Difference between mesh vs. solid plate 300B?

Not sure, but I think @charles1dad might have compared some within brand solid vs mesh 300bs... perhaps he'll notice the thread.

In my opinion Emission Lab mesh plates are worth the money. BUT, while they rarely have problems, they aren't good about honoring warranty on those rare occasions.

That may be who you are dealing with.  I have been an EML dealer for a couple of years and have found Jac to be extremely helpful in terms of warranty issues.  The only time it is a problem is if someone just ignores his advice and mis-uses a tube.  For instance, the EML Mesh tubes have a max plate dissipation of 28w vs. the standard 40w.  If you put them in the wrong amp and/or don't adjust the bias down and they burn up, he isn't going to honor the warranty.  Otherwise, the couple of failures I have dealt with were addressed immediately.  Jac even facetimed with a customer to help him modify the bias on an amp when he had an issue.  

I’ve listened to both the original EML solid plate 300Bs and EML mesh plate 300Bs for several years each without any problems. The solid plates are very good, while the mesh plates present a richer, more musical sound. Worth the money? Yes!

The only only ones that rate higher on my list are the Takatsuki and current production Western Electric, but only by a small margin. I’d put the Elrog at about the same level as the EML mesh, but the Elrog has reliability issues. If the current WE = 100, then the Takatsuki is a 99, the EML Mesh is a 97, and the EML solid plate is a 95. Having used two pairs of the original (1940s and 1960s) WE 300B for years prior to the EML’s, I would put the latter at 94 -- they have a great midrange, but fall short in the bass compared to the modern ones.

As usual, other listeners may differ in their pecking order or preference, so don’t take my word as an absolute. Your driver and input tubes, plus the overall build quality of your amps, have a great deal of influence, too.

Be careful to observe the caveats about the meshies -- 22W dissipation is recommended, with 28 watts the maximum. Most commercially built amps run much higher -- 38 watts isn’t unusual -- so you can’t just plug them into any ol’ 300B amp without doing the math.

For the 300B mesh plates, I run my 300B amps with about 350V on the plates, -73V on the grids, and with an 1100K ohm, 22W cathode bypass resistor. The cathode bypass cap is normally 20uF, 200V. That’s just a guide -- your amp may require different values to achieve 22 watts. The Emission Labs data sheet is helpful in that regard.

BTW, the choice of rectifier is critical to the sound of any 300B (or other power tube). For the vintage WE 300B, I preferred the vintage metal banded Amperex (Holland) 5AR4/GZ34. For the EML solid plate 300B, the best sound was with the Sylvania/Philips USA "Big Boy" 5AR4/GZ34, and for the EML 300B mesh, I prefer the 1950s/60s RCA 5U4GB. As usual, your listening pleasure may vary, but it pays to experiment with different numbers and different brands within each number until you reach the magic spot.

http://www.emissionlabs.com/datasheets/EML300B-mesh2.html

Cheers,
Joseph