Red spots appeared on 300B anode

Dave the bass · Post by **Dave the bass** » Thu Feb 03, 2011 8:15 am

Thats odd innit that, I always thought the the idea of having a mesh anode was to augment cooling of the plate. I thought that was why that idea was introduced in the late 20's/30's.

DTB

IDM · Post by **IDM** » Thu Feb 03, 2011 8:20 am

I checked all the voltages this morning and come up with one amp at 29.7W the other at 30.2W. So clearly both to high for mesh plate tubes so I will go back to my EH gold grid solid plates.

I am surprised as I have read articles about using mesh plates in the LD2+ but not mentioning short valve life or need to adjust the amps for lower power dissipation (or maybe I read the wrong articles).

If I wanted to use mesh plates is it a case of adding and RC stage to drop HT to reduce power or again is it more complex? Secondly will the mesh plates be damaged?

Thanks for all the help
Ian

Post by **Nick** » Thu Feb 03, 2011 9:39 am

Dave the bass wrote:Thats odd innit that, I always thought the the idea of having a mesh anode was to augment cooling of the plate. I thought that was why that idea was introduced in the late 20's/30's.

DTB

I don't think its ever been about cooling, remember its in a vaccum so normal ideas about loosing heat by convection don't apply. In a valve its about radiation, so its all about surface area. The mesh has more surface area at first sight, but most of the extra area "points" at other parts of the anode so is no use for heat loss through radiation.

I am not certain what the reason for the use of mesh is, maybe there are some old texts out there as there were old valve with mesh plates so its not just fashion. I suspect its to do with the electrons arriving at the anode, some will pass through and be pulled back. It may affect secondary emission.

Post by **Nick** » Thu Feb 03, 2011 9:41 am

IDM wrote:I checked all the voltages this morning and come up with one amp at 29.7W the other at 30.2W. So clearly both to high for mesh plate tubes so I will go back to my EH gold grid solid plates.

I am surprised as I have read articles about using mesh plates in the LD2+ but not mentioning short valve life or need to adjust the amps for lower power dissipation (or maybe I read the wrong articles).

If I wanted to use mesh plates is it a case of adding and RC stage to drop HT to reduce power or again is it more complex? Secondly will the mesh plates be damaged?

Thanks for all the help
Ian

Reducing anode voltage will work, the other option to reduce current is to increase the cathode resistor. Replacing the 1k to a 1k8 would give you what you wanted.

Dave the bass · Post by **Dave the bass** » Thu Feb 03, 2011 10:30 am

Nick wrote:
I don't think its ever been about cooling, remember its in a vaccum so normal ideas about loosing heat by convection don't apply. In a valve its about radiation, so its all about surface area. The mesh has more surface area at first sight, but most of the extra area "points" at other parts of the anode so is no use for heat loss through radiation.

I am not certain what the reason for the use of mesh is, maybe there are some old texts out there as there were old valve with mesh plates so its not just fashion. I suspect its to do with the electrons arriving at the anode, some will pass through and be pulled back. It may affect secondary emission.

Ta Nick, I didn't know that.

I know modern 'mesh' plates are sometimes actually a punched plate with holes in like those TJ's I've got. I've got a few old 'real' mesh plate valves and they really are a woven mesh anode, they're small signal though not power o/p valves.

Wonder if our-SteveS may know about how/why the mesh plate was introduced? He's the old valve fella isn't he (not him!... the Valves).

DTB

Mike H · Post by **Mike H** » Thu Feb 03, 2011 1:32 pm

See here:

http://www.emissionlabs.com/datasheets/ ... -final.htm

and here (note 6):

http://www.emissionlabs.com/datasheets/ ... .htm#Note5

I'll go along with the idea that wire mesh increases the surface area for the electrons to land on, and they also have access to the outside surface of the anode as well as the inside. Probs has to be kosher wire mesh though, a plate with holes in isn't the same. Unless they're LOTS of little holes.

Post by **Nick** » Thu Feb 03, 2011 2:13 pm

But why would increasing the surface area make any difference? The anode current is controlled by the emissivity of the cathode and the potential gradient seen by the cathode. It would be like the flow from a hose pipe being altered by pointing the hose at a bucket with a hole in the bottom, instead of a normal bucket.

The increased physical damping seems more likely to me.

ed · Post by ed » Thu Feb 03, 2011 2:37 pm

It would be like the flow from a hose pipe being altered by pointing the hose at a bucket with a hole in the bottom, instead of a normal bucket.

ooooheeey! best analogy I've seen today

pre65 · Post by **pre65** » Thu Feb 03, 2011 5:29 pm

Does the same formula work when there is no cathode resistor ?

Or even for A2 ?

Post by **Nick** » Thu Feb 03, 2011 5:57 pm

Plate dissapation = current through the plate * voltage between the cathode and the plate.

current through the plate = number of electrons arriving at the plate in a unit time.

voltage between the cathode and the plate = the speed the electron is accelerated to as it travels between the catode and the plate.

speed of electron * mass of electron = kinetic energy of a single electron

energy of a single electron * number of electrons = total energy being imparted to the plate by the electrons being stopped dead by the plate in unit time.

total energy of electrons is converted from kinetic energy to heat.

If that was the equation you meant.

if there is current flowing from the cathode to other places, grid, screen and so on, then the current through the cathode is not the same as the current through the plate, so you need to measure plate current to know plate dissapation.

Also true for dissapation from grid or screen. Voltage from cathode * current through grid or screen = dissapation from grid or screen.

heat energy = plate dissapation.

Mike H · Post by **Mike H** » Thu Feb 03, 2011 6:16 pm

A-ha, so you believe the electrons will just go to the nearest wires of a mesh plate anode and be done with it, no overshooting and coming back from behind and all that nonsense.

Post by **Nick** » Thu Feb 03, 2011 6:22 pm

A-ha, so you believe the electrons will just go to the nearest wires of a mesh plate anode and be done with it, no overshooting and coming back from behind and all that nonsense.

Well, no, I expect the electrons to behave in a generally newtonian fashion, but I don't see how that would affect anything.

pre65 · Post by **pre65** » Thu Feb 03, 2011 6:31 pm

Well, I was thinking of working out the dissipation of my 833a, but I'm even more confused now.

I know it's well within it's limits so I will leave it at that.

Mike H · Post by **Mike H** » Fri Feb 04, 2011 1:36 pm

Va - Vk x Ia. Seemples!

Post by **Nick** » Fri Feb 04, 2011 2:15 pm

Mike H wrote:Va - Vk x Ia. Seemples!

Thats what I said

Just needs to remember Ia â‰ Ik in the case of A2

audio-talk

Red spots appeared on 300B anode

#16

#17

#18

#19

#20

#21

#22

#23

#24

#25

#26

#27

#28

#29

#30