EL84 grid voltage?

Davy · 12th September 2006, 08:20 PM

Hi everyone,im new to this forum and was wondering if anyone here had experienced the following.
I've got a small but good kit amp which uses four EL84's in p/p mode.The manuel says voltage at pin 2(G1) should be zero,im reading low voltage varying between 0.1v to 0.6v with the individual valves.The amp work's&tests fine otherwise.The valves are made by jj,and seem ok in every other respect,fairly low hours use (around 300).Is this a usual occurance with this valve?i'd welcome your thought's on this.

Cheers

Davy M

anatech · 12th September 2006, 08:28 PM

Hi Davy,
The grid voltage should always be at it's reference, in your case it's 0 VDC. Check your coupling caps and for any possible leakage to that point across a terminal strip, PC board or tube pins. Check that your reference resistors (to ground) are not going open.

-Chris

gingertube · 12th September 2006, 11:28 PM

Davy,
This NOT unusual. The grid 1 resistor to 0V is often 470K for EL84.
0.6V across 470K is ONLY 1.3 uA grid current or capacitor leakage current. Good idea to keep an eye on that voltage though. If say in a few years time it starts to read 2 volts or more then it may indicate a problem.
Cheers,
Ian

tubelab.com · 13th September 2006, 01:31 AM

Measure the voltage with the output tubes removed. If you get zero volts the leakage is in the tubes (somewhat normal) if you get the same, or more voltage, the leakage is in the coupling caps (not normal).

anatech · 13th September 2006, 01:36 AM

Hi Davy,

Quote:

Measure the voltage with the output tubes removed.

Just watch your B+ doesn't get too high with the outputs removed. Good point about the tubes being leaky Ian. I haven't seen this that often.

-Chris

tubelab.com · 13th September 2006, 01:49 AM

Quote:

Just watch your B+ doesn't get too high with the outputs removed.

Didn't think about that, remove the tubes from one channel at a time. The other channel will still be drawing current.

anatech · 13th September 2006, 01:57 AM

Hi tubelab,
You know what guys in a hurry would say? Ouch! Phhheeww ! phhheww! Hot, triscut!

If you have a variac, turn it to abpout 90 VAC and then watch the B+ voltage as you turn it up. Just another (lazy) way of doing this. I am very probably going to do that test that way.

-Chris

DougL · 13th September 2006, 05:19 AM

I would check the values of those grid resistors.

If they do not match the schematic, replace them.

The kit could have grabbed some high value resistors rather than the correct value. (By the way, Pentode or ultra-linear grid resistors can be much higher value than grid resistors in triode connected circuits)

Doug

Davy · 13th September 2006, 06:16 PM

Thank's for all the replies folk,it's nice to see people takin the time to help stranger's.
I probably should have been more specific in my first post(sorry),the amp test's fine,i even sent it to an expert to check it over and he cant find a reason either.
When he fitted annother set of EL84's it measured 0v at G1 as it should do,it seem's to be related to the valve's im using.
Could this be an age related thing(the valves have low hours use,only a few hundred).Or could something regarding the amp be prematurely ageing them.I've just recently increased the grid stoppers from 6.8K to 10K,dont know yet if it will help though.

Cheers

Davy M

anatech · 13th September 2006, 06:25 PM

Hi Davy,
This may sound silly, but did you clean the bases of those older tubes?

Try cleaning them with methyl hydrate and a cotton swab. It is possible for the grid wires to pick up cathode material I think. I have never seen this condition myself.

-Chris

12th September 2006, 08:20 PM	#1
Davy diyAudio Member Join Date: Sep 2006	EL84 grid voltage? Hi everyone,im new to this forum and was wondering if anyone here had experienced the following. I've got a small but good kit amp which uses four EL84's in p/p mode.The manuel says voltage at pin 2(G1) should be zero,im reading low voltage varying between 0.1v to 0.6v with the individual valves.The amp work's&tests fine otherwise.The valves are made by jj,and seem ok in every other respect,fairly low hours use (around 300).Is this a usual occurance with this valve?i'd welcome your thought's on this. Cheers Davy M

13th September 2006, 01:31 AM	#4
tubelab.com diyAudio Member Join Date: Jul 2005 Location: South Florida	Measure the voltage with the output tubes removed. If you get zero volts the leakage is in the tubes (somewhat normal) if you get the same, or more voltage, the leakage is in the coupling caps (not normal). __________________ Too much power is almost enough! Turn it up till it explodes - then back up just a little.

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12th September 2006, 08:28 PM	#2
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi Davy, The grid voltage should always be at it's reference, in your case it's 0 VDC. Check your coupling caps and for any possible leakage to that point across a terminal strip, PC board or tube pins. Check that your reference resistors (to ground) are not going open. -Chris

12th September 2006, 11:28 PM	#3
gingertube diyAudio Member Join Date: Oct 2004 Location: Adelaide South Oz	Davy, This NOT unusual. The grid 1 resistor to 0V is often 470K for EL84. 0.6V across 470K is ONLY 1.3 uA grid current or capacitor leakage current. Good idea to keep an eye on that voltage though. If say in a few years time it starts to read 2 volts or more then it may indicate a problem. Cheers, Ian

13th September 2006, 01:57 AM	#7
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi tubelab, You know what guys in a hurry would say? Ouch! Phhheeww ! phhheww! Hot, triscut! If you have a variac, turn it to abpout 90 VAC and then watch the B+ voltage as you turn it up. Just another (lazy) way of doing this. I am very probably going to do that test that way. -Chris

13th September 2006, 05:19 AM	#8
DougL diyAudio Member Join Date: Feb 2004 Location: Wheaton IL. Blog Entries: 25	I would check the values of those grid resistors. If they do not match the schematic, replace them. The kit could have grabbed some high value resistors rather than the correct value. (By the way, Pentode or ultra-linear grid resistors can be much higher value than grid resistors in triode connected circuits) Doug

13th September 2006, 06:16 PM	#9
Davy diyAudio Member Join Date: Sep 2006	Thank's for all the replies folk,it's nice to see people takin the time to help stranger's. I probably should have been more specific in my first post(sorry),the amp test's fine,i even sent it to an expert to check it over and he cant find a reason either. When he fitted annother set of EL84's it measured 0v at G1 as it should do,it seem's to be related to the valve's im using. Could this be an age related thing(the valves have low hours use,only a few hundred).Or could something regarding the amp be prematurely ageing them.I've just recently increased the grid stoppers from 6.8K to 10K,dont know yet if it will help though. Cheers Davy M

13th September 2006, 06:25 PM	#10
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi Davy, This may sound silly, but did you clean the bases of those older tubes? Try cleaning them with methyl hydrate and a cotton swab. It is possible for the grid wires to pick up cathode material I think. I have never seen this condition myself. -Chris

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