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Old 18th Oct 2020, 7:29 pm   #1
Electronpusher0
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Default Solid State display for the Mullard High Speed Valve Tester

I sold my Mullard High Speed Valve Tester (MHSVT) to a fellow member but could not resist when offered a Mk 1 version that had been stripped of valves and without cards but at the right price.
The challenge became one of trying to get it functional at minimum cost. This eventually morphed into a challenge of replacing the CRT meter with an LED bar graph display.

The first move apart from replacing capacitors and out of tolerance resistors was to replace the rectifiers with silicon devices and to replace the gas discharge voltage stabiliser with a chain of zeners.
It is a sad fact that some of the valves used in the MSVT have become highly desirable to audio aficionados and are now fetching astronomical prices. The most valuable valve, or at least the one fetching the highest prices, is the EL37 used as the series regulator for the Anode supply (V5).
The EL37 was made obsolete by Mullard in 1954 and replaced by the EL34. Functionally these valves are identical with the same power and gm rating. The only difference when using them is that in the EL37 the G3 is connected internally to the Cathode on pin 8 whereas in the EL34 the G3 is brought out independently to Pin 1. Pin 1 has no connection on the EL37 and this pin on the valve base is used as a convenient connection tag on the E7600 (and possibly on others).
The EL37 can be effectively replaced by the EL34 if pins 1 and 8 are connected together, having first removed any wires on Pin 1 on the valve base. Obviously any wires removed from Pin 1 should be kept connected together and isolated from other circuitry.

In the Mk1 the amplifier for the CRT meter is an ECC35 dual triode, it was replaced in later models by two EF91s
The ECC35, introduced in 1945, has also become very desirable for audio and carries a matching price tag. In this application the ECC83 is a suitable replacement electrically, of course the ECC35 has an Octal base and the ECC83 a B9A. However a suitable base adaptor can be made to allow the ECC83 to be plugged into the Octal base, see photo of the adaptor I made for my MHSVT.

I had a spare EF51 in the junk box so at this point I had a working MHSVT. Unfortunately I just cannot leave a good thing alone.
It occurred to me that the CRT and associated dual triode amplifier and rectifier(s) are only there to provide a robust voltmeter of approximately 1.3V fsd.
I got to considering how I would replace this with a modern solution. This the result.

The circuit described here uses a 20 led bar graph to provide the same function of the CRT.
It uses two LM3914 ICs to drive four red LEDs, two Yellow LEDs and fourteen red LEDs.

I purchased a few 10 way bar graph LED displays in Red, Yellow and Green, looking at them I wondered if they would still function if cut down, only one way to find out, and indeed they do. Consequently I cut them down to the requisite 4 red, 2 yellow and 4 green. One green display I kept at 10 way. I glued them all together to form the complete display.
The schematic of the complete display is attached.
The supply is derived from the now redundant heater supply for the ECC35 (EF91s). This is rectified, smoothed and stabilised to 6V by a 78L06. I am afraid I am guilty of the sin that all designers commit, I designed in the component I had in stock. Itís possible a 5V regulator will do but I have not tested this.

A +2.5V regulator is used to offset the M- connection, this is done to allow the display to work as a centre zero display if the switch marked SAfa is closed. This switch is part of the MHSVT. This can be provided by either a LM336 or TL431.
The set up procedure is written on the schematic.
A switch is provided to allow the display to work as either a moving single bar, which emulates the MHSVT or as a bar graph display.
The 39k resistor in series with the input extends the input voltage tolerance to +/- 100V.

The whole circuit must be isolated from the 0V of the tester, the only connection to the rest of the circuit is by connection to the 115K resistor.
I constructed the circuit on vero board and mounted it to a plastic sheet with a cut out for the display, this was mounted using the original crt mounting points.
The window was replaced with a new piece of Perspex with red, yellow and green film applied.
I am happy to report it works perfectly. See pictures

Peter
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Old 18th Oct 2020, 7:35 pm   #2
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Default Re: Solid State display for the Mullard High Speed Valve Tester

More pictures.
I guess the only thing to do now is to replace the last two remaining valves with silicon.
Oh and find a set of cards (I made a set of setup cards and one for the EL84)

Peter
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Old 18th Oct 2020, 8:20 pm   #3
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Default Re: Solid State display for the Mullard High Speed Valve Tester

I like it.
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Old 18th Oct 2020, 8:37 pm   #4
turretslug
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Default Re: Solid State display for the Mullard High Speed Valve Tester

An ingenious, appropriate and sympathetic piece of updating.
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Old 18th Oct 2020, 11:57 pm   #5
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Default Re: Solid State display for the Mullard High Speed Valve Tester

Excellent work
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Old 19th Oct 2020, 9:21 am   #6
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Default Re: Solid State display for the Mullard High Speed Valve Tester

That's a really nice job, dare I say I think I prefer it to the original!
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Old 19th Oct 2020, 9:28 am   #7
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Default Re: Solid State display for the Mullard High Speed Valve Tester

What a great job!
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Old 19th Oct 2020, 9:38 am   #8
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Default Re: Solid State display for the Mullard High Speed Valve Tester

Thank you all for the positive comments.
I admit I was rather apprehensive about upsetting those who prefer to keep things as original as possible.

Peter
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Old 19th Oct 2020, 11:06 am   #9
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Default Re: Solid State display for the Mullard High Speed Valve Tester

Great job Peter. If mid 20th century valve testers are to work reliably, well into the 21st century, then such innovations are the way forward.

Regards, David
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Old 19th Oct 2020, 11:29 am   #10
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Default Re: Solid State display for the Mullard High Speed Valve Tester

Very elegant solution, using readily available components. Excellent design and implementation.
Jeremy
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Old 19th Oct 2020, 12:11 pm   #11
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Default Re: Solid State display for the Mullard High Speed Valve Tester

In my opinion anything that makes an instrument usable is always ok. As long as it works correctly I have no problem with modern ways of getting an instrument up and running. I wish it was just as easy to replace an AVO VCM meter with lets say a TFT display (it has been done, but it wasn't an easy design/replacement).

However for people who collect items for their design and history it is in my opinion always best to have an original instrument, but these instruments often just sit on a shelf to be looked at rather than seing everyday use.

Also, if you want to show how the original instrument worked it is in my opinion necessary to have the original parts so that you can show that it works in its original design - especially of you want to show that the new design also works, it is hard to compare and determine that if you can't show that properly.
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Old 19th Oct 2020, 1:33 pm   #12
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Default Re: Solid State display for the Mullard High Speed Valve Tester

Quote:
Originally Posted by Dekatron View Post
I wish it was just as easy to replace an AVO VCM meter with lets say a TFT display (it has been done, but it wasn't an easy design/replacement).
I have breadboarded a 7 segment type digital display to replace the meter on a 2 panel AVO valve tester.
https://www.vintage-radio.net/forum/...7&postcount=14
This works in simulation but I have not been able to try it "for real" as I do not want mess with my 2 panel tester. That one is a full restoration in wooden box.

I think I will put a post in Wanted for the control panel of a 2 panel tester with a bust meter.

Peter
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Old 19th Oct 2020, 2:07 pm   #13
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Default Re: Solid State display for the Mullard High Speed Valve Tester

That's a really good job. An excellent way to rebuild a piece of kit that would normally be BER due to unobtanium parts.

I have often wondered about a possible EL34 conversion, but fortunately I have a spare EL** for mine...
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Old 19th Oct 2020, 2:42 pm   #14
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Default Re: Solid State display for the Mullard High Speed Valve Tester

Absolutely stunning - and shows the way forward as unobtainium takes over the world - much much better to have a working unit even if it isnít 100% original rather than having it sitting on the shelf gathering dust, or worse - back in the day we would not have hesitated to adapt etc when confronted by an unobtainable component - whatís different now?
That said it is always preferable to restore to original if possible but not at any cost

Well done

Rgds to all
John
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Old 19th Oct 2020, 3:29 pm   #15
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Default Re: Solid State display for the Mullard High Speed Valve Tester

Looks right, though, doesn't it?

Nice job!

David
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Old 19th Oct 2020, 8:41 pm   #16
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Default Re: Solid State display for the Mullard High Speed Valve Tester

That is rather good! I often thought the original use of the CRT was a little underwhelming, I've toyed with the idea of making the rather boring dot into a line instead, just to use a little more of the phosphor!

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