[doggie]

I ordered a pair of 300Bs from a reputable and helpful dealer. When they arrived I noticed that both had heaters seemed to be working fine and evenly colored but once the high voltage was applied one tube had a line of red-orange dots/hotspots on the metal across from the filament in a vertical centerline and evenly spaced.

The dealer will take the pair back but I am wondering what might cause that issue. I did try swapping channels but the problem followed the tube.

Thanks!

[FullRangeMan]

Any photo?
but once the high voltage was applied one tube had a line of red-orange dots/hotspots on the metal across from the filament in a vertical centerline and evenly spaced.
By what you said looks the grid, what is brand/model?

[doggie]

It was a Shuguang Treasure 300B-Z. I am not really up on tube anatomy but I believe that the grid is composed of wires wound around but not touching the filament?

These did not have the glowing dots. The very bright dots seemed to be on the metal casing that surrounds the grid and filament. The cathode? The other tube just has a glowing filament as one would expect.

I am going to return them as I have low confidence in the "hot" tube.

The vendor has been very understanding.

[JakeJ]

Yes, there is something wrong with that tube.  Was it the anode or the cathode?  Maybe a bad weld on one of the pins?  And since the problem followed the tube that proves the amp is OK.

Sounds like the vendor is a stand up person and you will likely get a new pair shipped out ASAP.  The nature of tubes.

[FullRangeMan]

It was a Shuguang Treasure 300B-Z. I am not really up on tube anatomy but I believe that the grid is composed of wires wound around but not touching the filament?

These did not have the glowing dots. The very bright dots seemed to be on the metal casing that surrounds the grid and filament. The cathode? The other tube just has a glowing filament as one would expect.

I am going to return them as I have low confidence in the "hot" tube.

The vendor has been very understanding.

Yes the grid is a wire around the cathode,
There should be no glowing dots is a tube,
The 300B is a direct heating triode so there is no filament,
the Cathode is heated directly by DC voltage.

[doggie]

Thanks for the perfect graphic. So the cathode(heater) looked good and the grid also. It is the plate that is glowing on one tube.

They will go back but unfortunately that model is no longer in stock by this vendor...or even made, which is why I got a good deal on it as a close out.

I am getting a pair of Gold Lion reissues from another vendor. It is another well reviewed reasonably priced current 300B. The first vendor does not carry them and only has much more expensive offerings from Psvane.

I appreciate the input.

[FullRangeMan]

It is the plate that is glowing on one tube.
Incredible, this is rare endeed I have never see a prob like this.
Do mind post a photo?

[doggie]

The photos do not really show the issue accurately. They make the spots look flared but in fact there is a vertical line of distinct glowing dots that are spaced at about the same interval as the grid wires.

My only guess is that the internal structure shifted in shipping, moving it closer to the plate and creating hot spots. 






Here is the other tube for comparison...

[FullRangeMan]

Thanks for posting you most kind.
The first photo show a shining spot area indeed.
keep us posted.

[Scott F.]

Nope, that's not normal. I'd stop using that tube immediately. If it grounds to the plates, you'll have a major repair on your hands.
Best to send it back and get a replacement.

[doggie]

Thanks for the helpful input. The tubes are boxed and will go back on Monday. New pair of Gold Lions on the way.