Mesa Boogie Dual Rect Rebuild

[Tubetec]

Low resistance at the input will load the incoming signal , cathode follower output from the pre might help .
Maybe if they have a series resistance on the output of the preamp , the low load at the input to the poweramp acts as an attenuator say in conjunction with 100k in series with the output of the preamp.

 

[okgb]

More so I'm curious what you guys think about the output tube biasing? I'd sure love to run it without a fan though I'll look for a quieter one. Looks like I need a dc 5-8 volt fan?

 

[CJ]

reducing screen voltage a bit can sometimes keep the tubes from running away, the problem is they use a choke on most high power amps which means the plate v is close to the screen v, but you can add a series resistor, maybe 100 ohms at 10 watts, so if all tubes draw about 100 to 150 ma, then you lose 10 to 15 volts off the screens, which might be just enough to keep the tubes from drifting towards meltdown.

they make low noise fans , most are DC, so that means adding some parts, then you have to figure out where to mount it, a lot of work,

 

[abbey road d enfer]

More so I'm curious what you guys think about the output tube biasing? I'd sure love to run it without a fan though I'll look for a quieter one. Looks like I need a dc 5-8 volt fan?

Well, you know, EL84 has a max recommended voltage of 300V. Vox and Fender push it to about 330V. Mesa Boogie are having their customers bearing the consequences of their choice to push B+ to 380V.
Purely and simply, this is malpractice; no responsible designer should do that, period.

 

[Matador]

Mesa Boogie are having their customers bearing the consequences of their choice to push B+ to 380V.

That Lonestar I just mentioned that was biased to 18W per tube had B+ running at 410V.

 

[musipol]

As Abbey said, this is malpractice, running EL84's at those B+ voltages and dissapations. In my experience there is no sonic benefit...........................and if you want more power use a different tube. It doesn't make any sense.

 

[emrr]

I'm flashing back to a custom Mesa from the late '70's that 'sounded bad', seemed to have crossover distortion problems at certain gain points. Eventually came out that everyone who'd owned it had the same complaint, it wasn't broken, it was never right. I wasn't the tech on that one, I didn't know much at the time, just a witness.

 

[abbey road d enfer]

I'm flashing back to a custom Mesa from the late '70's that 'sounded bad', seemed to have crossover distortion problems at certain gain points. Eventually came out that everyone who'd owned it had the same complaint, it wasn't broken, it was never right. I wasn't the tech on that one, I didn't know much at the time, just a witness.

It's not surprizing.
Since they outrageously exceed the voltages on some amps, they have to run the tubes at lower current in order to limit plate and screen dissipation, which results in low conduction angle.

 

[okgb]

A variac would be a clunky but easy to bring the voltage down.

I wonder if this is what randall refers to when he says mesa does something that no other amp manufacturer has figured out.....

 

[Rob Flinn]

This sounds a bit like the issues I have with my AMPEG VL503. It was working when I last powered it up .....