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  #1  
Old 12th October 2015, 10:38 PM
oceanobsession oceanobsession is offline
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Default valve testing el84 only

Hi folks , ive been looking at just testing the el 84 , ive seen this jig and would
base my tester on this , but I need some help understanding it this is the link

http://golbornevintageradio.co.uk/fo...d.php?tid=4877

its basically a 9ba socket and 10x 4mm plug sockets wired to each pin , then you supply the voltages ad 2 resisters and read off the anode current in ma
which is 4.8ma for the el84, can anyone look at this link and explain it to me .
cheers phil.
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  #2  
Old 13th October 2015, 12:23 AM
bikerhifinut bikerhifinut is offline
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Default Re: valve testing el84 only

It's a simple jig where the 4mm sockets are wired to the marked corresponding pins on the valve sockets plus one for a top cap and an earth connection in case the croc clips on the resistors slip
My advice to you.................... DON'T do this, I get the impression you don't really understand how the thing works.
By the way an EL84 should pass 48mA not 4.8 under normal operating conditions.
But don't get the idea that an EL84 wouldnt work under different conditions. I have a Push Pull amp using EL84's out of an ECKO Gramophone where the valves are set to draw 32mA each at a reduced Ht of 210V ish.
if all you want to do is check that a set of EL84's are working then why not just check the voltage across the cathode resistor in the amp they are meant to be in and check against the amps spec. A simple bit of Ohms law will quickly tell you what current the Valve is drawing.

That's a lash up jig to test a batch of known valves for a couple of simple parameters using a bench PSU and a couple of Voltmeters.
It's not a Valve tester as such.

Andy.
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  #3  
Old 13th October 2015, 08:31 AM
bob orbell bob orbell is offline
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Default Re: valve testing el84 only

Good grief, that's for a DIY DIYourselfer , I completely agree with Andy, don't bother, our super moderator Richard has a valve tester, it may be worth asking if he will test your valves, he is a very nice chap, grovel grovel BOB
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  #4  
Old 13th October 2015, 11:07 AM
Richard Richard is offline
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Default Re: valve testing el84 only

Yes as Andy and Bob say, if it's cathode biased, check the cathode voltages in the amp. Also check the g1 voltage which should be 0V (but may be a few tens mV if pio coupling caps are used) as el84s often start to draw grid current as they age then run away. Let them get hot and anything over half a volt there needs checking (try a known good valve). Happy to check yours on the Avo tester if you PM me.
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  #5  
Old 13th October 2015, 12:51 PM
oceanobsession oceanobsession is offline
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Default Re: valve testing el84 only

Thanks folks for your advice, my amp is a manley stingray so it has 8 el84
you can switch it to run in triode 18w or ultra liner 40w , each valve has a pot to adjust the bias current you set them at 250mv , if you remove a valve that
has been set at 250mv and place another one in the same position and without
adjustment it reads 150mv would that be any indication of a worn valve or if
it were to read 350mv would that be any indication of a worn valve , I have a
lot of ei el84 which I would like to know there condition , this is why I would like to build a jig to test only the el84 , im great with the soldering iron , I have a fluke multimeter and a twin variable voltage supply uptp 30 volts
2 amp current which would power the heater element , I suppose you would
need to find 250dc for the other voltages and would I be right in thinking there is also a couple of resisters , would you use the data for class a where I think 3 pins are not used , does the valve have an earth pin , im fully aware that 250v dc will kill and is more dangerous than ac , ive been looking for a
tester but there silly money for old crap , the taylor 45c would be my choice
though , thanks for the offer to test the valves but im in Birmingham so a fair
way from you , but thanks anyway . its a pity you cant design a el84 base
socket with out wires to plug in to the amplifier that you already have because
you have all the voltages you need , and then read via the resisters the ma of
the anode . that's all folks ive a lot to learn but im trying cheers phil.
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  #6  
Old 13th October 2015, 01:18 PM
Richard Richard is offline
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Default Re: valve testing el84 only

Search for the Mullard circuits online. There are only a handful of components needed to build a single cathode bias power amp
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  #7  
Old 13th October 2015, 01:26 PM
bob orbell bob orbell is offline
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Default Re: valve testing el84 only

You could always post your valves to Richard, after all, if you buy new valves they come by post. BOB
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  #8  
Old 13th October 2015, 11:50 PM
oceanobsession oceanobsession is offline
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Default Re: valve testing el84 only

Test Conditions 250 Va
250 Vg2
Ia = 48 mA
Rk = 135 ohms

These are the test conditions for the el84 pins 1 6 8 are not used , heater element is pin 4 and 5 , would you earth one and supply 6.3 volts dc to the other one ,

This leaves the pins 2 3 7 9 I think 7 and 9 has 250v dc or ac ,

how would you connect pin 2 and 3 I think you need resisters to measure anode current cheers phil.
ok ive just read that the control grid requires a negative voltage and a variation in this alters the anode current by a large amout but for test proposes it would be fixed at
-7.3 volts

Last edited by oceanobsession; 14th October 2015 at 12:37 AM.
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  #9  
Old 14th October 2015, 08:59 AM
Richard Richard is offline
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Default Re: valve testing el84 only

Or increase rk to 270R but neither will let you see if an old valve is drawing grid current. Search kel84 PDF online and you'll see the typical el84 p-p output stage and pinouts.
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  #10  
Old 14th October 2015, 10:02 AM
John Caswell John Caswell is offline
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Default Re: valve testing el84 only

Oceanobsession,
Contrary to what you say "el84 pins 1 6 8 are not used".
This is incorrect, they are internal connections and can be used for anything depending in the whim of the manufacturers.
I have had several makes on the bench at the same time and all have differing internal connections to them, mainly to support the structure.
Proceed with care!!

John
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