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Old 17th Jun 2020, 2:02 pm   #29
AdrianH's Avatar
Join Date: Mar 2020
Location: Blackburn with Darwen, Lancashire, UK.
Posts: 851
Default Re: Yes, yet another home made valve tester.

Hello David;

I will try and answer all your questions in this post under the Homemade section and ask mods to close the other.

At present the small buck converter provides a variable voltage from +50 to +350 Volts DC, it needs to source around 15mA to be a stable output. There is a further 22uF HV cap in the tester, it may not be a perfect DC at present not seeing any flicker in the meter over 15mA draw.

if the likely anode current is small I load it with an external resistor which is not metered so I only measure anode or grid current.

There is also a negative rail output as well, this is not regulated as such but follows the positive rail, so when the positive rail is at +50 Volts the negative rail will be around -60 Volts, +150 and -162Volts etc. So the negative rail is used to provide a negative grid bias with a series resistor and Zener diodes.

I am only pushing the Buck converter at 40mA to 50 mA positive rail at present and it will happily sit at the for some hours. I have another of the same type that I plan to add to provide a independent g2 supply rail. I also have some of the better units meant for valve pre-amps with filament supplies built in, but the down side is they only have one positive rail and it starts at 150 Volts.

There are two banana sockets on the tester supplied from the single (at present HT rail,) one is directly from the rail and the other goes through the current meter. The scales are 4 and 40mA. So depending on which way I connect the terminals I can measure g2 or anode current and will typically do anode first, then screen. The buck shuts down with excessive draw.

So at present when it comes to testing valves that go over the meter range (40mA at present) I have been looking at the charts and checking against the lower current ranges of the vales, such as a few EL84's I have.

Of course the supply current is not that great for testing some power valves yet, and the plan is in the future to build a mains HV PSU. I guess I could see if the transformer for the Sussex and the PSU boards are still available, or my preferred option is to attempt rewinding my own.

No your valve is still sat in it's cardboard box yet until I add the separate rail.

When it came to ferrites for me it was a case of making sure, rather than adding two, undoing adding another etc. for the extra cost it was not an issue.

I have just received another batch of 2nd hand valves from Ebay that I can play with, if I damage any of these there is no great loss to anyone, the package even has valves in there which look to be new and would be spares for my Pye P76F and Vidor CN430 radios. So happy on that score.

But and this is a big but! I doubt if I will ever be a collector of valuable radios and/or valves, I plan to build a Radord design audio amp, a receiver, test the valves in my AR77 and generally try to build circuits as I go. The buying, refurbishing and then selling on which I saw on the Facebook side of this site does not appeal to me.

Avo meters. I have a model 8 mk7 sat next to me, it is duff, current ranges work but the voltage ranges do not, yet! It came from where I worked many years ago in a clear out as it was faulty and everyone went to DVM's including me. It may get fixed or offered on the site. I think the plastic tracks have a break in them and yes I could fix it

Hope that answers a few of your queries they have reminded me to add a 100mA fuse in line of the HT.

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