The "Sussex" Homebrew Valve Tester.

[Top Cap]

Very interesting posts and mirror my own ideas about the GM meter. I thought, why not cal the 100mV test signal using the GM meter then perhaps the value obtained across the 10 Ohm resistor will reflect more accuracy? I still favour the idea of a transformer coupled signal to replace the 10 Ohm resistor but this would have to be carefully set up using a measured 1kHz current through its primary winding and possibly a variable resistor on the output side to make the GM meter indicate correctly. Then I think the tests would have to be done again with DC currents also flowing in the primary to see how the transformer holds up with respect to core saturation effects. Still trying to think of a way to to that without the specialised test gear required, I would imagine one would have to feed the AC via blocking capacitors whilst feeding the DC through the primary winding. Out of my comfort zone here and the previous ideas might be just rubbish lol

[crackle]

It was primarily originally part of the debugging to clean up the traces of the anode. But I have left it as a permanent feature. I did wonder if it would be useful to be able to inject a HF signal to test RF valves and whether it would make any difference to the results of the valve characteristics.
Something else I have noticed when measuring the grid AC voltage (grid to cathode) is; my DMM gives some strange readings for the oscillator output when the grid voltage is much more than 4 volts of DC. The Ac reading goes up to 150 or more mV AC.
So always check your grid audio signal with grid volts at "0".

Mike

[KeithsTV]

Just to let you all know how I'm progressing with the updated manual.

I've completed sections on a bit of valve theory plus the circuit operation which are being proof read. I'm working on the construction, commissioning and operation sections now. I'm hoping to have a draft copy at the Golborne meet in April so if you are at the meet and are curious, find me and you can have a preview of the manual.

Keith

[crackle]

Excellent Keith, dont forget to mention about the problems found with D16
Mike

[KeithsTV]

Already included Mike.

BTW
I'm still after some pictures of completed Sussex testers to include to show in the gallery in the manual.

I wonder if the problems people are having when measuring the AC voltages is due to the way the DMM/DPM measures AC. Is it possible that if DC is present this would affect the rectifier in the meter? It may be worth trying a capacitor in series with the AC meter to remove the DC component.

Keith

[Paul Stenning]

Originally Posted by KeithsTV

Just to let you all know how I'm progressing with the updated manual.

Excellent - thank you for your hard work on this Keith, and everyone else who has contributed.

[Omegaman]

Hi All,

It's about time my Sussex, the third one, saw the light of forum.
I built one to Mikes original design, but was never really happy with it. The design is spot-on, just my construction that was the problem.
I built a second using my own pcb design to save on internal wiring and this was working well until testing a 6L6 one day when the valve flashed over and wiped out half of the circuitry. It was put onto the round tuit pile.
I eventually gave it to a young colleague who was looking for a tester. He repaired the damage and it's now working great for him.

Enter number three.
I decided as I only use the usual audio valves, and have an Avo anyway, that I would make it as simple as possible using a single switch to select the valve type required. Again, I used my own board layout and I'm pleased to say (Holding a piece of timber) that it's doing well. The circuit itself is as the original with some of the updates added.

For those who may wish to use it, the board track pattern and silkscreen are posted below as PDF files ready for printing. If anyone would like the actual file I can send it via email.
The board layout was done in Sprint Layout 5.0.
I also used Sprint to make a front panel legend that was printed onto plain paper and then covered with clear sticky backed plastic. Blue Peter style!!

The grid voltages are set with a bank of 10 500K multi-turn presets. All the presets are in parallel giving the 50K required by the circuit. The front panel selector merely selects the grid voltage required according to valve type. No grid voltage meter is fitted.

As the anode and screen voltages required were all 250V, these were preset using a 150K and 750K resistor for each instead of the multiway switches.
The valve sockets are all wired in parallel with an SPDT switch to select anodes for the dual triodes.

The transformer I wound myself. It only has a single 6.3V heater winding instead of multi taps. I put a 10V winding on for powering the fan. In the picture the single diode and smoothing cap can be seen atop the transformer.

I had some problems with the protection diode for the Gm meter so left it out. After seeing the destruction to my second tester caused by the flashover, I don't think it would help in that case, anyway.

The unit is shown testing a new Mullard EL34 and I'm quite happy with the readings given. I'm not too worried about ultimate accuracy as I'm now getting used to the readings given and interpreting them accordingly.

If anyone would like to know anything further, please ask.

[Omegaman]

A few more pictures....

Keith, if you'd like to use anything for your manual, design or otherwise, please feel free. I can send you the original images, not re-sized.

Regards

[Ed_Dinning]

Hi OM, this is probably a way forward for those who only test a few types as it removes a lot of the complexity from the wiring.

Ed

[Omegaman]

Hi Ed,
That's what I was thinking. I tried to make it as simple as possible. (Like me, really!). I didn't need gas test facilities, although the positions for the components are on the board
This was also the basic idea behind using one board for all the circuit. The board could actually have been made about half the size I used, but I like to use big components with proper markings on them. All the resistors are 1W carbon, none of these microscopic multi colour banded thingies. My Ladybird book shows the colour codes for 4 bands so they're good enough for me!! The parts are well spaced out on the board so it's not too cramped which also meant I could use decent sized tracks.

[KeithsTV]

Thanks Howard if I need any larger images I'll be in touch. I should add that any photos or other information will be acknowledged in the manual.

Keith

[KeithsTV]

Here's a question that's puzzling me. When testing a valve on the Sussex, or any other tester, with the anode and grid voltages set as per the manual you should get a specified anode current and gm. If the valve is "not at its best" and gives a lower reading of either anode current or gm, at what percentage of the specified readings would you say the valve should be rejected?

This is, as you might guess, for the manual as I don't want to put too narrow a tolerance on the accept/reject criteria.

As well as taking a draft copy to Golborne I'll also bring one down to NVCF so if you can find me at either event you can get a preview of the manual and I'd appreciate any comments, good or bad.

Keith

[Radio Wrangler]

That rather depends on what equipment the valve is going to be used in, there is quite a wide variation in tolerance. I seem to recall that most new mainstream valves I tested in the 60s were within 10% of their maker's figures for Gm. Some valves down to a quarter of their original Gm can work in oscillators and low-gain stages.

Probably the most acceptable approach would be to go for the same pass/fail bounds as on the AVO meter scale. No one will argue against that.

David

[crackle]

My policy is:

Radios;
If it works reliably in the application it is intended for don't replace it.
If it does not work reliably then replace it, and keep for future.
(I have found some valves will not work in one radio but will in another.)

Hifi;
Replace if owner has the money and wants it replaced.

Guitar amp;
Replace if owner has the money and wants it replaced.

Old valves which have been replaced;
If it works at all keep it and save as last resort.

Mike

[Top Cap]

Someone did post the Pass and Failure info on a thread, related to the Red and Green scales on the AVO Valve tester and which can be applied to the Sussex.
If for example the AVO manual gives a Target gm of 15 at the test bias setting, then a
PASS = an indication of Target * 0.7 (lowest acceptable) to Target * 1.3
In our example 15*0.7 to 15*1.3 or more simply 10.5 to 19.5, this being the area of the Green Pass meter scale.
A FAIL = an indication of Target * 0.5 or less.
In our example 15*0.5 or more simply 7.5
(This would be the upper edge of the AVO Red scale and anything less than 7.5 would obviously indicate an even greater fail)

[KeithsTV]

Thanks for that Les.

Keith

[Mogliaa]

Originally Posted by Top Cap

Very interesting posts and mirror my own ideas about the GM meter. I thought, why not cal the 100mV test signal using the GM meter then perhaps the value obtained across the 10 Ohm resistor will reflect more accuracy? I still favour the idea of a transformer coupled signal to replace the 10 Ohm resistor but this would have to be carefully set up using a measured 1kHz current through its primary winding and possibly a variable resistor on the output side to make the GM meter indicate correctly. Then I think the tests would have to be done again with DC currents also flowing in the primary to see how the transformer holds up with respect to core saturation effects. Still trying to think of a way to to that without the specialised test gear required, I would imagine one would have to feed the AC via blocking capacitors whilst feeding the DC through the primary winding. Out of my comfort zone here and the previous ideas might be just rubbish lol

Hi Les,
I've been playing with multiple testers and tracers for some years now and after a bit of experimentation I ended up with the following circuit to measure transconductance using a transformer to couple the anode to the sensing resistor. I use now a 10kHz signal (as later AVO tester does) with great results. Here is a description if you are interested on it:

http://www.bartola.co.uk/valves/2013...ctance-tester/

The coupling transformer is very easy to wind (I did it so it's very simple), calibrate and very accurate. Circuit is simple enough to modify it to fit the Sussex tester.
Cheers,
Ale

[Top Cap]

Thanks for posting that Ale, it could turn out to be most useful.

[Diabolical Artificer]

Re what constitutes a good/bad valve. I recently tested some Chinese EL34's using the AVO valve tester manual settings, ie 250V on anode and g2, 13.5V bias, which should give 100mA anode current. These valves were sold as Premium. I was surprised to see most were in the region of 85mA anode current, especially as the EL34 is an output valve.

I've been trying to find what data Mullard etc. used for passing a valve A1, apart from heater continuity, gas emission etc. Am also curious what parameters are/were used to match valves.

Andy.

[KeithsTV]

Looking at the AVO manual data for the EL34. At 250V anode & G2 and 13.5V bias the anode current is 75mA. The Mullard data sheet has two conflicting value of Ia. The data for Class A with 250V on anode and G2 and -13.5V on G1 shows the anode current as 100mA yet in the Ia vs Vg1 plot later in the same data sheet the anode current is approx. 75mA. So which is correct?

Keith