Re: The "Sussex" Homebrew Valve Tester.
 

Hi Mike,
Thanks for the info, I did think that I was missing the plot as I couldn't see anything in the BOM. I have a 50k dual gang pot.

Regards

Rob

Re: The "Sussex" Homebrew Valve Tester.
 

Rob,
Would suggest a multi turn, makes setting the grid bias much easier

Mike

Re: The "Sussex" Homebrew Valve Tester.
 

Hi Mike,
Thanks for the update and I will put this on order

Regards

Rob

Re: The "Sussex" Homebrew Valve Tester.
 

Originally shown as 30k, I think the more easily obtainable 47k will be ok.

With regards the D1/D2 options I totally agree. It might be useful for finding an open circuit inside an EB91 but how often does this crop up?

I'll keep an open mind about it though and may include something later just to
make everything look like an AVO VT when using its handbook info.

The pcb has arrived (Thanks Martin) so it is time to get drilling.
Les

GEE Sorry - I was well and truly caught out by the page turn over LOL

Re: The "Sussex" Homebrew Valve Tester.
 

Sorry,

Isn't it VR3 on the BOM?

Andy

Re: The "Sussex" Homebrew Valve Tester.
 

I have made some updates to my original circuit sketch which may be of interest.
Having received the PCB, I had it guillotined to separate the main part from the section that contains the heater switch relay RL2 and the short circuit detector. I needed to do this anyway to allow the main board to fit in the planned enclosure.
I have not separated the 2 smaller circuits but kept them together as another board.
There is space between the two small circuits to allow RL3 to be mounted, I shall probably use RL1 (when it has been drilled) as a template for the holes as there will be no track to use as a guide. A small dab of adhesive should be sufficient to hold the relay in postion along with careful bending of the relay connections.
I plan to cut the track feeding the pin C3 so that I can mount the 10k resistor (I call it R29) that feeds LED 4.
I have added some track detail on the main board to assist the placement of the 3 wire links. Note that there are two C12's on the original board drawing, the one located close to VR2 should be C7 and this has been corrected on my sketch.
Also, I have added the two extra diodes (D1b and D2b) which I hope to mount using the board pins at AA, AB, AC and AD.
Just hope I have not made any mistakes but the BOM will have to be changed if my new component allocations are used, namely the components on the small circuits like R29, R30, R31 and R32.
Les

To save you looking for it the link to the circuit and pcb component side is:-

http://homepage.ntlworld.com/lez/Sus...nstruction.JPG

Re: The "Sussex" Homebrew Valve Tester.
 

Hi Andy,
I was under the notion (or misapprehension) that vr3 was a pcb mounted component and not fitted to the front panel with flying leads to the board.
My mistake.

Regards

Rob

Re: The "Sussex" Homebrew Valve Tester.
 

With this thread in mind, is it worth building in a high-voltage test for heater-cathode leakage?

Re: The "Sussex" Homebrew Valve Tester.
 

Hi
I dont think that much needs to be done to the H/K leakage, AVO only use 50v for insulation testing,very similar to the Sussex. Using the LED, a 47k leakege just illuminates the LED, bearing in mind that in most cases, the cathode resistor is usually a low value, which is by-passed, the impedance at the cathode is low, so 47k leakage would have very little effect on performance, and this can be seen on the LED.
In the case of the main leakage LED, any leakage is amplifed by the transistor and on my tester a 10M resistor just illuminates the LED.
If you want a more sensitive leakage indiactor on the H/K duplicate the main leage circuit. Be warned however, a 1M H/K will light the LED brightly, and you could be discarding valves which are perfectly serviceable.

Mike

Re: The "Sussex" Homebrew Valve Tester.
 

Hi Mike, as requested by another Forum member I have added transistor lead out info with ghosted outlines for TR6,7 and 8 on my sketch at the link in my last post. While doing this I think I found an error on C8 and D6 as these are reversed against the circuit diagram. I have corrected this on my sketch but just hope this is now correct as I am banking on the circuit diagram being correct i.e. C8 +ve at VR2 and -ve at TR7 emitter.
Cheers
Les

Re: The "Sussex" Homebrew Valve Tester.
 

Andy,
You are correct with your observations, at this end I put it down to Anno Domini !!!.
Thanks for a very professional looking presentation.
Mike

Re: The "Sussex" Homebrew Valve Tester.
 

Hi, Mike,
Some questions that I hope you would care to give me some answers :
a. should the regulator keep the anode voltage at roughly the same value before and after a tube is inserted for testing
b. does the regulator eliminate most noise and ripple of the power supply that affect the accuracy of any measurement (if you have taken some measurements on noise and ripple, at what values could these be considered as acceptable?)
c. I read elsewhere that when measuring gm (transconductance), a sine wave signal should be applied to the grid. How do you supply that signal to the Sussex tester?
Thanks.
John

Re: The "Sussex" Homebrew Valve Tester.
 

JOHN
In answer to your questions
(a) The voltage stays pretty constant whether a valve is in or out of the tester.
(b) The Constant current supplied to the zener chain supplies a smooth supply to the anode and screen voltage switches, which in turn contol the gate of the mosfets. I havent measured ripple and noise, but valves tested and compared with a VCM163 indicate that all is OK
(c)The sine wave is applied to the grid through C7 and is adjusted by VR2

Hope this helps

Regards
Mike

Re: The "Sussex" Homebrew Valve Tester.
 

Hi, Mike,
Thanks very much for your quick reply. The reason for my questions is that I have built a prototype of the anode voltage regulator (only the part from D3 to TR1, TR2, TR3, and output via R5, R6, and R7) on a breadboard with all the components (except I replaced the voltage selector switch with a 1mega ohm pot and the mosfet is a IRF730) as stated on the schematic, but it doesn't work properly as expected. May be I should report some values I have measured:
Power transformer secondary: 250 vac
After a half wave rectifier: 324 vdc
Output from the anode regulator: 250 vdc, can be adjusted to all the way to 0 v. Voltage remained stable even if left on for over an hour.

I then tested the regulator with a 12AU7, and later with a 6922. The cathode (pin 3 and 8 were connected to ground). The moment the tube was inserted the voltage dropped to 90 vdc at the anode, grid negative voltage was set at -6.5vdc. Anode voltage could still be adjusted but only downward by the 1 megaohm pot. Adjusting the grid voltage could changed the anode voltage but the max value was still 90 vdc. It seemed the regulator just woundn't hold the voltage at the preset point. I also noticed that the mosfet didn't get hot at all. Should it get hot when working properly?

Any idea on what went wrong?

Regards,
John

Re: The "Sussex" Homebrew Valve Tester.
 

Hi All,
Given the size of the custom transformer for this project (thanks Ed) do you think that there is a potential for heat dissipation problems within an enclosed case?

Thanks

Rob

Re: The "Sussex" Homebrew Valve Tester.
 

John
If the voltage can be varied with with no load, it looks as though there may be current limiting taking place. How much current is being drawn? Check around TR3 and associated components. I assume the gate voltage was OK and also the voltage on the pot slider.
On mine, I tested an EL34, with the voltage set at 250, the actual voltage with a 75mA load was 247, negligible difference.
Other than that I have no other ideas to offer, may be some else has
Regards
Mike

Re: The "Sussex" Homebrew Valve Tester.
 

The circuit was designed for full wave rectified input, would applying just half wave cause the problem?
Do you have the Drain of FET TR2 correctly supplied from the un-reg HT line?
Are R6 and R7 correctly connected in parallel and both 10 Ohms?
What voltage on the gate of TR2, if low then the limiter TR3 may be coming in early.
Les

Re: The "Sussex" Homebrew Valve Tester.
 

I'm waiting for my transformer to arrive before selecting a case.

I might even get it made for me by frontpanelexpress.
I hate the thought of butchering some perfectly good piece of aluminium and ending up with something scruffy

:-)

Re: The "Sussex" Homebrew Valve Tester.
 

I received mine in the week and its relatively big hence my comment about cooling. I'm sure that this subject was raised early on.

Re: The "Sussex" Homebrew Valve Tester.
 

...on the subject of cases I looked around and couldn't find anything suitable so I have ordered some sheet aluminium and will build a case to suit.