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Bass Amps & Effects / Tung Sol Reissue 7581A in Mesa D180
« on: May 08, 2015, 11:50:52 PM »
Howdy. My Mesa D180 seems short on power as it is only as loud as a GK MB200. The tubes are the original fitment Mesa 6L6 GC STR415 the tough ones from Sylvania which were getting silly expensive until they ran out. They lasted! So, I don't know much about tubes but I'm learning. ..
I wish there was better info cached on D180. From what I can gather they were originally designed for 6550 then the quality of those nosedived so the next best thing was the overspec 6L6. I can't see how the 6L6 can survive where a 6550 was meant to be without lowering the plate voltage. They monkied with the driver tubes a couple of times and added graphic EQ which is capable of a bit of a level boost. I have no time line or poweramp schematic.
This amp has a strange current limiting master volume section. Correct me if I am wrong here but I think that it brakes the voltage hitting the power tubes causing them to distort with less drive from the pre amp.
After only a few years and less than 1000 units sold Mesa abandoned the fancy Master and turned it into a regular one. At the same time rumour has it they went back to 6550 tubes and called it Bass 400, before again reverting to 6L6. I'm guessing that nasty Chinese tube quality meant too many failures and it was anaemic competition for the SVT so they doubled up on tubes for the 400+
The 400 schematic shows 540v applied across 3 tubes, is that 180v each? I think not! But 540v is way more than any 6L6 is rated at. A little knowledge is a dangerous thing.
Given the tube store's glowing review of their new tung sol 7581A as the duck's nuts 6L6 I figure it should be a good substitute for the original STR 415. I have no idea what to ask for in the way of bias current.
Note, installing bias trim pots isn't an option as there is no tech support here.
I wish there was better info cached on D180. From what I can gather they were originally designed for 6550 then the quality of those nosedived so the next best thing was the overspec 6L6. I can't see how the 6L6 can survive where a 6550 was meant to be without lowering the plate voltage. They monkied with the driver tubes a couple of times and added graphic EQ which is capable of a bit of a level boost. I have no time line or poweramp schematic.
This amp has a strange current limiting master volume section. Correct me if I am wrong here but I think that it brakes the voltage hitting the power tubes causing them to distort with less drive from the pre amp.
After only a few years and less than 1000 units sold Mesa abandoned the fancy Master and turned it into a regular one. At the same time rumour has it they went back to 6550 tubes and called it Bass 400, before again reverting to 6L6. I'm guessing that nasty Chinese tube quality meant too many failures and it was anaemic competition for the SVT so they doubled up on tubes for the 400+
The 400 schematic shows 540v applied across 3 tubes, is that 180v each? I think not! But 540v is way more than any 6L6 is rated at. A little knowledge is a dangerous thing.
Given the tube store's glowing review of their new tung sol 7581A as the duck's nuts 6L6 I figure it should be a good substitute for the original STR 415. I have no idea what to ask for in the way of bias current.
Note, installing bias trim pots isn't an option as there is no tech support here.