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PostPosted: Thu Feb 01, 2018 5:27 pm 
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Bottle Rocket

Joined: Thu Feb 01, 2018 5:11 pm
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Hi I am new to this board and kindly ask for some help understanding the Tube layout of the Mesa 50/50 rack power amp (mine was made in 1999).

2 Questions:
(1) Have I labeled the input, phase inverter, and power amp tubes correctly in the image below?
(2) If I only want to use channel A, can I remove 2 of the 4 6L6GC power tubes (that is, the 2 power tubes for channel B) and still use the amp? Or if two of the power tube sockets are empty the amp will not work at all?

Thanks so much for your help.

Image


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PostPosted: Thu Feb 01, 2018 6:59 pm 
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Mark II
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Hi H.G.G.

Your tube designations are correct... Please find the tube layout below

Image

According to Mesa Fifty/Fifty Manual...

If you opt to use only one channel of the Fifty/Fifty with speakers connected to only one channel's output(s), you must turn the unused channel's Level control to zero! Otherwise the channel will be operating without a load, and damage to the tubes and/or the transformer will result.

Just to be on the safe side, I would put a load-box on the channel that is not being used.

Image

I hope this helps

Seeya

Joe

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PostPosted: Thu Feb 01, 2018 8:56 pm 
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Bottle Rocket

Joined: Thu Feb 01, 2018 5:11 pm
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Thanks so much Joe, for the very helpful info. I do have a load I can use on channel b.

However, what about if I physically removed tubes v6 and v7 (the power tubes associated with channel b)--can I still use channel a? Or will the amp not work if its missing 2 power tubes?

Thanks again,
HG


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PostPosted: Thu Feb 01, 2018 10:23 pm 
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Mark II
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Hi HG,

Yes you can remove the Power Tubes for Channel B and No damage will occur

I hope this helps

Seeya

Joe

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PostPosted: Mon Feb 05, 2018 12:34 am 
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Bottle Rocket

Joined: Thu Feb 01, 2018 5:11 pm
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Thanks again. If I remove the power tubes to channel b, does channel b still need a load to prevent damage to the transformer? Or no?


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PostPosted: Tue Feb 06, 2018 8:58 am 
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Mark II
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Hi HG

If you remove the channel B output tubes you don't need a load on Channel B Speaker output... Please let me know if I can help with any other questions.

Seeya

Joe

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PostPosted: Tue Feb 20, 2018 6:11 pm 
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Bottle Rocket

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Thanks a lot man, really appreciate your help.


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PostPosted: Wed May 23, 2018 12:18 pm 
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Bottle Rocket

Joined: Sun Apr 08, 2018 12:58 pm
Posts: 13
Hi.

Sorry to bump this topic (although it's fairly recent).

I'm retubing my 50/50, and I was wondering if the tube layout might be different depending on the version of the power amp. On the Mesa schematics, I see they say it's Version C.

Mine looks exactly like the OP's picture, with a blue light.

And on both the schematic and the owner's manual they only mention there is only 2 preamp tubes (strange since the schematic says: V1-V2: 12AX7, V4-V7: 6L6GC). There is obviously a V3 tube that is omitted...

I'd like to know if ALL VERSIONS of the 50/50 have the same tube layout. I'm asking because last time I retubed I was told the power tubes were A-B-B-A...

And is it confirmed that the Preamp tubes are, from left to right: PI Channel B, PI Channel A, Driver tube.

I have identical tubes (matched & balanced with the same specs) to use for the Channels' PI, and another Matched & Balanced tube to use as a driver to ensure the best balance possible between the two channels' volume/drive. So since I went through this expense I want to be sure which 12AX7 goes where, and that I have the proper power tube layout (thinking about maybe using a different brand of matched pair for each channel for sonic variety).

Thanks for your help!


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PostPosted: Wed May 23, 2018 3:34 pm 
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Bottle Rocket

Joined: Sun Apr 08, 2018 12:58 pm
Posts: 13
Update:

I've been told by Mesa that indeed the A-A-B-B power tubes layout is correct for ALL versions of the 50/50.

Also, that the Driver tube the one closest to the input jack. (No, didn't say Jacks) So it seems obvious that the Driver is indeed V3 if we say V1 is the 12AX7 located to the left, V2 in the middle, V3 at the right side.

Stands to reason the Phase Inverter for each channel is located closest to that channel's power tubes, but the Mesa rep didn't answer my question regarding that.

One thing that Mesa rep told me that I find interesting is that "There is NO need to use balanced tubes in any of the 3 12AX7 positions."

Now that is one statement I'm quite ambivalent about:

-1- Of course Mesa doesn't sell Balanced (nor Matched) preamp tubes, so it's obvious that they won't encourage people to get some that are...

-2- True, in Combo/Head applications the use of a Balanced Phase Inverter is highly debatable for reasons I won't get into here, but mainly because the Improvement, wether in tone or feel, is highly controversial and if not impossible to notice, there are reasons why a balanced PI in a combo could adversely affect tone (marginally).

-3- BUT... In a Stereo Power Amp context, that's a different story. Tubes manufactured today are VERY inconsistent: the specs variance from one tube to another can be as high as 40-50% !

So just sticking any 12AX7 (including Mesa's, which, while tested for more consistent specs, still possess a non-negligible variance) in the Driver stage (each side of the 12AX7 drives a channel's PI) and in the Phase Inverter position for each channel WILL result in a noticeable difference in volume/drive between both channels!

But then again if you plug stereo into a 4x12", you're never going to notice this output difference since the speakers are so close to one another. And if you play live, and in 99% of cases you only get to mic one speaker, then again none of this matters... Same thing with recording mono (only one speaker mic'ed).

But at that point... What's the point of having a STEREO power amp? With stereo effects processors...? A good rack rig is a huge investment... so better use it for what it's meant to be, or put another way, to use it to it's full potential!

Finally. Fitting your 50/50 (or any stereo pa) with a Matched & Balanced Driver (each side of the 12AX7 will send the same signal level to each channel) and with 2 identical Matched & Balanced Phase Inverters will assure you that both channels/sides of your power amp will put out the same level of volume. So when playing live or recording in stereo, using 2 cabs and putting enough distance between them to hear the effects in stereo ( you can create awesome sounds with stereo effects!), you'll want to have a good volume balance... (Especially for recording)

So. This was my two cents on using any 12AX7's or some that are Matched & Balanced in a Stereo Power Amp.

Rock on!


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