Mesa Boogie Nomad 100- tube questions

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Bluesbabe

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Right now I'm only using Channel 1 on my Nomad because I need the clean tone for my jazzbox.
I need to change my worn-out tubes.
I had an idea. If I only put in the pre-amp tubes for Channel 1, would the other channels simply not work?
Presumably that means I could save on tubes and keep the unused tubes for later.

What about the 6L6 power tubes?
If I set the amp on 50W and only put in the "inside" set of tubes, would that mean I could save the other two tubes for later?
My studio is small and I'm not playing out these days. I think the 100W sound is nicer, but I don't need anywhere near that much power.

Somewhere on the forum I saw a reference to the 50W tubes being the "inside" set. Is that true?

Would all of this be a wise idea? Mesa Boogie amps seem like high-strung racehorses. Very beautiful but delicate.
I'm wondering if the amp needs to be all set up as it comes just to work without problems.
 
Unfortunatly, you can only pull V3. V1,V2,V4 and V5 are all in the signal chain of channel 1. I can't remember what two tubes work in 50 watt mode. It is the two middle or two outside tubes and I'm leaning towards the two midlle tubes. You got a 50/50 shot. - Pat.
 
Thank you for replying.
It was just an idea.
I should just play it safe and put all the tubes in.
I may sell my other amp and then I would need those channels anyway.
 
From the manual: "The two inside power tubes are put in a standby mode when the 50 watt position is selected reducing the RMS power to approximately 50 watts." This could mean that these two tubes have the high voltage disconnected, or more likely the signal to these two tubes is disconnected and the high voltage is left on. If the latter, the idle current will increase on the two outer tubes if you pull the two inner tubes. So by trying to save two tubes, you might be putting more stress on the other two tubes, reducing their life prematurely, (and altering the tone compared to just using the half-power switch). Of course you could check the bias to see what happens with two tubes pulled, my guess is that only the signal is disconnected, and if so, it's probably best to leave all tubes in and use the switch to reduce power.
 
I'm pretty sure the switch breaks the connection to the cathodes, so, the tubes that are not on are not conducting at all.
 
The 50 watt switch was put there to eliminate the need to pull 2 tubes to reduce power. Simply use the 50 watt mode, and swap the inner pair with the outer pair every 6 months to keep even wear on each pair. Thanks!
 
shytfayse said:
I'm pretty sure the switch breaks the connection to the cathodes, so, the tubes that are not on are not conducting at all.

You are probably right. After reading that section of the manual again, I noticed; "For optimum performance it is recommended that you change the speaker or cabinet from the 8 ohm to the 4 ohm jack when switching down to the 50 watt position. This ensures a proper impedance match by keeping the relative load the same."

This infers that the two tubes are indeed disconnected on the output side, rather than the signal side as I guessed.
 
I just picked up a used Nomad 100 1X12. Although the manual and Mesa techs say the inner tubes go into standby in the 50 watt mode, mine works opposite. The outer tubes go into standby and the inner tubes remain on in 50 watt mode. I tested it by removing each pair and it definately works opposite of what the manual and Mesa say it should. The Mesa tech said the only explanation is that the previous owner modified the amp somehow. There are no physical signs of this thing being touched since it looks like it's brand new. Plus it makes no sense that someone would modify an amp and include changing which tubes work in the 50 watt mode. Any ideas?

Also, when I got it, it had a mix of Groove Tubes and Marshall power tubes that appeared to be from the Nixon administration era. I re-tubed with Mesa 6L6s (yellow) and it sounds great. However, I would like to experiment a little with the tone. I have a set of Electro Harmonics 6CA7s in my Hughes and Kettner and I love the tone I get from them. I'm not a tube guy but I read that these tubes are described as EL34s on steroids, which describes my experience with them in my H & K. Are the EH 6CA7s compatible with the Nomad in the EL34 bias mode and if so, how do I know which ones to select that are compatible with the MB fixed bias? I know for example that with Groove Tubes' proprietary bias rating system, the 4,5,or 6 GT ratings are compatible with MB fixed bias. But how do I know what "non-Mesa" or "non-Groove Tubes" tubes will be compatible with the MB fixed bias?
 
You may never know unless you can get a bias probe on it. I have heard the GT 5 (middle of the road) will work/closest to Mesa Spec. I have not tested this myself. Drop the tubes in and if they don't red-plate and you like the sound - go with it. - Pat.
 

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