SET Asylum

Another thing to consider with Mesh

66.161.252.127

Gordon Rankin

In Reply to: RE: Careful with NOS and Mesh Plate 45s posted by somejoe on April 12, 2010 at 16:55:32

Gang,

Since this is a direct coupled design... there have been some issues with mesh plate tubes when the amps are power down. In some cases they tend to arc a lot easier than solid plate tubes.

The sellers of EML and TJ/Full Music/Sophia have all stated problems with direct coupled amps.

Thanks
Gordon
J. Gordon Rankin

Follow Ups:

RE: Another thing to consider with Mesh - tube wrangler 19:06:03 04/13/10 (0)
Woven-mesh plates, because of the interweaving, have both closer, and further-away spots from the other tube elements.
On Power-Down, arcing can indeed occur, and occasionally does. This is just another of the many, many reasons for why you don't run tubes near their so-called "design-Center" -- or "normal" ratings. You run less current, and even less plate voltage where you deem it necessary.
Regardless of this, in Direct-Coupling, arcing can, and does occur on power-down with a mesh-plate tube. (TRUE mesh-- that is-- INTERWOVEN WIRE-- for a plate)..
In my own case, using EML and the older AVVT mesh-plates, I have tubes with over 40,000 hours on them that have occasionally arced ALL THEIR LIVES! They are STILL excellent.
Here's what actually happened: Only the high-points of the mesh ever arced, and that simply sheared-off when the tube LIGHTLY arced. Then, it won't do that again for months, until it gets a new spot that's a little too close to grid, or filament, and will arc again until THAT gets shaved-off a little. This is because mesh moves over a bit as gravity, etc., causes it to not remain in the exact same spot as the tube ages. Also, as interwoven mesh (wires) sag a bit, that's one cause of enough displacement of a tiny "tit" of the mesh to get too close. (from slight bulging of horizontal wires in the mesh).
Why didn't the tubes ever fail? That's because of conservative voltages and currents, and because the metals that are used for all 3 elements in the tubes are rugged.
It's easy to see: (1) lower plate voltages than stated. (2) Lower operating currents than stated. (3) No large caps in your PS. (4) Cathode biasing ONLY, with foil/film caps ONLY-- that are small values.... don't worry about the Cathode-Bias cap being too small. Worry instead, about your Plate PS having too high a DCR, and caps that have too large values.
Now, you have wonderful, reliable sound!
And, no, you won't find anything that sounds better than a TRUE mesh-plate tube, and no, you will not find anything that can compete sonically with Direct-Coupling.
There you are.
---Dennis---

Edits: 04/13/10 04/13/10 04/13/10 04/13/10

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