Greetings all,
MESA Express 5:25 http://music-electronics-forum.com/a...-5-25-5-50.pdf
After redplating a new (within 4 hours of play time) MESA EL84 in my express, and now waiting for a replacement set, I poked around a bit w/a bias probe hoping to get a better idea of what was going on in the amp.
The B+ was 400v, cathode current with the new set of tubes (the one that had a redplate) was 32mA (both tubes). I didn't leave the amp on long as I didn't want the tube to redplate again. This is dissipating 12.8w at idle(!).
I put in a set of JJ's, they were running just under 32mA. I tried a few different sets of various el84's, most of them were in the 30-35mA range.
This strikes me as pretty darn high b+ for an el84, especially considering the MESA tubes are putting 32mA of current through them. I have a peavey classic 50 as well, and it too is putting 400v on the plates of the el84's.
Anyway, wanting to check the MESA out further, this morning I opened the amp up and took some measurements. Seeing 410v on the center tap of the OT (point A), and 402v on each of the plates. The screens are at 330v. I didn't measure screen current, probably should have.
Couple questions: does the B+ seem excessive for an el84? The schematic shows 401v on node A of the B+ (and then 382v on the plates, which doesn't seem likely). Is the fact that the screens are so much lower the reason this is OK? Or is it just not OK?
Adding a bias pot would allow me to run the tubes cooler, but I'm thinking that much difference could raise the B+ as much as 10 to 20 volts, and what would that then mean for the screens on the next node?
Thanks for any thoughts on this.
MESA Express 5:25 http://music-electronics-forum.com/a...-5-25-5-50.pdf
After redplating a new (within 4 hours of play time) MESA EL84 in my express, and now waiting for a replacement set, I poked around a bit w/a bias probe hoping to get a better idea of what was going on in the amp.
The B+ was 400v, cathode current with the new set of tubes (the one that had a redplate) was 32mA (both tubes). I didn't leave the amp on long as I didn't want the tube to redplate again. This is dissipating 12.8w at idle(!).
I put in a set of JJ's, they were running just under 32mA. I tried a few different sets of various el84's, most of them were in the 30-35mA range.
This strikes me as pretty darn high b+ for an el84, especially considering the MESA tubes are putting 32mA of current through them. I have a peavey classic 50 as well, and it too is putting 400v on the plates of the el84's.
Anyway, wanting to check the MESA out further, this morning I opened the amp up and took some measurements. Seeing 410v on the center tap of the OT (point A), and 402v on each of the plates. The screens are at 330v. I didn't measure screen current, probably should have.
Couple questions: does the B+ seem excessive for an el84? The schematic shows 401v on node A of the B+ (and then 382v on the plates, which doesn't seem likely). Is the fact that the screens are so much lower the reason this is OK? Or is it just not OK?
Adding a bias pot would allow me to run the tubes cooler, but I'm thinking that much difference could raise the B+ as much as 10 to 20 volts, and what would that then mean for the screens on the next node?
Thanks for any thoughts on this.
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