The "Sussex" Homebrew Valve Tester.

[Pamphonica]

I have had quite a few PM enquiries recently about sourcing thumbwheel switches.

I got mine from Electrovalue, a fine place for odds and sods of surplus stuff. But I cleaned him out and he says he definitely has no more. The ones I had made up only 3 or 4 sets. All I have left is a lot of end-cheeks!

For reference they are about 2.25" square and have very wide-spaced tracks on the pcb. The action is rather jerky and stiff but at least they click positively into place.

I have yet to test them on higher heater currents but they look fairly meaty.

For anyone new to thumbwheel switches it's worth noting that virtually all currently available (surplus) switches are BCD coded (binary, 4-contact) not decimal, which is useless. Make sure you see a full set of 10 contacts on the back, plus common.
Contacts marked 1,2,4,8 instead of 1,2,3,4... are a giveaway of the BCD type.

If I find a good source of big switches I will post here.

All the best
Jeremy

[Brian R Pateman]

These from RS should be OK. Bit pricey though.

Bear in mind that the ratings are for make or break under load. Presumably you are going to set the switches up off load as with the AVO VCMs. The current rating isn't so important under those circumstances.

[a_strong]

Herewith a first attempt at producing numbered skirts and background plates for DIY rotary valve pin function selector switches. The diameter of the skirt is about 28mm and the plan is to print them onto OHP transparency material and them laminate them for a bit more rigidity before finally cutting them out and supergluing them to the underside of suitable control knobs.

Alan

[a_strong]

Hi all - egg on face time, spot the deliberate mistake. Back to the drawing board!

Alan

[eddie_ce]

I have just stumbled onto this thread. What a marvellous project - thanks to all involved. Another example of what a fine community we have here.

I was in the process of building another design, but after reading this thread (took best part of yesterday evening ) have binned the idea and will go for the "Sussex".

However, I would like to use the stage I have already bulit - the heater supply.

I have the following questions:

1. Are there any reasons for not using a DC heater supply - my circuit will supply up to 2.5A DC?

2. Can you give me an idea where to obtain the DPM's - the links mentioneD in the thread all seem to be dead and as there are so many types of DPM on offer I would like to make sure I buy the correct types.

Thanks guys
Eddie

[swordholder]

Eddie,
I dont see any reason why DC could not be used for the heaters, I didn't originally use it for obvous reasons, cost size etc.
Re the DPMs I found these
http://myworld.ebay.co.uk/ithingsonl...id=p4340.l2559
although I havent used them, they shoud be OK

Mike

[RF Burn]

Eddie,

I have used this source for my Sussex DPMs

http://stores.ebay.co.uk/1stincity-U...=p4634.c0.m322

All the ranges needed for the Sussex VT are available.

Adrian

[a_strong]

I have just bought four meters from http://stores.ebay.co.uk/Asia-Engineer?_rdc=1.

If you are buying more than one meter Email the seller (Giorgio) for a price for the lot, rather than ordering them individually. I saved a bob or two. Postage can be a bit slow. Although they were despatched straight away mine took three weeks to arrive.

Alan

[Ed_Dinning]

Hi Eddie, make sure you have 3 seperate and isolated supplies for the DPM's, or you will have shorts between the tester supplies.

Ed

[domm1kta]

Hi all,

Bit late to the party but going to build this as and when can get all the parts. I have a HRO that is feeling poorly and a couple other sets could do with some attention and testing the valves is required. The simple bit I guess was the 3 DVM and they are on order from HK already as I was buying a couple for a separate meter project.

Daft question but has anyone compiled a BOM with item numbers from Farnell et al? I have seem a few 'lists' but not the item numbers.

72

Dom
M1KTA

[domm1kta]

Hi Mike et al,

All parts (except DVM and transformer) arrived (Rapid supplied), with an additional eBay purchase. Also I was sent a pair of PCBS and will hand one to another interested party. Will probably start to melt solder on this before next weekend.

72

Dom
M1KTA

[Andymic]

I'm 're-engineering' my VT-1 to get rid of my rats nest of cables.
Will be three, maybe four PCB's.
One for the switch network, one front panel, one main PCB and perhaps a valve socket PCB.
I've finished the layout of the front panel PCB (moved some of the main board components to this PCB to simplify wiring. I've also finished the switch matrix PCB.
I'm using Phoenix connectors for board interconnects.
Next up is the main PCB.

I'll post some images when I get further on with this.

Andy

[eddie_ce]

Hi all,

I'm a bit unclear as to the measurement of Gm.

Am I correct in assuming that

a) the level of audio-signal injected into the grid circuit is fairly critical and should be a stable 100mV rms?

b) the audio frequency is not that critical. What is the nominal frequency of the Sussex phase-shift oscillator?

Next question, how does the gas test work?

Thanks guys.

Eddie King

[a_strong]

HI Eddie - Les (Top Cap) has kindly produced an excellent document summarising the Sussex design. This can be found at post #502 which should appear on page 26.

He says this about the GM test.

Start quote:

"Gm is a measurement of gain and is in fact the change in Anode current (mA) from a change in Grid voltage (V). In the Sussex we apply 0.1V to the grid of the valve under test and measure the change in Anode current across a 10 Ohm resistor R12. Let us assume a valve under test has a gain of 25, if we were using 1V as a grid voltage we would expect a change in Anode current of 25mA and therefore there would be 250mV developed across R12. But as we are using 0.1V signal then the voltage developed
will be a tenth of this i.e. 25.0mV. This figure is what will be displayed on the front panel Gm meter.

Due to failures encountered with this meter under fault conditions, for example if the Anode supply FET goes short circuit, a protection diode placed across R12 is being trialled.

The diode has a forward turn on voltage of 600mV approximately so should not affect the 100mV max expected in normal use.

Whether a simple diode on its own will be sufficient has yet to be proven!"
End quote.

If you cannot find the Les's document I can email it to you if you send me a personal message.

Regards - Alan G3WXI

[Station X]

Here's the link to the handbook:-

http://homepage.ntlworld.com/lez/Sus...%20version.doc

[Andymic]

I've been a busy bunny

This is what I've done so far......

I know it'll need a few tweaks but I want to finish a board for the valve base interconnects after which I'll sit them all side by side and do a net check against the circuit.

Andy

[Mr_Klaatu]

Looking great !

Should enable a very neat and compact build.

I'll be interested in a set if you decide to do a run of these.

[G8UWM-MildMartin]

Hi Andy,
Congratulations:
They look an excellent enhancement to the project.
(And come with near-perfect timing as I have only 2 of the batch of 50 to the original design left.)

[RF Burn]

Andy,

Nice work, however, do check the track/track/pin spacing on those boards, there look to be some pretty close bits on there considering the high voltages that are in use.

Adrian

[Andymic]

Adrian

Yes, I've set my routing rules for a minimum 0.8mm track spacing.
Isn't this enough?

Andy