GK MB Fusion 500 preamp tube replacement?

[chris4001asat]

So I've been noticing a problem lately. A slight.....Airyness? in the background of my amp. When I play soft, it's very noticeable, when I get louder, the volume overcomes the...fuzz. I was thinking the speakers might be going, so I tried it with a different cab, and it was still there. So I thought preamp tubes. I went and bought three, came home and opened up the top, and saw it would require some disassembly to replace them. I decided to plug it back in, and it was gone. Maybe there was just a loose connection or something. Should I go ahead and replace them anyway since its halfway apart? It's only about two and half years old.

 

[B-string]

Well it could be induced when the cover is back on in one of the tubes. On the other side I saw post by I believe @tombowlus a while ago Mr Gallien showed him what to do to pivot the tube board up for tube replacement. I know it's tight in there and have never done a tube swap in one.

 

[lakefx]

I had a dirty tube socket cause issues with mine a few months ago. I had to go through the process you would do to replace the tubes, but I just cleaned the sockets with some contact cleaner. It's really not too bad, but you need a low profile bit driver to fit between the tubes and remove the nuts. Once you do that, the tubes can be pivoted up and replaced. Getting the nuts back on once you're done is just as bad as getting them off.

 

[tombowlus]

Well it could be induced when the cover is back on in one of the tubes. On the other side I saw post by I believe @tombowlus a while ago Mr Gallien showed him what to do to pivot the tube board up for tube replacement. I know it's tight in there and have never done a tube swap in one.

Yup. One or two screws, and it tilts out for tube replacement.

 

[Killed_by_Death]

I dislike that you can't see the tubes glowing while they're in operation.
Are two of the tubes a push/pull while the 3rd tube is just for the "contour"?

 

[B-string]

I dislike that you can't see the tubes glowing while they're in operation.
Are two of the tubes a push/pull while the 3rd tube is just for the "contour"?

No. All six triodes are gain stages. V1 is the first gain stage on one triode and Contour recovery/gain on the second triode. The other two tubes (4 triode stages) are gain for the other eq controls (Treble, High Mid, Low Mid and Bass).

 

[Killed_by_Death]

Thanks!

I've got a couple of 12AX7B tubes laying around that I might swap in, if I don't like the initial overdrive sound.

 

[boamedt]

Take pics of what to do, im sure it would help out some of those thinking it might be to involved to do themselves

 

[Turlu]

Can anyone post instructions on how to change the tubes in those Fusion 500 ??? Which screws Tombowlus is referring to ???

 

[tombowlus]

So here is a shot of the MB Fusion (that's Bob Gallien holding it, btw). That whole "rack" with the three tubes pivots "up," so that you can easily remove/replace them.

 

[Killed_by_Death]

Can anyone post instructions on how to change the tubes in those Fusion 500 ??? Which screws Tombowlus is referring to ???

Make sure to use a JIS screwdriver when you're removing the screws that hold the cover on.
I made the mistake of using a normal Philips screwdriver & stripped the heads.

 

[Turlu]

Yes, I can see that tombowlus and thanks for the help! However, how can you get the three tubes to go 'pivot up' and then change them ??? Are there any screws one can remove ??? I never seen such a compact and tight space to play around tubes, although my knowledge is fairly low I must say !!!

So here is a shot of the MB Fusion (that's Bob Gallien holding it, btw). That whole "rack" with the three tubes pivots "up," so that you can easily remove/replace them.

 

[tombowlus]

Yes, I can see that tombowlus and thanks for the help! However, how can you get the three tubes to go 'pivot up' and then change them ??? Are there any screws one can remove ??? I never seen such a compact and tight space to play around tubes, although my knowledge is fairly low I must say !!!

As I recall, it's one phillips head screw.

 

[chris4001asat]

I had to remove a couple of.....things to be able to pivot the tube rack up.

 

[Turlu]

Yes, it looks like two screws need to be removed. They are very hard to reach however...

 

[Turlu]

To all,

Here is the answer received from GK service department below:

To replace the tubes on the MB Fusion 500, here are the steps:



1) Remove all screws on the top cover.

2) Remove top cover (do not remove the cables to the switches on top cover unless necessary, if you remove them please mark them accordingly to not mixed them up)

3) You will need a 5.5mm Hex Nut Driver that would fit in-between the tube shields.

4) Remove tube shields by pressing down and twisting on the shield. (Ones you remove them you do not have to put them back)

5) Remove tubes.


GK Service was absolutely great in responding to my request !!! Thanks Jose!!!!

 

[Turlu]

Hi all,

Which one of the tube is for Master A ? Is is the one from the left ??

Basically, what functions ( or inputs, etc. )are associated each of the tubes?

If one were to change only one tube, which one would it be to get the most of the newly installed tube ??

 

[B-string]

Hi all,

Which one of the tube is for Master A ? Is is the one from the left ??

Basically, what functions ( or inputs, etc. )are associated each of the tubes?

If one were to change only one tube, which one would it be to get the most of the newly installed tube ??

View attachment 908804

I'm pretty sure the basic circuit is the same as the Fusion 550 with an addition of the A/B gain selection before V2. Each tube has two triode sections used as:
V1: First gain stage and contour.

V2: Post "Gain control" /"Bright switch", High Mid.

V3: Low Mid, Bass and Treble.

The most affect happening from V1 as the first gain stage, but V2 also carries a full range gain circuit post of the second section "Contour" stage of V1.

 

[Turlu]

Thanks B-string and it makes sense.

So, I can assume that V1 is the tube position farther on the left side of the amp, am I correct?

 

[B-string]

Thanks B-string and it makes sense.

So, I can assume that V1 is the tube position farther on the left side of the amp, am I correct?

Yes as viewed from the front, closest to the input jack.