A better ¨homebrew¨ Valve Tester?

[neutronic]

I found the specification of this Tube- or Valvetester and think it is very impressive.
Cheap components and high performance.

Maybe something to ¨brew¨ during the Wintertime.
The garden is no excuse at this time of year.

www.dos4ever.com/uTracer3/uTracer3.html

Best Wishes for 2013 from Jard N.

[Herald1360]

This looks like a nice piece of engineering- along the lines of "an engineer is someone who can do for a tenner what any fool can do for a ton".

[fetteler]

Originally Posted by Herald1360

"an engineer is someone who can do for a tenner what any fool can do for a ton".

Well I've just had a look and I must say it is impressive and is my idea of how a tester should be realised these days...

When I first heard the quote it was a shilling and a pound though. I guess that's inflation

Cheers,
Steve.

[GrimJosef]

It is indeed a nice design. However like so many testers it doesn't test the valves under true operating conditions. The anode and screen supplies are pulsed. So the average power dissipated in the electrodes of, say, a power valve will be very much less than when it's actually in use. This will change the valve's temperature and thus its behaviour, particularly if there are any traces of gas present. So you'll end up making very precise and detailed measurements of parameters which may not be the same as those you'll see in practice.

Cheers,

GJ

[Ed_Dinning]

A re-programme and some power supplies with muscle should make this the sort of tester that is needed.
The valves could then be operated under actual conditions, possably pushed into overload if the pulse facility is retained and give real results.
Looks like you can also get graphical printouts as well if needed.

Ed

[Chris Leyson]

At first glance, a very nice design. However, there are a few features that I'm not happy with. Firstly, the boost converters and current limiters are software controlled, and secondly, perhaps more important, the heater supply !!
Not sure I would want to supply a 1.25V directly heated cathode with 19V even if it is pulse width modulated, would much prefer a proper buck converter here.
If I was building the V3 uTracer the first mod would be the heater supply.

Chris

[MichaelR]

I think it is always prudent to isolate the grounds of computer and the PCB connection.You could get some ground loop issues. A suitable dc to dc converter and some opto isolators will do the trick if you power the tester separately .

Maybe somebody could explain to me what difference his PWM measurement technique ( in terms of measured results ) are likely to be . If it is significant then what is the purpose of this as a tester ?. It appears his results tie in with published specs.

Mike

[PJL]

Originally Posted by MichaelR

Maybe somebody could explain to me what difference his PWM measurement technique ( in terms of measured results ) are likely to be . If it is significant then what is the purpose of this as a tester ?. It appears his results tie in with published specs.

Mike

1. The desire is to reproduce the test conditions used to produce the valve datasheets so results can be compared. We are assuming that the datasheet results are measured when the valve has reached thermal equilibrium although this cannot always be the case as datasheets show test conditions at which the valve would thermally runaway. In this tester, thermal equilibrium is not reached as the current is pulsed and therefore there is minimal anode dissipation.

2. Using PWM on the filament of a directly heated valve will also result in abnormal behaviour as there will be a much higher than normal voltage gradient along the filament. Many directly heated valves are intended for DC filament operation.

There are some other issues:

3. The heater voltage is limited and would not be suitable for some valves designed for operation in a heater chain.

4. Most NOS valves have some air inside from long term storage which is 'burnt' off during normal use. This tester is unlikely to raise the temperature sufficiently for this process to occur. My understanding of the theory is that leaving the heater of a gassed valve on without any anode current is about the worst thing you can do. You really want to run as much anode current as you dare to heat the valve up quickly before the cathode is poisoned.

5. It is a curve tracer and not a tester. It does not check grid leak or hk insulation.

Having said this, the AVO VCM's do not use DC test conditions either and these are regarded as the 'standard'.

As others have said, the principal of PC operation is definitely the way to go as curves can be traced. The desirable features that are missing are a) an option to apply a DC test condition to the valve prior to the curve trace which would require a large external PSU b) control signals to operate an improved heater arrangements, maybe using a transformer and relays. c) grid current and hk leakage measurements.

[Top Cap]

I am quite happy with the Sussex. It may appear a bit long winded but I simply make up an Xcel spreadsheet using the data available in manufacturers literature. I then enter the Ia obtained at different bias settings and enter these into the YELLOW boxes as required. The spreadsheet draws the graphs, the Green being the expected and the Red being that which was measured. At the required AVO checkpoint, I make a special note of what was obtained against their pass figures, for the EL34 it is -13.5Vg, Ia = 75mA and gm = 11.
I have similar spreadsheets for EL84, ECC81,82,83, EF86 and 6N1. I make new ones as required but I normally spend most time with the types mentioned.
On the double Triode spreadsheets I have 3 graphs drawn so you can see at a glance how both Triodes are matched against themselves and the expected.
Not the most ideal arrangement but no special circuits required, just a list of figures which I record on the Data input (sht.2) of the spreadsheet. These can be copied and pasted directly onto the graph sheet.
I wish I was more capable with Excel as I am sure better graphs could be drawn, but I am no expert and happy to get what I have achieved so far.

[julie_m]

OK, time to spill the beans ..... Just before the late Mike published the Sussex design, I was already thinking about building a computerised valve tester. I've been designing something involving Sussex parts and a commercially-available USB interface board (but probably now going to use a Raspberry Pi right inside the machine).

So far, I have been struggling to get a together a sensible variable HT PSU that doesn't blow up ..... but I'm now ready to order parts for the latest prototype and, if that is successful, I will post here.

[MichaelR]

Hi Ajs,

Are you going to use the Raspberry pi to do all of the on board measurements or control a number of separate pics.

This could be an interesting project.

Mike

[julie_m]

I'm going to use the Raspberry Pi's GPIO pins as a 6-bit address bus, 8-bit data bus and read and write strobes, to interface to an expansion board using a bunch of 74HC TTL ICs. It won't actually take too much modification from my original design which was based on a Vellemann K8055. This will drive an array of relays, D-to-As and the variable HT PSU. It will have to have its own ADC. (The original design used the two K8055's two ADCs in a range-extending setup, with ADC2 fed 1/16 of the signal fed to ADC1, and just used a multiplexer on the extender board to select a measurement point.)

But I want to get the HT supplies for the anode and G2 sorted first. My last attempt kept going unstable and blowing itself up -- not ideal for something that's going to be plugged into an expensive piece of equipment

[mikeydee]

Great idea @AJS - I recently bought a Rasp PI to see how well it can run the home media centre - XBMC software. It's fairly good as a media player in terms of handling the formats but there are better set-top boxes options out there now (e.g. android) ... Well to cut to the chase I was wondering what other uses I could think of for a Rasp PI and since I am in the middle of building a Sussex I came to the conclusion that writing a program to drive a tube, take a few measurements and store the results should be possible. Perhaps an evolution of an existing tube tester would be a good approach since all the heavy lifting in terms of HT and heater supply is already done.

I will be interested to see how this idea develops...

[Nicko]

Originally Posted by PJL

2. Using PWM on the filament of a directly heated valve will also result in abnormal behaviour as there will be a much higher than normal voltage gradient along the filament. Many directly heated valves are intended for DC filament operation.

3. The heater voltage is limited and would not be suitable for some valves designed for operation in a heater chain.

Hi - I wonder if you could expand on these two points? What "abnormal behaviour" would you expect - can you quantify the effect or are you speculating?

The Heater supply is limited to the supply voltage of the PSU used (typically 19V) - what's the issue there?

Thanks

[Nicko]

Originally Posted by Nicko

Hi - I wonder if you could expand on these two points? What "abnormal behaviour" would you expect - can you quantify the effect or are you speculating?

The Heater supply is limited to the supply voltage of the PSU used (typically 19V) - what's the issue there?

I'll answer my own questions to PJL as no-one else wants to

First of all, I'll fess up - I own (and have done for many many years) very nice Hickoks: 600, 6000A and 539C. I also built a uTracer as a "to me" Christmas present and I'm very impressed with it...

2. If you read the uTracer docs you'll see that 1mS before the Ia/Ig/Va/Vs/Vg etc. readings are taken, the heater is disconnected and grounded. Ergo, how the heater is warmed up is completely irrelevant - there is no PD across the filament as both ends are at 0V and the heater up to temperature when readings are taken, i.e. no "abnormal behaviour".

3. The heater voltage (and current) for the uTracer is not limited to the laptop supply used (typically 19V @ 1.5A for the heater) - you can connect any external heater supply you wish - you don't have to use the internal one. So-called "supply chain" valves are therefore no problem...

Regarding other points raised:

4. There appears to be considerable debate about if cathode poisoning is indeed any sort of issue with old valves, and even if it is, whether the mechanism mentioned is a possible culprit.
See https://www.vintage-radio.net/forum/...ad.php?t=28839 and many other debates!

5. Currently, the uTracer is indeed a curve tracer, not an overall tester - however, as the h/w is essentially "dumb" and controlled completely by the Windows client s/w, I know that Ronald has plans in that direction...

HTH

[Dekatron]

Nicko, thank you for your extensive answer to these questions!

I have used the uTracer for a while now and it gives the exact same measurements as the AVO Mk III, Mk IV, CT160, CT160A and VCM163 and also the Taylor 45D and 45D2 plus the much more expensive RoeTest, I own all of these so I have been able to compare their measurements on many different valves.

[Nicko]

Originally Posted by Dekatron

Nicko, thank you for your extensive answer to these questions!

I have used the uTracer for a while now and it gives the exact same measurements as the AVO Mk III, Mk IV, CT160, CT160A and VCM163 and also the Taylor 45D and 45D2 plus the much more expensive RoeTest, I own all of these so I have been able to compare their measurements on many different valves.

Hey! You're in Linköping! - We cycled through there a while ago on the way to Motala to do the Vätternrundan... Lovely area...