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
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
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
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
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
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
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).
Hi Davy,
-Chris
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
Didn't think about that, remove the tubes from one channel at a time. The other channel will still be drawing current.
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
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
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
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
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
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
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
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
davy wrote:
Yessir. When these tubes don't get fully evacuated (none truly do) the gas inside allow a current path to the grid. When you see blue inside power tubes when they're really working..... ...gas.
with 470k as the grid resistor, the circuit is going to be fairly sensitive to the "gassy-ness" of the tubes, uh I mean valves 😀 The grid resistor will have to fight the gas path leakage. Any chance that the grid resistor could be changed? I am not recommending one way or the other. Another issue that might come up is the grid capacitance or the Miller capacitance. That's another parasitic that grid resistor has to fight. If you've got low gas tubes/valve measuring 0v on the grid, use them. Just be cautious when it's time to replace those.
Yessir. When these tubes don't get fully evacuated (none truly do) the gas inside allow a current path to the grid. When you see blue inside power tubes when they're really working..... ...gas.
with 470k as the grid resistor, the circuit is going to be fairly sensitive to the "gassy-ness" of the tubes, uh I mean valves 😀 The grid resistor will have to fight the gas path leakage. Any chance that the grid resistor could be changed? I am not recommending one way or the other. Another issue that might come up is the grid capacitance or the Miller capacitance. That's another parasitic that grid resistor has to fight. If you've got low gas tubes/valve measuring 0v on the grid, use them. Just be cautious when it's time to replace those.
A lot of modern tubes just don't seem to have the vacuum quality that older tubes had. It was rare to see the blue glow inside (among the grid wires) in a tube in the 60's and the ones that glowed were considered "gassy" and discarded. A blue glow on the inside surface of the glass is normal.
New production tubes all seem to glow when they are pushed hard. They all seem to show some signs of grid current also. I have bought several JJ EL34's and they all exhibit this effect. None have failed despite my continued operation above the maximum power rating. I haven't tried their EL84 yet.
I would say that if the amp works OK, ignore it. It may be wise to check the grid voltage every few months, and if it doesn't get worse, ignore it.
New production tubes all seem to glow when they are pushed hard. They all seem to show some signs of grid current also. I have bought several JJ EL34's and they all exhibit this effect. None have failed despite my continued operation above the maximum power rating. I haven't tried their EL84 yet.
I would say that if the amp works OK, ignore it. It may be wise to check the grid voltage every few months, and if it doesn't get worse, ignore it.
Thank's all,gives me more to think about
.Im going to get a spare set to compare,back up's always a good thing eh.The new grid resistors may solve this so i'll be keeping an eye on things for a while.
Cheers
.Im going to get a spare set to compare,back up's always a good thing eh.The new grid resistors may solve this so i'll be keeping an eye on things for a while.Cheers
tubelab.com said:
Be nice when we can start building tubes in space... just open the part og the process that puts them in the glass to the outside world 🙂
dave
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