burning smell from my Champ

[Graffiti Yellow]

I decided to switch 6V6's in my silverface Champ tonight with an older tube I had. When I turned it back on there was a burning smell almost immediatly. So I shut everything down, cooled everything down and put back the original tube and the same thing happened when I turned it on again.

The few minutes I played it everything seem to work good and it sounded fine.

Any ideas on what might be wrong.

[Blue Strat]

Champs run the 6V6 on the verge of meltdown and should have the cathode bias resistor (for the 6V6) increased to 1K to run the tube (almost) within spec instead of 40% over.

All power tubes vary in idle current. The one that caused the smell is either shorted or has a higher idle current than the other tube.

[Trout]

Champs run the 6V6 on the verge of meltdown and should have the cathode bias resistor (for the 6V6) increased to 1K to run the tube (almost) within spec instead of 40% over.

All power tubes vary in idle current. The one that caused the smell is either shorted or has a higher idle current than the other tube.

Thats actually a very good Idea, A resistor swap is easy and low cost compared to a complete smoke refill kit! :D

[Blue Strat]

complete smoke refill kit! :D

ROFL! :D:AOK

[John Phillips]

Champs run the 6V6 on the verge of meltdown and should have the cathode bias resistor (for the 6V6) increased to 1K to run the tube (almost) within spec instead of 40% over.If you do this you MUST replace the cathode bypass cap (25uF 25V) with a 50V-rated one, because the cathode voltage rises from about 20V to about 30V.

It's also a good idea to fit a screen resistor, which Champs don't have as stock.

[Blue Strat]

If you do this you MUST replace the cathode bypass cap (25uF 25V) with a 50V-rated one, because the cathode voltage rises from about 20V to about 30V.

It's also a good idea to fit a screen resistor, which Champs don't have as stock.

Good call on the cap value. Do you really think the screen resistor is necessary? The dropping resistor in the power supply virtually covers this aspect (and adding a screen R is likely to cut the output power).

[John Phillips]

I'm not 100% sure, but they don't seem to eat cheap new-production 6V6s as badly with one.

OTOH, maybe you just shouldn't use crap tubes ;). There's no excuse in a single-6V6 amp really, it's not as if the right ones cost a fortune.

[Blue Strat]

I'm not 100% sure, but they don't seem to eat cheap new-production 6V6s as badly with one.

OTOH, maybe you just shouldn't use crap tubes ;). There's no excuse in a single-6V6 amp really, it's not as if the right ones cost a fortune.

There's a 1K in the power supply chain which feeds the screen.

Cheap new production tubes don't like to be run at 150% of max dissipation. NOS tubes don't seem to mind as much.

[John Phillips]

There's a 1K in the power supply chain which feeds the screen.Yes, but that node has a filter cap on it, so if you drive the tube really hard to the point the plate voltage falls (which it might, even though the amp is single-ended, because the bias point is far from symetrical), for a short time the screen voltage will be held above the plate voltage until the cap discharges. I'm guessing this is probably what kills cheap 6V6s. If you fit a screen resistor as well, that's much less likely to happen. At least, it seems to make them more reliable...

[Blue Strat]

Yes, but that node has a filter cap on it, so if you drive the tube really hard to the point the plate voltage falls (which it might, even though the amp is single-ended, because the bias point is far from symetrical), for a short time the screen voltage will be held above the plate voltage until the cap discharges. I'm guessing this is probably what kills cheap 6V6s. If you fit a screen resistor as well, that's much less likely to happen. At least, it seems to make them more reliable...

Good analysis:AOK

[Trout]

There's a 1K in the power supply chain which feeds the screen.

Cheap new production tubes don't like to be run at 150% of max dissipation. NOS tubes don't seem to mind as much.


Wouldnt the JJ 6V6S be durable enough to hold up though? Its so close to a 6L6 in its ratings I would think for a " Cheap" tube it could get by pretty easy.

Trout

[BoostAddict]

How about the newer Champion 600's? I don't have to swap resisitors or caps after swapping in some EH 6V6gt's do I?

[Dana-L]

Yes, but that node has a filter cap on it, so if you drive the tube really hard to the point the plate voltage falls (which it might, even though the amp is single-ended, because the bias point is far from symetrical), for a short time the screen voltage will be held above the plate voltage until the cap discharges. I'm guessing this is probably what kills cheap 6V6s. If you fit a screen resistor as well, that's much less likely to happen. At least, it seems to make them more reliable...
Good analysis:AOK
What resistor value would you guys suggest for this?

Incidentally, I have a Groove Tubes Soul-O Single and I noticed that they installed a cap rated at 400 VDC in the power valve cathode bypass position. Inexpensive insurance, I suppose.

Cheers,

-Dana

[John Phillips]

What resistor value would you guys suggest for this?The standard 470-ohm 2W is fine. In fact, if you're a cheapskate and/or don't mind carbon-comp screen resistors, you can just use the one you replaced with a 1K in the cathode-bias position ;).

[Dana-L]

Thanks, John.

BTW, I always appreciate and enjoy your posts.

Cheers,

-Dana