations from Pentodes, works nicely for EL34's.

In the case of dropping voltage I would suspect the over current protection, check the voltage across the low ohm resistor across the base and emitter to see if it gets close to the base-emitter voltage which would result in that the transistor starts to conduct lowering the output voltage.

Also check to see if the voltage drops before the mosfet, if it does then you probably have some problem between the transformer and the mosfets, like too low capacitance in the reservoir capacitors or in the voltage doubling capacitors.

You could also have problem with external signals entering the tester which will affect the frequency applied to the grid, you could check this by removing the signal from the oscillator just using the grid voltage - if it becomes stable without the oscillator signal you probably have an overlaid signal there or an unstable oscillator frequency or voltage. You might have to render the oscillator inoperative and just not remove the output signal from the oscillator to properly check this.

I don't own a "Sussex" tester but other testers I've built works in a similar fashion and experience similar problems, even the most expensive ones like the RoeTest and the AVO VCM-163!

/Martin

I made a mistake in my text above, there is no voltage doubling circuit in the Sussex power supply like it is in the RAT-tester (which the Sussex is similar in design to, see the RAT-tester here: http://triodeel.com/tester.htm). So you should not have any problems with the voltage doubling circuit.
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Last edited by AC/HL; 11th Oct 2013 at 2:09 pm. Reason: PS note merged
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Old 11th Oct 2013, 2:04 pm   #962
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Default Re: The "Sussex" Homebrew Valve Tester.

Martin, I believe you are right. I seemed to have missed out on TopCap's suggested mod to alter the current limiting. By doing this the voltage drop problem has gone.

I am hoping my other issues are related to parasitics so I will experiment with the AVO suggestions that you have kindly given.

Thanks everyone.

Last edited by AC/HL; 11th Oct 2013 at 2:10 pm. Reason: Quote not needed now
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Old 12th Oct 2013, 11:05 pm   #963
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Default Re: The "Sussex" Homebrew Valve Tester.

I am glad that you found the reason for the voltage drop and that it wasn't anything serious.

You might have to use grid stopper resistors and capacitors on the grid/and or anode to stop oscillations even if you add ferrite beads to the wiring as some valves vreak havoc no matter what you do to stop them from oscillating, especially if they pick up some signal from the outside.

Adding ferrite beads is a first good step as that stops most oscillations. If you test expensive or very rare valves it is always a good measure to add a grid stopper resistor as it will help in stopping the oscillations which could otherwise lead to damaged valves.

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Old 13th Oct 2013, 11:52 am   #964
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Default Re: The "Sussex" Homebrew Valve Tester.

Hi Martin,

Appreciate if you can help with my debugging.

I am not sure if I am facing parasitic oscillation is there any way I can tell with the equipment that I have - (basic electronics kit scope, DVM, AVO 8 etc)

I have tried some of the suggested anti oscillation tips: grid stopper 4K7 added right on the valve socket, screened cable, ferrite bead and 1nF decoupling cap. You can see that the signal going into grid below looks clean to me. I have made these changes one by one and not really seen any impact.

I have a few other symptoms that might be useful info:

When the "test" position is selected, the Gm does appear to shoot up to 9 on the meter just for blink of the eye before settling down at about 2.

The Ia is a high at ~90mA but by increasing the grid bias from -13.5 to -16 I can get the Gm to go up to about 4 and the Ia to 75mA. Moreover using my DVM in parallel with the Gm meter I take a reading of about double (Gm=8).

I have scoped the signal on the grid and anode and attached pictures. The sine wave output contains lots of waves on top of it - is this evidence of parasitics?

You mentioned a decoupling cap on the anode - could you suggest a value to try out and suitable connection point.
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Old 13th Oct 2013, 1:31 pm   #965
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Default Re: The "Sussex" Homebrew Valve Tester.

I haven't read the whole thread on your build and problems but before we go into any debugging we need to know that the valve you are using for your test is working ok in another Sussex tester, or at least that it works good in for instance a VCM163 which uses a similar test method with an overlaid sine wave signal on the grid.

If you can't check that I suggest that you use a few different high current valves and see how they behave.

Also before you check with a valve in the socket, have you checked the output signal from the oscillator, is it stable no matter what grid voltage you set, is it stable if you put a resistor across anode/cathode or from grid to cathode to present some load to the circuit? You should test this to make sure that the oscillator output signal is not affected by the load on the circuit. You could also check the voltage for the oscillator circuit, both with a DVM and your scope to see if it changes while you observe the change in gm.

If you have tested this already I suggest that you either replace the gm meter to check if that is working or not or check it outside of the circuit to check if it is reading correctly. When you are checking it you can use your DVM in its place to see if the tester behaves in the same way.

I can't really say what value to use for any of the decoupling caps, but it needs to have a value that doesn't dampen the amplified signal from the grid, then you will get to low readings. It is the same with grid stopper resistors, sometimes you need a high value and sometimes a low - it all depends on what interference/oscillation you have in your circuit.

If you want to see how oscillations can affect your tester you can try with EF80 or any of the ECF8X or ECL8X valves as they are quite prone to oscillate if the circuit isn't dampened enough - I must warn you though as it might damage your tester and/or your valve as these oscillations sometimes wreaks havoc with the mosfet power stages. I have destroyed my share of valves and also valve testers with valves that started to oscillate in this way so I always recommend people to switch off their valve tester as soon as possible if they suspect that the valve is oscillating. This is also the reason why I use a grid stopper resistor for every Pentode that I check, even if it isn't needed.

Protecting the valve and the valve tester is not trivial and usually the hardest thing to design when you make a valve tester, displaying the measurements with meters or via an interface to a computer is nothing compared to designing a high voltage power supply that you have full control over which you can also shut down in an instant if you (or itself) detect a fault situation.
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Old 13th Oct 2013, 10:44 pm   #966
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Default Re: The "Sussex" Homebrew Valve Tester.

@martin
Unfortunately I don't have the ability to obtain a calibrated valve but I have enough new and old EL34 that are giving the same results.

Unless anyone in Bristol has one. . . ?

My grid bias signal is 280mv peak to peak and it measures the same whether in or out of circuit with a valve. The waveform is very steady over the range of bias voltage. However the bias does wobble around by a about 50mv. Seems to be caused by the pot shaft itself. I added some felt pads and this has eased the problem though not eradicated it. Is this something I should be concerned about?

I checked out my Gm meter independently from the circuit driving it from the signal generator and it was in alignment with my dad when measuring this very clean sine wave.

I also tried out a 220nf Decoupling cap on the Anode but there was no impact.

I still don't get why I see the 42vdc on the heaters. Could anyone confirm whether that is correct.

Thanks for reading and all the helpful responses.

Head scratching time.

Mike
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Old 13th Oct 2013, 10:57 pm   #967
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Default Re: The "Sussex" Homebrew Valve Tester.

Just one more idea before bedtime here - do you have the possibility to feed the heaters from an external transformer bypassing the internal transformer and any dc levels?
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Old 14th Oct 2013, 8:29 am   #968
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Default Re: The "Sussex" Homebrew Valve Tester.

Yes good idea. Isolating the heater could help identify the problem.

I have a few old Transformers lying around so let me get back to you on that.
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Old 18th Oct 2013, 11:12 am   #969
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Default Re: The "Sussex" Homebrew Valve Tester.

A line or two to say Thanks for the messages from Ed Dinning and G8UWM-MildMartin.

Yesterday I recieved the circuit board safely from Martin (thanks) and when finances recover I might well order a transformer from Ed, the handling costs levied by the Post Office / Customs & Excise nearly doubling the costs into Norway. I am toying with the idea of either winding it myself or having the transformer sent to a UK address and collect it next time over in the UK.

Does anyone sell the switches and other components for this project, please could anyone put me in touch with them? I have the valve bases and many sundry items.

Thanks
Tony

Last edited by Anthony Thomas; 18th Oct 2013 at 11:13 am. Reason: spelling.
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Old 24th Oct 2013, 6:51 pm   #970
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Default Re: The "Sussex" Homebrew Valve Tester.

Quote:
Originally Posted by Anthony Thomas View Post
Does anyone sell the switches and other components for this project, please could anyone put me in touch with them? I have the valve bases and many sundry items.
Tony,

I know it is a big task, but if you read through all of the posts, you will get masses of help. For starters, Top Cap's (Les Carpenter) document on his build is very good and downloadable from a link on the first post by Swordholder. Also, Andymic posted a "Bill of Materials" on post #104, listing ordering details for many items from Rapid Electronics. I went through a torturous search myself to find some decent thumb-wheel switches; posts #679-#681 give the details of my best find.

Good Luck (especially with the Norwegian Post and Customs), Colin.
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Old 24th Oct 2013, 8:46 pm   #971
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Default Re: The "Sussex" Homebrew Valve Tester.

Thank you Colin for your reply.

I tried to download the bill of materials in posting # 104 but that proved beyond my capabilities, I have trouble with most tasks involving downloading pdf files these days. Will have to get some help with this

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Old 11th Nov 2013, 3:45 pm   #972
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Default Re: The "Sussex" Homebrew Valve Tester.

As this is such a major and popular project I have made the thread "sticky" so everyone can find it easily.
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Old 11th Nov 2013, 6:27 pm   #973
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Default Re: The "Sussex" Homebrew Valve Tester.

Quote:
Originally Posted by Anthony Thomas View Post
I tried to download the bill of materials in posting # 104 but that proved beyond my capabilities, I have trouble with most tasks involving downloading pdf files these days. Will have to get some help with this
Stumbled across this today and found that the BoM in post #104 looks like it's about to disappear- it's not a pdf it's some online spreadsheet thing.

Anyway, attached is a version in Excel. (zipped since xls isn't a forum format)
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Old 12th Nov 2013, 9:53 pm   #974
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Default Re: The "Sussex" Homebrew Valve Tester.

Quote:
Originally Posted by mikeydee View Post
Head scratching time.
Hi Mike,

I think I finally cracked the problem with my Sussex and EL34s.

Putting a scope on the anode voltage I found that it oscillated madly when testing any EL34. Putting my hand around the valve (before it got too hot!) stopped the oscillation and the meter readings became steady.

I disconnected the wire to pin 5 of the Octal valveholder (the grid on an EL34) and inserted a 3k3 stopper resistor (not a critical value, it was just lying on the bench).

Now I test EL34s and get believable readings. I put five through their paces just now. I also tested some 6L6GTs and they work fine too.

I'm hoping that's the last of the niggles out of the way now!

Richard
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Old 12th Nov 2013, 11:48 pm   #975
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Default Re: The "Sussex" Homebrew Valve Tester.

Ferrite beads can also be effective. Fair-Rite number 43 material is one of the best.

In a valve tester you are biasing the device to measure its mutual conductance under the bias conditions which maximise it. This means right where the gain is at a maximum, which means perfect conditions to cause spurious oscillation.

David
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Old 13th Nov 2013, 7:15 pm   #976
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Default Re: The "Sussex" Homebrew Valve Tester.

Just wondering - Would any of you keen regular Sussex users like to carry out a Sussex/Leymarcavo comparison Test ?
Not a competition, just draw up a simple graph of Ia/Vg of an EL37 which I'll supply by post.
Apart from my own CT160 & recently aquired MK3, test graphs have been aquired from other guy's VCM's. Two CT160's, one CT163, and a MK4.
Just print off a suitable graph from the ones I attach, then superimpose your results. Any difficulty in printing off copies, let me have your email address & I'll send graph copies that way. Obviously, I'll also need postal addresses.
Just a couple of Sussex guys participating would be fine.
The VCM's used so far havent been tweaked specifically for this particular exercise. So if you've an AVO VCM you'd like to include, please do so, warts & all. My old & much handled (recently De-hybridized)CT160 no C118 for instance needs some tlc, calibration-wise.

Regards, David
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Old 13th Nov 2013, 7:33 pm   #977
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Default Re: The "Sussex" Homebrew Valve Tester.

Quote:
Originally Posted by Herald1360 View Post
Anyway, attached is a version in Excel. (zipped since xls isn't a forum format)
Thanks for that

will print it out now, all I need now is the schematic now and I will be happy.

Tony
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Old 19th Nov 2013, 2:25 pm   #978
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Default Re: The "Sussex" Homebrew Valve Tester.

Quote:
Originally Posted by Anthony Thomas View Post
.... all I need now is the schematic now and I will be happy.
Tony,
Apart from the Handbook (a Word document which includes a redrawn schematic) that has been appended to the very first message in this thread, the original schematic from Swordholder is there as a pdf file.
Colin.
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Old 21st Nov 2013, 12:12 am   #979
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Default Re: The "Sussex" Homebrew Valve Tester.

Hi
I wondered if you guys on here would like to ponder over a problem I had and which I dont fully understand exactly what happened and the sequence of events which lead to the bang.

The problem was with the "HT Supply - Rectification and pre-regulation:" circuit as it is labelled in the manual.
I have attached a copy of the circuit.

The original design was to have a 250v AC HT and used MPSA92 transistors rated for 300v for TR1 which seemed a little close for comfort. And as I was using an old transformer rated at 290v AC I decided to see if the transistor could be uprated. So I searched around and purchased some higher voltage transistors to go in, KSA1625 rated 400v.

I finished the HT supply and the Anode and screen current limiter circuit boards and wired them together. Then came the big moment. Connected up the HT from the transformer and wow it all worked, I could switch the anode voltage range from 0v to 300v, in 12 steps, it was all working well I thought.
Then on a later occasion I turned it on and after a short while there was a bang and something hit a radio on the top shelf went ping and fell to the ground. I cant remember now what I was actually doing but I am sure it was not anything which caused it to blow.. It obviously all got turned off pretty quickly.

The obvious sign of what went bang was that TR1 was only half there. I decided to drop the idea of using the higher voltage transformer and get the custom made transformer from Ed Dinning. I put it all away as other things were happening.

6 months later I have just got around to ordering the transformer for a present from Gill to me for Christmas.
It took a while to refresh myself as to where I had got to. I decided to test the zener diodes before putting in the spare KSA1625 and lucky I did. They were all short circuit..

This is the bit I don’t quite understand, what blew first the transistor or the zeners.
My theory is the transistor went faulty, (possibly shorted EC) causing the zeners to overload, they probably all fused in quick succession causing a short through TR1 and at that point TR1 decided to fuse and blew apart.
Does that sound reasonable?
But what do you think was the real cause.
The original design transformer was 250v AC giving a rectified and smoothed HT of 353v DC. Which was 53 volts over the rating of the MPSA92. So it seemed logical that the transistor was not working at the full HT voltage.
My transformer was 290v AC giving an HT of 410v DC. Just 10 volts over the rating of the transistor.
Why did the 400v transistor give up. Was it just faulty?

Ideas please.
Thanks
Mike
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Old 21st Nov 2013, 6:12 pm   #980
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Default Re: The "Sussex" Homebrew Valve Tester.

Sussex - LeyMarcAvo EL37 comparison test :-
Just one guy has kindly offered to participate, so far. Would really like two at least. Prahaps it wasn't clear in my initial post that I would pay all the postage involved. I'd pop some 1st class stamps in the padded envelope along with the valve. Only costs £2.80 each time.
If folk have reservations about a high Ia thumper of a valve - please suggest an alternative. Prahaps a 6AU5GT with a more modest 50mA Ia?
This isn't a competition between designers or constructors of homebrew valve testers. The only competition is between eras of technology.1950's/60's sourced equipment v 21st century. After all, this is called a "Vintage Radio" Forum, and at 68 I'm feeling pretty "vintage" myself.
Alternatively, if someone has a valve of their own which they'd rather use, again - I would re-imburse the postage.

Regards, David
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