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Internal tube change and cleaning. 2:50, 50/50, 2:90;

Posted: Tue Jan 23, 2018 7:41 pm
by slybird
I own the 2:50. I listed the other amps assuming they are all somewhat similar on the inside.

I am ordering new power tubes soon from Mesa. I have owned the amp since about 2003 or a bit longer. During my ownership I have never opened it up. It is probably due for a cleaning to get rid of the years of dust, change the internal tubes, maybe install a quieter fan and finally install a fan switch.

Here are my questions.

Are those SPax7 tubes a good idea for the internal tube position?

Any safety concerns when opening the amp? I know about the capacitors, is there anything else?

As long as I have it open are there any mods I might consider?

I like the idea of running 6v6 tubes. Is a bias mod a good idea?


Thanks much.

Re: Internal tube change and cleaning. 2:50, 50/50, 2:90;

Posted: Thu Jan 25, 2018 5:15 pm
by vipmetal
I had Stereo 2:50 power amp for 12 years. I installed NOS Mullard 12AT7 tubes in there, for input (phase inverter) and for presence tubes. It helped to have better/slower raise in volume pot. Not so much didn’t like in presence tubes. But my memory is now fading away abiut other changes, if there were any. Just to let you know that at home use it was good to have lower gain tube at first slot of the power amp.

Re: Internal tube change and cleaning. 2:50, 50/50, 2:90;

Posted: Wed Feb 14, 2018 3:03 am
by synaesthesia
I did that too 12AT7, and I think I did that to the studio pre too. it just gives me a wider range of control over the area where my tonal prefs. live, otherwise for me it saturates too much too quickly....

Re: Internal tube change and cleaning. 2:50, 50/50, 2:90;

Posted: Sat Aug 18, 2018 9:56 am
by joey_truelove
slybird wrote:I own the 2:50. I listed the other amps assuming they are all somewhat similar on the inside.

I am ordering new power tubes soon from Mesa. I have owned the amp since about 2003 or a bit longer. During my ownership I have never opened it up. It is probably due for a cleaning to get rid of the years of dust, change the internal tubes, maybe install a quieter fan and finally install a fan switch.

Here are my questions.

Are those SPax7 tubes a good idea for the internal tube position?

Any safety concerns when opening the amp? I know about the capacitors, is there anything else?

As long as I have it open are there any mods I might consider?

I like the idea of running 6v6 tubes. Is a bias mod a good idea?


Thanks much.
6V6s are likely to go bad before you know it, seeing that these amps are spec'ed at 450V on B+, some are even closer to 500V with that 20% variance Mesa allows for. The earlier Fifty/ Fifty featured a low voltage mode where 6V6s would have had an easier time to operate. If going with 6V6s you probably want to have the OTs replaced even so, with something of a slightly higher reflected value.

The filter caps is your main concern. What you could do is when the amp is properly heated up and running: pull the chord out of the wall. This will drain the filter caps. If still unsure: use a volt meter to measure across one of the filter caps. On one hand this would give you and indication of their status and it would also drain the cap should there be a charge present.

Not so many mods to choose from, I'm afraid - not that makes much sense, at least. Unless of course you have some know-how of valve amplification and a vision to take the amp in a certain direction. You could undo the deep resistor, you could go with lesser or higher filtering... One mod would be lowering the B+ and grid voltages to allow for those 6V6s - here an adjustable bias mod would probably be required to get them into the zone.