More noob tube questions

[Raggi73]

I'm really unsure about how my 3ch Dual rectifier (a 2007 modell) is working.
Today at practice my amp started making a bad distorted noise while playing (using mainly ch3). It disapeared after I turn the Standby off/on but only do appear again minuts later. It keeps coming more often as the amp became warmer. Also the 4. 6L6 tube (closest to the 5U4G) where glowing blue-ish on the bottom. I replaced the 6L6 tubes with someone I bought a while back, but the problem came back. I also tried using ch2 on my amp, but the same problem. These tubes where not bought in a match-quad pack, but as two and two package and not sure they where matched eighter.

Question 1: Is it prefered to buy a matched Quad-pack with tubes (6L6) as the amp dosn't have a BIAS switch.
Question 2: Is it the same for the 12ax7 tubes? (matched quad-pack)?

I have used the original Mesa tubes and will probebly continue with that.
I figure the problem was the BIAS as out bass player had the same problem with his Ampeg rig some years back.

Hope someone can answer this. (excuse my English)

 

[ryjan]

Are your tubes spec'd for use in a mesa amp?

 

[Raggi73]

Sorry. the brand is MesaBoogie.

 

[fluff191]

Yes it is ideal to buy a matched Quad of POWER TUBES.

No you do not have to matched anything for preamp tubes.

I would start with a fresh set of power tubes and then report the results so we can help further if needed.

 

[94Tremoverb]

Don't get another set of power tubes yet, they may not be the cause. In fact, if you have already changed them once and the exact same problem occurs, it's almost certain that they're *not* the cause. Don't just change tubes until you know what the problem is! At best you just spend money you don't need to. Blue glow in power tubes is almost never a fault, by the way - although if it's too bright it can sometimes indicate one.

It could just as likely be a preamp tube or possibly a rectifier (although that would most likely blow the fuse). You only need one preamp tube to find and fix a fault with any of them since they are all 12AX7s. A preamp tube costs about 1/10th of a set of power tubes and a rectifier only about 1/4 as much, so don't spend more money than you need to until you know for sure.

First, remove both the 5U4 rectifier tubes and set the amp to Silicon Diode. Does the problem still occur? If yes, it's not the rectifier tubes. If no, put *one* of them back and run the amp at low volume with it set to Vacuum Tube. If the problem does not occur, test the other tube the same way. It's OK to run the amp with only one rectifier tube as long as you don't crank it up. If it's not the rectifiers:

Does the problem occur with the FX loop bypassed using the switch on the back of the amp? If not, try changing V4 (fourth along from the input jack end, remembering that V1 is under the metal cover). The FX loop also affects the Output Level and Solo section so these don't work with the loop bypassed. If yes:

Does the problem occur on Channel 1? If yes, try replacing V5, and if that doesn't work try V2 and then V1. If no:

Replace V3 - this tube is only used on Channels 2 and 3, not channel 1.

To be perfectly honest, if you want to short-cut this whole process you could try changing V3 right away, since it's the most likely cause. It's a 'cathode follower', which makes some tubes (Russian made) fail more often in this position. If you're replacing it, try to avoid using a Russian tube - Chinese is fine. The same applies to V4, but not the other preamp tubes.

If after all that the problem still persists, you *could* try changing the power tubes, but at that point it could just as likely be a non-tube fault.

 

[Raggi73]

94Tremoverg, thanks.
I have ordered two sets of 6L6 duce that should be matched from the supplier and two 5u...tubes. I need these as backup anyway.
I also have a set with 12ax7 preamp tubes that I will try to replace first as you described in the post here.

"First, remove both the 5U4 rectifier tubes and set the amp to Silicon Diode", you are saying I can remove them both (5u4g tubes), set the amp to Silicon Diod and turn the amp on at low level?

 

[94Tremoverb]

It's fine to use the amp at any level with the switch set to Silicon Diode and no rectifier tubes. You just must not turn up too far with only *one* rectifier tube and all four power tubes in - although if you want, you can also run the amp safely with one rectifier and only two power tubes (the inner or outer pair) - this is the standard 'half power' mode. The reason I would suggest not testing the amp like that is that if the fault goes away, you don't know whether it was the rectifier or the power tubes you pulled that were the cause.

I don't think it's likely to be a rectifier issue, really - that would most likely blow the fuse - but it's simple to test and worth making certain first, since the rectifier affects everything else in the amp.

 

[Raggi73]

Hi.
I just bought JJ electronics 6L6gc matched quads and 2 5u4gc rectifier tubes.
Also thinking about doing the BIAS mod and maybe the Precence mod at the same time.
Thoughts about this?


My music style is mainly Metal/trash.