When I was trying to find out why a 6922 in a common cathode circuit was oscillating I also noticed that I had the heater voltage too high (6.5V instead of 6.3V), so I proceeded to adjust it to 6.3V (when fully heated) and added grid stoppers to try to fix my original problem. My question is, what are the consequences of a higher heater voltage, could that have been originally responsible for the oscillations? If it's possible (or maybe I'm way way off, I'm new to tubes) and that could have been the cause I'll take out the grid stoppers and pobe with my scope again (as it is, it's rock solid, but I'd like to be able to eliminate the grid stoppers), but if there's no way that was causing it, I won't waste me time. Again, thanks for all the input from you guys.
Most data sheets say +/-10%. 6.5V is only 3% high, and for all either of us know could well be meter error (..depends on the meter).
The only way heat could be related to oscillations is by heat varying the transconductance of the tube - and this is N/A because of the emission curve of oxide-coated type cathodes. Grid stoppers are a seperate deal, and the need depends on the circuit.
Tim
The only way heat could be related to oscillations is by heat varying the transconductance of the tube - and this is N/A because of the emission curve of oxide-coated type cathodes. Grid stoppers are a seperate deal, and the need depends on the circuit.
Tim
JoeBob said:
Where did you measure? You'd be surprised by how much voltage can be dropped down a bit of twisted heater wire. It's vital to measure at the valve base.
Mullard quote 5% tolerance on heater voltages, but mains voltage varies, so AC heaters need to be set as closely as possible (and ideally measured with a true RMS meter). Regulated DC supplies can be set to precisely the correct voltage, so it seems foolish not to do so...
Actually I did measure on the tube socket pins, and it is from a regulated DC power suply, silly me was absent minded when I set the voltage, that's why. I was just wondering if this was the cause of the problems I had been seeing, I guess not, so I'll have to look elsewhere to eliminate the grid stoppers.
Likely you'll have to do more than change components around. With fidgety tubes you'll probably have to change the layout altogether, be it longer or shorter wire runs between parts.
You might try neutralization, check out ham-radio books for how to do that. (Not like it's worth the price of a grid stopper.
)
Tim
You might try neutralization, check out ham-radio books for how to do that. (Not like it's worth the price of a grid stopper.
)Tim
STOPPERS...AGAIN.
Hi,
Since you have a scope you could try to lower the value of the gridstoppers untill oscillation shows up again.
Make sure the body of the resistor is soldered as close as possible to the grid, otherwise the resistor is not quite as effective.
Don't forget the 6DJ8 is an RF tube and oscillates quite easily.
Another way is to use a small ferrite rod to kill off parasitic oscillation but they can be quite tricky to implement correctly.
Cheers,
Hi,
Since you have a scope you could try to lower the value of the gridstoppers untill oscillation shows up again.
Make sure the body of the resistor is soldered as close as possible to the grid, otherwise the resistor is not quite as effective.
Don't forget the 6DJ8 is an RF tube and oscillates quite easily.
Another way is to use a small ferrite rod to kill off parasitic oscillation but they can be quite tricky to implement correctly.
Cheers,
LIFE EXPECTANCY.
Hi,
I even think that is a rather optimistic prediction.
The effect is easily visible when you have a tranconductance tester.
Just take a new 6DJ8 and measure it at 6.3V, than set the heater voltage at 7V.
Leave it like that for half an hour and than measure the transconductance again at 6.3V and you'll notice that it will have sunk to almost unusable levels.
OTOH, running small signal tubes at slightly less than the rated heater voltage will extend their usable life considerably and won't hurt the sound at at.
If you use DC on the heaters a regulator such as a 7806 or 7812 should give you 6 VDC or 12 VDC across the heater pins, so that's very convenient.
Cheers,
Hi,
I even think that is a rather optimistic prediction.
The effect is easily visible when you have a tranconductance tester.
Just take a new 6DJ8 and measure it at 6.3V, than set the heater voltage at 7V.
Leave it like that for half an hour and than measure the transconductance again at 6.3V and you'll notice that it will have sunk to almost unusable levels.
OTOH, running small signal tubes at slightly less than the rated heater voltage will extend their usable life considerably and won't hurt the sound at at.
If you use DC on the heaters a regulator such as a 7806 or 7812 should give you 6 VDC or 12 VDC across the heater pins, so that's very convenient.
Cheers,
Hi guys!
Don't know if this helps but maybe? Here's what I found on
http://www.tone-lizard.com/lesson6.htm
Thers also a nice diagram from AUDIO CYCLOPEDIA (I don't know how to send pictures yet ((maybe I did it this time)), but please inform me. I'm all ears):
From the chart above, you should be able to determine that if a tube has a usable life of approximately 6,000 hours with it's filament operated at 100% of the rated voltage, the tube's life expectancy is reduced to 1,000 hours just by lowering the filament voltage by 10%. This translates to a 6.3VAC tube being run at approximately 5.67VAC, or the input voltage lowered from 120VAC to approximately 108VAC. Ask your local tube guru if they have read Audio Cyclopedia. However, even 'experts' can disagree on anything and everything. GE published data confirming tube life is decreased substantially by raising the filament voltage, although tube life can be increased by lowering the filament voltage very slightly.
gammelnalle
Don't know if this helps but maybe? Here's what I found on
http://www.tone-lizard.com/lesson6.htm
Thers also a nice diagram from AUDIO CYCLOPEDIA (I don't know how to send pictures yet ((maybe I did it this time)), but please inform me. I'm all ears):
From the chart above, you should be able to determine that if a tube has a usable life of approximately 6,000 hours with it's filament operated at 100% of the rated voltage, the tube's life expectancy is reduced to 1,000 hours just by lowering the filament voltage by 10%. This translates to a 6.3VAC tube being run at approximately 5.67VAC, or the input voltage lowered from 120VAC to approximately 108VAC. Ask your local tube guru if they have read Audio Cyclopedia. However, even 'experts' can disagree on anything and everything. GE published data confirming tube life is decreased substantially by raising the filament voltage, although tube life can be increased by lowering the filament voltage very slightly.
gammelnalle
Attachments
Define "life"
I've seen graphs like this before, and they all show the estreme effects of straying from the specified voltage. These days, there's really no excuse for being more than 0.5% in error. I've just ordered UK £100 of bits, and a DVM is being thrown in free! (It's probably horrible, but there's no such thing as too many meters.)
But what if you want to deliberately run the cathodes a little cooler (perhaps to reduce noise)? One theory says this reduces life. Another says not. If you don't need the full current from the valve, then reducing cathode temperature could increase life. Valve longevity is not a simple thing, and the longevity graphs I have seen are very rarely accompanied by specifications of conditions. That's why people disagree.
To my mind, valves last forever anyway (provided you don't abuse them). Envelope temperature is far more significant than the easily soluble problem of heater voltage.
I've seen graphs like this before, and they all show the estreme effects of straying from the specified voltage. These days, there's really no excuse for being more than 0.5% in error. I've just ordered UK £100 of bits, and a DVM is being thrown in free! (It's probably horrible, but there's no such thing as too many meters.)
But what if you want to deliberately run the cathodes a little cooler (perhaps to reduce noise)? One theory says this reduces life. Another says not. If you don't need the full current from the valve, then reducing cathode temperature could increase life. Valve longevity is not a simple thing, and the longevity graphs I have seen are very rarely accompanied by specifications of conditions. That's why people disagree.
To my mind, valves last forever anyway (provided you don't abuse them). Envelope temperature is far more significant than the easily soluble problem of heater voltage.
Thanks for te answer. I don't want to argue against you , (My english aint that good ) but where can I read about this:
"But what if you want to deliberately run the cathodes a little cooler (perhaps to reduce noise)? One theory says this reduces life. Another says not."
You also say 0,5%... Then you MUST drive them with DC (or...?) I come from the world of guitar-amps and my kind of people say that thats is crap... "you don't need DC"... But I believe that it can't be wrong to take the best pieces from where ever I can find it to improve .....
cheers,
björn
) but where can I read about this:"But what if you want to deliberately run the cathodes a little cooler (perhaps to reduce noise)? One theory says this reduces life. Another says not."
You also say 0,5%... Then you MUST drive them with DC (or...?) I come from the world of guitar-amps and my kind of people say that thats is crap... "you don't need DC"... But I believe that it can't be wrong to take the best pieces from where ever I can find it to improve .....
cheers,
björn
VALVE LIFE.
Hi,
Indeed it isn't.
Valves can last forever, well a lifetime at least.
Some die soon, others refuse to give up, which one reason for valves not having a MTBF rating.
From long time experience I can only say that running them slightly under the rated heater voltage seems to have no impact on cathode emission whatsoever and it makes for an easy life especially when you want regulated DC voltage on their heaters.
Going too hot on the heaters however is not good practice IMHO and will burn them out fast, it also changes their curves.
Cheers,
Hi,
Indeed it isn't.
Valves can last forever, well a lifetime at least.
Some die soon, others refuse to give up, which one reason for valves not having a MTBF rating.
From long time experience I can only say that running them slightly under the rated heater voltage seems to have no impact on cathode emission whatsoever and it makes for an easy life especially when you want regulated DC voltage on their heaters.
Going too hot on the heaters however is not good practice IMHO and will burn them out fast, it also changes their curves.
Cheers,
gammelmalle said:
Yes, I prefer to heat critical valves from DC. (It is possible to regulate AC - Tektronix used saturable inductors in series with the heater mains transformer, and I own a rather splendid servo-motor-driven Variac that regulates the AC to my test bench.)
Absolutely right. Musician's amplifiers do not really need regulated heaters etc (although I would dearly love to try moving the head amp to the guitar body and send a line level signal to the amplifier). Remember, you deliberately provoke valve distortion as part of the sound of your instrument. Hi-Fi uses valves for their clarity, and that often requires stable operating conditions. More to the point, regulating a DC heater supply is actually cheaper than doing AC heater wiring properly.
Yes distorsion is part of SOME of the guitarists sounds, I wouldn't call Hank B Marvin's (Shadows) sound "distorted" I think is pure clean.... But I think I know what you mean.
I also think that I (or whoever plays) should be in command of the "distorsion" or modification (tone control) . Therefore I think its important to learn more about tubes.
I havent still seen any "proof" of that "my" diagram is wrong. (I dont say its right, but its the only written "proof") I must say that I don't understand how the lifetime of a tube will be reduced by lowering the heater supply.....
cheers
björn
I also think that I (or whoever plays) should be in command of the "distorsion" or modification (tone control) . Therefore I think its important to learn more about tubes.
I havent still seen any "proof" of that "my" diagram is wrong. (I dont say its right, but its the only written "proof") I must say that I don't understand how the lifetime of a tube will be reduced by lowering the heater supply.....
cheers
björn
Paradox.
Hi,
Cathode poisoning is one of the reasons.
Manufacturers usually allow a deviation of the stated heater voltage of +/-10%, although this is tolerable in general measurements this will show up different curves with varying heater voltages.
It may take a lenghty explanation of what exactly goes on here, maybe someone more clever than me can condense it in a few lines....?
Anyway, to understand it you need to get to grips with electrons flowing, all the basics really, once you come to grips with these principles you'll see that starving heaters for voltage and, more importantly current is NOT a good idea.
Cheers,
Hi,
Cathode poisoning is one of the reasons.
Manufacturers usually allow a deviation of the stated heater voltage of +/-10%, although this is tolerable in general measurements this will show up different curves with varying heater voltages.
It may take a lenghty explanation of what exactly goes on here, maybe someone more clever than me can condense it in a few lines....?
Anyway, to understand it you need to get to grips with electrons flowing, all the basics really, once you come to grips with these principles you'll see that starving heaters for voltage and, more importantly current is NOT a good idea.
Cheers,
Cool cathode!
Reduction in valve life with reduced cathode temperature is easy. The Richardson/Dushmann equation shows that electron emission is strongly temeperature dependent. As the cathode ages, it becomes less and less able to emit electrons, and eventually, we decide that it has reached the end of its useful life. If we run at reduced temperature (due to lower heater voltage) we reduce electron emission, so the limit at which useful emission ceases is reached earlier. Of course, if you are only drawing a very small current, you may never reach this limit because the valve fails for some other reason. This is why I was somewhat sceptical of sweeping statements about valve life.
May I soundly applaud you on your desire to find about more about the valves you use!
Reduction in valve life with reduced cathode temperature is easy. The Richardson/Dushmann equation shows that electron emission is strongly temeperature dependent. As the cathode ages, it becomes less and less able to emit electrons, and eventually, we decide that it has reached the end of its useful life. If we run at reduced temperature (due to lower heater voltage) we reduce electron emission, so the limit at which useful emission ceases is reached earlier. Of course, if you are only drawing a very small current, you may never reach this limit because the valve fails for some other reason. This is why I was somewhat sceptical of sweeping statements about valve life.
May I soundly applaud you on your desire to find about more about the valves you use!
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