The Last Bass Outpost

If I understand you, subbing a 12au7 in V4 would limit the power output???

Meantime it sounds really good just not as loud as I expected. I haven't owned it long. Replacing the original 30 year old tubes seemed a reasonable proposition. It would be a three day affair to get a tech on it in between the driving. Courier service is more risk than warranted. If I knew of a truly worthy tube tech to take it to I could plan vacation around it. Mesa NZ told me to play it until it acts up.

Thanks, a 12AU7 would be a cheap enough experiment.

Any thoughts on how that driver tube interacts in a current limiting master volume setup? To get cleanish tone they all require master over 9. Mesa instructs for bass that you leave it on 10. Believe it or not it was sold as dual purpose guitar amp suitable for bass. The less than clean sounds are neato stuff

No chance of adding bias. One trip to tech costs like 2 sets of tubes. The 7581A I am looking at is not vintage NOS btw. http://blog.thetubestore.com/tung-sol-introducing-impressive-7581a-vacuum-tube/

http://www.thetubestore.com/Tubes/6L6-5881-Tube-Types/Tung-Sol-7581A

What do you think of my conjecture that it should hold up similarly to the old STR415 set?

My main worry is if the tube is rated for a higher 35w power output it may jigger the balance of the Master vol system further than it already is. Flipside, if it would soften the descent into ramptant fuzz instead, that would be great! Damn Mesa for ''losing'' the schematic. Note I don't expect 180w to turn into 210w so to speak.

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.