Tube Size

[Mongillo19]

In my quest to figure out my Mark V Tweed noise issue, I took a look at the tubes. My first thought was a preamp tube since it was only on the tweed chanel but taking a look at the power tubes, while all 6l6, they definitely weren't a matched set.

First thing I noticed was the size difference between an alleged matched set (outer tubes on the mark V). The one was noticeably different but was still a 6l6

I'm hoping putting the included matches sets of mesa tubes in will solve my issue

 

[Achilles]

Pretty sure matched refers to electrical measurements but I see your point.

 

[la szum]

There are Rectifier tubes in there, too.

I am skeptical it would be a power tube and only be on the Tweed mode,
and not also be noticeable in the higher wattage Channel 3 modes, too.

 

[Mongillo19]

Pretty sure matched refers to electrical measurements but I see your point.

Yeah I don't know much about the electrical aspects but it just seemed off to me from a visual stance

 

[Mongillo19]

There are Rectifier tubes in there, too.

I am skeptical it would be a power tube and only be on the Tweed mode,
and not also be noticeable in the higher wattage Channel 3 modes, too.

Yeah the one I swapped out was not a rectifier tube which I thought it was at first!

I'm not expecting it to work but I'm trying it now!

 

[Mongillo19]

Swapping the power tubes did lower the noise floor a little but something is still up with tweed. It's not as bad. Clips below.

I'll start swapping out V1-V3 and see if it helps as those have CH1 in their description

Noise Issue.wav

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First part is tweed, definitely gets worse with gain increasing, then I switch to fat and clean

Is it dumb I enjoy troubleshooting amps lol obviously I'd rather not but if you can fix the issue it's a feel good moment it helps this isn't an amp breaking issue but I'll reiterate how great this amp is

 

[metropolis_4]

Did you try a new rectifier tube?

 

[Mongillo19]

Did you try a new rectifier tube?

I did not! When I dive back in I might. I guess the question is would only the tweed channel use the rectifier tube?

 

[metropolis_4]

I did not! When I dive back in I might. I guess the question is would only the tweed channel use the rectifier tube?

Very possibly, but I don’t know the V very well. It seems like a likely culprit to me if you’re only getting the noise on that mode

 

[Achilles]

I guess the question is would only the tweed channel use the rectifier tube?

Found this:

 

[Mongillo19]

Found this:

View attachment 21134

So probably not a rectifier tube. I believe I can switch it to diode on the back which would in theory bypass the tube rectifier right? Easy way to check if so.

I'm really leaning towards it being a preamp tube. I've used up all my spares but I'll just pull one I know works from another amp and go from there

 

[Achilles]

I believe I can switch it to diode on the back which would in theory bypass the tube rectifier right?

Sounds like it's most based around which wattage you're in - and the guy started out as saying the rectifier tube is being used as a diode in the circuit.

 

[Mongillo19]

Any thoughts on why the tubes are labeled 1A/1B? Maybe A is full power and B is Variac?

 

[laxu]

Any thoughts on why the tubes are labeled 1A/1B? Maybe A is full power and B is Variac?

It's just detailing how the gain stages work. 12AX7 tubes are dual triodes so they have two sides.

You can see how for example V4A is used as reverb driver and V5B as reverb return gain stage.

 

[GreatGreen]

Any thoughts on why the tubes are labeled 1A/1B? Maybe A is full power and B is Variac?

It's just detailing how the gain stages work. 12AX7 tubes are dual triodes so they have two sides.

You can see how for example V4A is used as reverb driver and V5B as reverb return gain stage.

Yep! You can kind of think of any single 12AX7 as two entirely independent gain stages that just happen to be stuck together in one tube of glass.

Amps will often use a single 12AX7's two gain stages at different points in the circuit, too. For example in the Mesa Mark VII's "Mark VII" mode, the first gain stage is V1A, the first side of the first 12AX7 in the circuit, then it routes through two more sides of two other tubes before being routed back through side B of V1. So the circuit goes V1A -> V3B -> V4A -> V1B -> V2B, with a bunch of different components between each stage.

 

[Mongillo19]

Yep! Amps will often use a single 12AX7's two gain stages at different points in the circuit, too. For example in the Mesa Mark VII's "Mark VII" mode, the first gain stage is V1A, the first side of the first 12AX7 in the circuit, then it routes through two more sides of two other tubes before being routed back through side B of V1. So the circuit goes V1A -> V3B -> V4A -> V1B -> V2B, with a bunch of different components between each stage.

You can kind of think of any single 12AX7 as two entirely independent gain stages that just happen to be stuck together under one tube of glass.

Interesting! Well I only tried swapping out V3 and it didn't do anything. The only other option is V1 but I feel like more than just CH1 tweed would be having issues. I've also noticed it only gets that static when the master hits around 9 o clock.

Oh well!