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Old 17th May 2010, 11:42 pm   #361
FRANK.C
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Default Re: The "Sussex" Homebrew Valve Tester.

Hi Mike
I wont be using that setup myself as I have already wound 2 extra heater transformers to give a range of voltages. I was just throwing the idea out there.

I have included a cathode/heater shorting switch as I was hoping to measure directly heated mains valves. But what you have said about not being able to measure Gm has took the wind out of my sail. Thanks for pointing that out it was something that I had never thought about.

I have along way to go before my "Sussex" is finished, it's a big project just to build, the fact that you designed it so well as well is really something else. You had to have spent a considerable time on it from start to finish

Frank
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Old 18th May 2010, 8:35 am   #362
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Default Re: The "Sussex" Homebrew Valve Tester.

Frank,
Yes, I was caught out myself when I made an adaptor to test 300B's.
In the end I created an artificial centre tap by soldering 150R resistors from each side of the heater to ground. This worked OK on the 300B which has a centre tapped heater, but I dont think the D series of valves have a CT filament. AVO seem to have overcome the problem by running the AC grid signal at around 18Khz and putting a tuned circuit in the GM meter amplifier. I didn't go down this path because it would make the tester more complicated, and also my interest are Ham Radio and audio where the D seies dont appear.
You are right, it wasn't a five minute job, I suppose I spent around a couple of months doing it, but, being tretired, I probably had more time on my hands than most.
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Old 18th May 2010, 11:48 pm   #363
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Default Re: The "Sussex" Homebrew Valve Tester.

My Sussex is just waiting a panel meter and final connections between PCB and the rest of the circuitry. All switches and valve holders are wired but I did have a job trying to get everything into such a small sized case. Hopefully nothing will short out when it all comes together but it has indeed been a challenge. I have no proper workshop facilities here and the worst task was cutting the meter rectangular holes with a hand file, the aluminium must be at least 3mm thick! I decided to add a Blue Cathode Top Cap connection so that when I get a D series valve to test I can simply use a patch lead between it and the Blue External Heater Terminal post as this is un-switched. My camera has gone away for service so I can only repost the drawing I have sketched for it, but it is pretty close to what I have.
Les
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Old 19th May 2010, 12:29 am   #364
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Default Re: The "Sussex" Homebrew Valve Tester.

Hi Top Cap,
Looks good and I know how much hard work it is with 3mm aluminium! I have the joy of cutting out the meter ports to look forward to this weekend.

Given that you are using a small footprint for your case have your ported this to allow for air circulation at all?

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Rob
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Old 19th May 2010, 6:05 am   #365
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Default Re: The "Sussex" Homebrew Valve Tester.

Hi Rob, the case comes with side vents either side, good job too as it is made of steel and thankfully nearly all metalwork, including the fan vent, involves the aluminium top. I had to design the layout the old fashioned way whereas the youngsters at work have a 3D CAD system costing 1000's which they use but still get holes in the wrong place
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Old 19th May 2010, 7:53 am   #366
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Default Re: The "Sussex" Homebrew Valve Tester.

Les,

That's very funny.

I can imagine you watching these 'youngsters' working and tutting to yourself.

Are we now just 'grumpy old men'?

Andy
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Old 19th May 2010, 8:55 am   #367
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Default Re: The "Sussex" Homebrew Valve Tester.

Just one quick question: Why use separate sockets for top caps?

As far as I know, they are only found on 8-pin valves; so couldn't you just have a single "top cap" socket, and use the rotary switch for pin 9 to select what goes to it?
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Old 19th May 2010, 6:01 pm   #368
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Default Re: The "Sussex" Homebrew Valve Tester.

Hi Folks, I still have there transformers available, including US primaries. But be warned that present deliveries to the US can take 4 weeks even when posted on a 5/6 day airfreight service. This appears to be due to customs clearance at the destination.

Ed
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Old 20th May 2010, 10:52 pm   #369
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Default Re: The "Sussex" Homebrew Valve Tester.

Well the last panel meter arrived today and I had a chance to do some more work on the VT. Still have a lot of wiring to do but it is almost there. I had planned to wire the Top Cap to switch 9 in my earlier plans for a VT before the Sussex came along but reckoned that one day someone would want me to test a PL81 or something similar so opted for the separate sockets, at least I have some handy test points . Still waiting for the pukka switch stators from Sifam so I made a few just to see what they will look like before they arrive.
The labels could have been better I suppose but at the end of the day, it is only for home consumption and not going into a shop window.
Les
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Old 20th May 2010, 10:59 pm   #370
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Default Re: The "Sussex" Homebrew Valve Tester.

Looking good Les.

Can anyone give me a 'heads up' on where 1 can get a pair of BUZ80's?
The guy on ebay who sent me a dodgy one seems to be unwilling to respond to my messages.

Andy
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Old 20th May 2010, 11:03 pm   #371
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Default Re: The "Sussex" Homebrew Valve Tester.

I bought my BUZ80's from Cricklewood. 1.50 each
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Old 20th May 2010, 11:11 pm   #372
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Default Re: The "Sussex" Homebrew Valve Tester.

Thanks Les.
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Old 20th May 2010, 11:21 pm   #373
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Default Re: The "Sussex" Homebrew Valve Tester.

Excellent work Les really looking good.

Frank
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Old 20th May 2010, 11:39 pm   #374
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Default Re: The "Sussex" Homebrew Valve Tester.

Hi Les,
That looks a very professional item - congrats.

I'm now green with envy.

Regards

Rob
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Old 21st May 2010, 8:43 am   #375
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Default Re: The "Sussex" Homebrew Valve Tester.

Les,
What a superb instrument, I'm really impressed. Just goes to show what can be done.
Just a thought for those interested in testing the "D" series of valve. \forget trying to get the heater volts from the Sussex, use a D cell connected to the external supply either internally or externally. This will give a DC voltage which will obviate any GM problems. A high capacity cell will give a very long life, as the valves only draw about 100mA and would only be used for short periods at a time. One side of the heater will of course have to be connected to cathode either via a removable link or switch.
Good luck to the remaining builders - at leat now you have a target to aim for

Mike
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Old 21st May 2010, 9:31 am   #376
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Default Re: The "Sussex" Homebrew Valve Tester.

Hi Mike
The D cell is a great idea.

You previous post has given me food for thought. Thanks for that information especially on how the AVO's get around that problem.

As I have already got a heater transformer wound I will have a play with making a filter to remove the 50 Hz. I would think that the rejection would need to be considerable as the grid signal is much smaller that the AC across the heater. I may well try filter + balancing resistors across the heater.

Chances are that I will fail miserably but it is all good fun.

Frank
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Old 22nd May 2010, 8:59 pm   #377
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Default Re: The "Sussex" Homebrew Valve Tester.

Well I finished wiring my unit, heater selection and G1 volts control ok and the oscillator works and set to 1V rms. But have a small problem with the Anode and G2 voltage selection. They both work at maximum output down to 225V but then further reduction makes no effect and even with both switches set to 0V, I get 220V. To re-cap then, from 0 to 200V the Anode and G2 give 220V but the last three positions work and give me 250,275 and 300V. I have checked the selector switches and the resistor values are correct throughout the full range, I had previously checked these anyway prior to fitting and measuring the Gate voltages show the correct incremental inputs. So for some strange reason both BUX80's are not allowing the Gate to control output unless I want more than 220V! Very strange and annoying as I shall have to remove the pcb stack and test in isolation, boy am I glad I fitted a connector to it as removing all the solder connections would be such a mission.
Les
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Old 22nd May 2010, 10:18 pm   #378
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Default Re: The "Sussex" Homebrew Valve Tester.

Les,
I hope it was a typo, you should be using BUZ80 which is a MOSFET, not a BUX80 which is a transistor.
A transistor will not work due to the high impedance of the driver voltage.

Regards
Mike
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Old 23rd May 2010, 5:53 am   #379
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Default Re: The "Sussex" Homebrew Valve Tester.

Yep, was a typo Mike and BUZ80's are fitted. All measurements taken off load, just wondering if some kind of load is required, even just a few mA. I am going to check out all wiring first as it is easy, at my age, to make a mistake
Les
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Old 23rd May 2010, 7:46 am   #380
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Default Re: The "Sussex" Homebrew Valve Tester.

Les,
Know the feeling.
Check the voltage on the output from the anode & screen volts switches, you could have a ground connection fault. BUZ80s are static sensitive, and are easily damaged outside the circuit, but pretty well bombproof in circuit.

Regards
Mike
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