Hi ,
I rigged up a 6922 (Sovtek ) tube as a simple cathode follower with +/- supplies and I get -2.4 volts ( measured) at the grid . Something must be wrong. I tried a 6922 EH tube also , with similar results.
I later checked the simulated circuit which also shows the expected zero volts at the grid ( with respect to ground).
What could be wrong. In one tube one section died after a few hours use. That grid now shows the negative supply voltage at the grid !
I can't think straight now. Anyone have any ideas what could be wrong? I checked the board several times to see if there was any connection errors. I can't find any.
Thanks.
I rigged up a 6922 (Sovtek ) tube as a simple cathode follower with +/- supplies and I get -2.4 volts ( measured) at the grid . Something must be wrong. I tried a 6922 EH tube also , with similar results.
I later checked the simulated circuit which also shows the expected zero volts at the grid ( with respect to ground).
What could be wrong. In one tube one section died after a few hours use. That grid now shows the negative supply voltage at the grid !
I can't think straight now. Anyone have any ideas what could be wrong? I checked the board several times to see if there was any connection errors. I can't find any.
Thanks.
Attachments
Re: Yes I will.
Absolutely no need, CFs are inherently stable and will find their own bias point. When you have *many* stages direct coupled and not stabilized the best, you might have to worry about such.
Besides, as soon as the tube starts conducting, it draws grid current just as the diode does if grid is positive. AND besides, the grid leak is hopelessly larger than the cathode resistor - even if a G-K short occured, or the diode there burnt out shorted, the resistor only has so much pull on the much stiffer Rk.
Tim
ashok said:
Absolutely no need, CFs are inherently stable and will find their own bias point. When you have *many* stages direct coupled and not stabilized the best, you might have to worry about such.
Besides, as soon as the tube starts conducting, it draws grid current just as the diode does if grid is positive. AND besides, the grid leak is hopelessly larger than the cathode resistor - even if a G-K short occured, or the diode there burnt out shorted, the resistor only has so much pull on the much stiffer Rk.
Tim
problem solved ..................I think.
Hi Tim and Sy,
Thanks for all the suggestions. I found the Sovtek 6922 seemed OK but he Electro Harmonix 6922 was not with one channel significantly different from the other .
It never occured to me that their actual operating characteristics could be different. From the curves on the Web for the 6922 , I expected certain operating voltages. I found them to be totally different !
I later used a current source of about 1mA ( original plate current was about 4mA) and supply of +/- 40 volts.
I got -0.74/-0.83 volts grid cathode voltage for the Sovtek 6922
and -1.2/-1.3 volts for the EH 6922 ! They certainly are very different tubes !
What looked like a faulty tube earlier ( EH6922) now settles down quite well at 1mA plate current . I heard that many like to run the 6922 at higher current as they sounded better. Well it can't be done with the tubes I have and low plate voltages. I have some NOS ECC88's , but I guess they can't really be compared.
What are the other dual triodes that work well at low voltage ?
Preferably newly manufactured. The NOS tubes can get really pricey and I've burned my fingers more than once by getting NOS tubes that were bad.
Thanks everybody.
Ashok.
Hi Tim and Sy,
Thanks for all the suggestions. I found the Sovtek 6922 seemed OK but he Electro Harmonix 6922 was not with one channel significantly different from the other .
It never occured to me that their actual operating characteristics could be different. From the curves on the Web for the 6922 , I expected certain operating voltages. I found them to be totally different !
I later used a current source of about 1mA ( original plate current was about 4mA) and supply of +/- 40 volts.
I got -0.74/-0.83 volts grid cathode voltage for the Sovtek 6922
and -1.2/-1.3 volts for the EH 6922 ! They certainly are very different tubes !
What looked like a faulty tube earlier ( EH6922) now settles down quite well at 1mA plate current . I heard that many like to run the 6922 at higher current as they sounded better. Well it can't be done with the tubes I have and low plate voltages. I have some NOS ECC88's , but I guess they can't really be compared.
What are the other dual triodes that work well at low voltage ?
Preferably newly manufactured. The NOS tubes can get really pricey and I've burned my fingers more than once by getting NOS tubes that were bad.
Thanks everybody.
Ashok.
No one said tubes had to be exactly on in terms of curves. Particularly if you have that unlikely bad egg. I would try a few to see if any match up to the expected data... then I'd check the circuit if none work. (My collection of 6U8s and 6GH8s aren't the most predictable as I've seen from trying them in Frankenhouse. Ain't used tubes great...)
Tim
Tim
Hi Tim,
You are right about tubes not behaving exactly as the curves suggest. Might be worthwhile getting a curve tracer. Unfortunately they are too expensive. I wonder if we can build one that can display on the computer screen ( with an external exciter box ).
I saw some curves from a Russian curve tracer ( using dos software). It would do. I couldn't find any info on it.
Cheers.
Ashok.
You are right about tubes not behaving exactly as the curves suggest. Might be worthwhile getting a curve tracer. Unfortunately they are too expensive. I wonder if we can build one that can display on the computer screen ( with an external exciter box ).
I saw some curves from a Russian curve tracer ( using dos software). It would do. I couldn't find any info on it.
Cheers.
Ashok.
I'd love someone to do a decent and buildable diy curve tracer for tubes. I've seen a few commercial efforts, but they're pretty expensive.
The funny thing is that we tube guys get terribly surprised when the curves for some tube are 20 percent different than the published curves. Yet the FET guys do just fine with parts that routinely vary in their critical parameters (like gm and idss) by five times. Kinda puts things in perspective.
The funny thing is that we tube guys get terribly surprised when the curves for some tube are 20 percent different than the published curves. Yet the FET guys do just fine with parts that routinely vary in their critical parameters (like gm and idss) by five times. Kinda puts things in perspective.
Yes, the phrase "sample variation" was invented for FETs.
It's worth remembering that the grid of a cathode follower is a stunningly high impedance point and if you had accidentally fitted a 6M8 grid leak (rather than a 680k), then connecting the paltry 10M input resistance of your DVM would be quite sufficient to cause the problems you are seeing.
It's worth remembering that the grid of a cathode follower is a stunningly high impedance point and if you had accidentally fitted a 6M8 grid leak (rather than a 680k), then connecting the paltry 10M input resistance of your DVM would be quite sufficient to cause the problems you are seeing.
Hi,
A computer assisted tubetester as used by Klaus Mobile and the folks at Audiomatica can be seen here:
TUBETESTER
Probably too much hassle for most of us but still:
CURVE TRACER
Years ago we hooked up a penplotter _to I can't quite remember which tubetester (AVO?)_ but I do recall quite distinctly that it took us about an hour to perform a successful plot as powertubes especially required at least half an hour to stabilize already...
Computerized or not, that warming up cycle won't change so I assume it's not something you'd want to do in a hurry.
Cheers,
A computer assisted tubetester as used by Klaus Mobile and the folks at Audiomatica can be seen here:
TUBETESTER
Probably too much hassle for most of us but still:
CURVE TRACER
Years ago we hooked up a penplotter _to I can't quite remember which tubetester (AVO?)_ but I do recall quite distinctly that it took us about an hour to perform a successful plot as powertubes especially required at least half an hour to stabilize already...
Computerized or not, that warming up cycle won't change so I assume it's not something you'd want to do in a hurry.
Cheers,
First you need a scope. Hook up an unrectified B winding (say 300VAC) to the tube's plate. Whatever voltage you want to swing up to. A variac or rheostat (NO LAMP DIMMERS!) will come in handy to change voltage if you don't have a suitable transformer on hand, or want it continuously variable. Make sure the rheostat can handle the tube current!
The tube will rectify on its own. You can put in a diode to protect it (or two if you want to be fancy-shmany and add a negative load so there is little DC in the transformer) but it's not worth it. Hook up a C- supply (variable grid bias with voltmeter) and A supply (heater; 6.3V filament transformer or whatever) and you're almost there.
Connect a small (10 ohm or so) resistor in series with the cathode, or transformer return (current that flows in the plate flows throughout the supply winding and back to ground, equal at all points because it's a series circuit). This senses current, connect to the VERTICAL (Y) scale on your scope. It goes 10mV/mA so 10mV/div reads 1mA/div, and so forth. You'll have to invert the display if you use the ground return method. Connect the HORZONTAL (X) scale to the plate, or if voltage swing is greater than you can read, add a voltage divider. 100k to 1k will give 1V/100V (not exactly but we don't care about the 1% in the divider, let alone 5% in the resistors or >3% in the scope, depending on what you have), which is 500V full screen (10 div) at 500mV/div.
Have fun!
Tim
The tube will rectify on its own. You can put in a diode to protect it (or two if you want to be fancy-shmany and add a negative load so there is little DC in the transformer) but it's not worth it. Hook up a C- supply (variable grid bias with voltmeter) and A supply (heater; 6.3V filament transformer or whatever) and you're almost there.
Connect a small (10 ohm or so) resistor in series with the cathode, or transformer return (current that flows in the plate flows throughout the supply winding and back to ground, equal at all points because it's a series circuit). This senses current, connect to the VERTICAL (Y) scale on your scope. It goes 10mV/mA so 10mV/div reads 1mA/div, and so forth. You'll have to invert the display if you use the ground return method. Connect the HORZONTAL (X) scale to the plate, or if voltage swing is greater than you can read, add a voltage divider. 100k to 1k will give 1V/100V (not exactly but we don't care about the 1% in the divider, let alone 5% in the resistors or >3% in the scope, depending on what you have), which is 500V full screen (10 div) at 500mV/div.
Have fun!
Tim
Hi all,
here' s another link to a DIY tube tracer:
http://xoomer.virgilio.it/0galavot/swpeng0.html
Cheers,
Bruno
here' s another link to a DIY tube tracer:
http://xoomer.virgilio.it/0galavot/swpeng0.html
Cheers,
Bruno
Just a thought, but 680K seems like a pretty large grid resistance to be using with a 6922 .. I think 100K would be more in order, and should help a lot with the unintended grid leak bias effect you seem to be experiencing. 
Cathode current as designed should be about 4mA if everything is working correctly.. Try increasing the cathode resistor to 15K or even 20K and see whether this helps or not. (reducing cathode current to as little as 2mA.) Also other dual triodes pin compatible with 6922 might be worth a try. (6BK7??)
Cathode current as designed should be about 4mA if everything is working correctly.. Try increasing the cathode resistor to 15K or even 20K and see whether this helps or not. (reducing cathode current to as little as 2mA.) Also other dual triodes pin compatible with 6922 might be worth a try. (6BK7??)
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