Hey all, just finished up this little build, played it some last night cranked into an attenuator and it sounded pretty good, despite some 120 hz hum (ordered more filter caps), came home today and played it unattenuated on 10, very gainy and distorted, hit a power chord on the bridge pickup and noticed a sudden volume drop and a big white flash inside the tube. I powered it down immediately, thinking it was a bad tube, discarded it and put another in and it seems ok but I am nervous...
Here's the schem I am working with, 263 volts on the plate, 13.3 on the cathode, so I am actually running the plate exactly at the datasheet recommended 250 volts with a slightly cooler bias, maybe 266 on the screen at idle (measured to ground) was this just a bad tube phenomenon? Should I lower my grid-ground resistor on the 6AQ5 to ensure stability?
Thanks for any ideas,
-A
What would cause arcing inside a tube? (more 6AQ5 stuff)
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What would cause arcing inside a tube? (more 6AQ5 stuff)
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Life is a tale told by an idiot -- full of sound and fury, signifying nothing.
...in other words: rock and roll!
...in other words: rock and roll!
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David Root
- Posts: 3540
- Joined: Fri Aug 04, 2006 3:00 pm
- Location: Chilliwack BC
Re: What would cause arcing inside a tube? (more 6AQ5 stuff)
Barring a bad tube in the first place, that might be it. Cut the gain a bit. I would normally use a 1k5 on the grid itself, but I've seen 5k6 used on 6V6GT grids, soo..?
In addition, often the screen is a weak link. You might try increasing the screen resistor to 1k or enough to get the screen voltage below the plate voltage, it's currently 3V higher than the plate. Again, I've seen that too (old Gibsons) but it should be the same or slightly lower. If the screen is more positive than the plate, it's competing with the plate for incoming electrons, which build up and eventually arc to somewhere else. In this case I wonder if 3V difference would cause that, but if the screen is already on the weak side, who knows?
In addition, often the screen is a weak link. You might try increasing the screen resistor to 1k or enough to get the screen voltage below the plate voltage, it's currently 3V higher than the plate. Again, I've seen that too (old Gibsons) but it should be the same or slightly lower. If the screen is more positive than the plate, it's competing with the plate for incoming electrons, which build up and eventually arc to somewhere else. In this case I wonder if 3V difference would cause that, but if the screen is already on the weak side, who knows?
Re: What would cause arcing inside a tube? (more 6AQ5 stuff)
Thanks David, I was thinking the same thing - maybe take the screen resistor up to 1K or 1K5, reduce the grid resistor down to 270k to not slam the tube so hard.
About the screen in this set-up, I read somewhere that under signal, somehow the screen is negative relative to the plate in this arrangement (which is in a lot of old oddball se amps, as you say) but I don't quite see how that works. I built a 5C1 like this and was freaked out that the screen was running above the plate, I went to a lot of trouble to lower it by a few volts and I could swear it just didn't have quite the same tone after that...
This amp not only blew that 6AQ5, but a smooth plate telefunken 12AX7 cracked along the base after sitting in the new socket overnight!
That's 2 for 2!
Thanks again!
About the screen in this set-up, I read somewhere that under signal, somehow the screen is negative relative to the plate in this arrangement (which is in a lot of old oddball se amps, as you say) but I don't quite see how that works. I built a 5C1 like this and was freaked out that the screen was running above the plate, I went to a lot of trouble to lower it by a few volts and I could swear it just didn't have quite the same tone after that...
This amp not only blew that 6AQ5, but a smooth plate telefunken 12AX7 cracked along the base after sitting in the new socket overnight!
That's 2 for 2!Thanks again!
Life is a tale told by an idiot -- full of sound and fury, signifying nothing.
...in other words: rock and roll!
...in other words: rock and roll!
-
David Root
- Posts: 3540
- Joined: Fri Aug 04, 2006 3:00 pm
- Location: Chilliwack BC
Cracked Telefunken.
Ouch! That could be expensive! My sympathies. I have three of those in my '62 Deluxe, used, not NOS, that I paid $15 each for about 5 years ago.
That's interesting about the tone with the high screen Vp. Have not heard that before. I think we need an EE to explain that! Perhaps it applies only in SE operation.
That's interesting about the tone with the high screen Vp. Have not heard that before. I think we need an EE to explain that! Perhaps it applies only in SE operation.