Testing an EL84 as a 6V6

Captn Dave · 28th June 2009, 04:10 PM

I have a B&K 500 tube tester which I think is a very useful tester, however it won't test 9CV tubes like the EL84. A major shortcoming I think.

I wired up an adaptor with a 6V6 base and a 9 pin socket wired to the 9CV configuration, pluged in an EL84 and found that I am getting no cathode current to speak of. (BTW- 6V6s test just fine with proper cathode current).

I have 190 vdc plate voltage, 154 vdc screen voltage and a grounded cathode. Bias is -9 vdc. Heater voltage is 6.3. These values are measured off the 9 pin socket and I'm counting pin numbers correctly. I have tried several known good tubes.

I would expect high current with such low bias yet practicaly nothing (0.4ma). Any ideas?

Cobra2 · 28th June 2009, 04:28 PM

You can...with the 610 adapter...

Arne K

Captn Dave · 28th June 2009, 04:51 PM

Arne,

Thanks for your comments. I've not seen one but If I'm not mistaken the 610 panel was intended for the B&K 650. Mine is the 500. I looked at the wiring instructions at the Tone Lounge and couldn't gleen much about the construction from them as they only show the 8 pin socket.

Anyway, I don't have a 610 and I am looking for the DIY solution. The poor mans 610 panel I suppose.

Dave

Cobra2 · 28th June 2009, 05:06 PM

Maybe this helps?

Arne K

Ty_Bower · 28th June 2009, 10:49 PM

I can't find any datasheets to confirm my theory, but I suspect with 154 volts on the screen, the tube is cut off when you apply the -9 volt bias. Try a little less bias, maybe -7 volts or so. Otherwise, try raising the plate and screen voltage.

Edit: here's some datasheets:
http://frank.pocnet.net/sheetsE2.html

Look at the triode curves. Plate and screen tied together, 190 volts on both, the -9 volt grid line puts you pretty close to no conduction.

Captn Dave · 29th June 2009, 01:16 AM

Ty,

You are exactly right. -9 was too much bias.

The model 500 has a voltage divider that provides -18.1, -8.1 and -2.9 volt bias taps. I split the divider fruther, by using two resistors equal to the sum of the original, to get a 6.1 volt tap which seems to be about perfect for the plate voltage. It seems to work very well now.

I did not fully appreciate how much the bias needs to drop as the plate voltage is reduced. Thanks for the help.

Dave

dorrisant · 18th July 2011, 02:37 PM

Dave,
Did you ever come up with sensitivity settings for the EL84s? I have a 500 and would love to be able to reliably test these tubes. Any diy info you could post would be much appreciated. Ex: which resistor did you split for your 6v tap? I have a complete manual with schematic and a calibration setup as well for reference.
Thanks,
Tony

DF96 · 18th July 2011, 03:45 PM

Quote:

Originally Posted by Captn Dave

I did not fully appreciate how much the bias needs to drop as the plate voltage is reduced.

For a pentode it is screen grid voltage which matters, not anode voltage.

28th June 2009, 04:10 PM	#1
Captn Dave diyAudio Member Join Date: Jun 2008	Testing an EL84 as a 6V6 I have a B&K 500 tube tester which I think is a very useful tester, however it won't test 9CV tubes like the EL84. A major shortcoming I think. I wired up an adaptor with a 6V6 base and a 9 pin socket wired to the 9CV configuration, pluged in an EL84 and found that I am getting no cathode current to speak of. (BTW- 6V6s test just fine with proper cathode current). I have 190 vdc plate voltage, 154 vdc screen voltage and a grounded cathode. Bias is -9 vdc. Heater voltage is 6.3. These values are measured off the 9 pin socket and I'm counting pin numbers correctly. I have tried several known good tubes. I would expect high current with such low bias yet practicaly nothing (0.4ma). Any ideas?

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28th June 2009, 04:51 PM	#3
Captn Dave diyAudio Member Join Date: Jun 2008	Arne, Thanks for your comments. I've not seen one but If I'm not mistaken the 610 panel was intended for the B&K 650. Mine is the 500. I looked at the wiring instructions at the Tone Lounge and couldn't gleen much about the construction from them as they only show the 8 pin socket. Anyway, I don't have a 610 and I am looking for the DIY solution. The poor mans 610 panel I suppose. Dave

28th June 2009, 10:49 PM	#5
Ty_Bower diyAudio Member Join Date: Mar 2007 Location: Newark, DE	I can't find any datasheets to confirm my theory, but I suspect with 154 volts on the screen, the tube is cut off when you apply the -9 volt bias. Try a little less bias, maybe -7 volts or so. Otherwise, try raising the plate and screen voltage. Edit: here's some datasheets: http://frank.pocnet.net/sheetsE2.html Look at the triode curves. Plate and screen tied together, 190 volts on both, the -9 volt grid line puts you pretty close to no conduction.

29th June 2009, 01:16 AM	#6
Captn Dave diyAudio Member Join Date: Jun 2008	Ty, You are exactly right. -9 was too much bias. The model 500 has a voltage divider that provides -18.1, -8.1 and -2.9 volt bias taps. I split the divider fruther, by using two resistors equal to the sum of the original, to get a 6.1 volt tap which seems to be about perfect for the plate voltage. It seems to work very well now. I did not fully appreciate how much the bias needs to drop as the plate voltage is reduced. Thanks for the help. Dave

18th July 2011, 02:37 PM	#7
dorrisant diyAudio Member Join Date: Aug 2010	Dave, Did you ever come up with sensitivity settings for the EL84s? I have a 500 and would love to be able to reliably test these tubes. Any diy info you could post would be much appreciated. Ex: which resistor did you split for your 6v tap? I have a complete manual with schematic and a calibration setup as well for reference. Thanks, Tony

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