Hey, all, I have a preamp tube issue on my Fillmore 25 head but the tubes are near impossible for me to get at even with removing the power amp tubes first. First off is there an easy way to remove the front grill? That would make it a breeze to get to the preamp tubes. if not, do people recommend removing the amp chassis from the case in order to do this? Or any other suggestion Thanks.
Change preamp tubes, Fillmore 25 head
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I’m guessing this is a head. I’m not sure about the front panel, grab a flashlight and look inside the head a see if there are screws in the back of the front panel. They might not be accessible with the chassis still in the head shell.
Have you tried flipping it upside down? I find it easier to deal with tubes when they are not hanging upside down from the chassis.
Yes, you can remove the chassis for the easiest access, just make sure you make note of the reverb tank connections when you disconnect them. Don’t go poking around inside the amp, the power supply filter cap hold about 450 volts even with the amp unplugged.
Hope this helps,
Dom
I’m guessing this is a head. I’m not sure about the front panel, grab a flashlight and look inside the head a see if there are screws in the back of the front panel. They might not be accessible with the chassis still in the head shell.
Have you tried flipping it upside down? I find it easier to deal with tubes when they are not hanging upside down from the chassis.
Yes, you can remove the chassis for the easiest access, just make sure you make note of the reverb tank connections when you disconnect them. Don’t go poking around inside the amp, the power supply filter cap hold about 450 volts even with the amp unplugged.
Hope this helps,
Dom
Thanks for the reply! After doing it the first time I would agree that flipping is probably better. I don't see any screws holding the front grill in and haven't tried taking the head out to look yet. Any chance someone here has done this and knows how the grill is attached? Thanks!
The grill appears to be attached from the inside. If it was removable like the JP2C (standard model) or the Mark VII it would have 4 screws on the front panel on the outside of the shell.
Make the attempt to remove the shields and preamp tubes without removing the chassis from the head shell. It may be tight as I am not sure how much room there is based on the images I have seen.
The 25W models do present a challenge due to their size.
If you opt to remove the chassis out of the head, be aware there is a chassis tension screw that places tension on the chassis after it is secured into the head shell. this is to prevent vibrations. Losen that screw first before removing the 4 screws at the top. If you fail to do this, you will end up tearing the foil shielding that is on the wood surface on the top of the amp. I captured an image from MESA's website and circled the screw.
The easiest method is to place the amp face down on a soft surface like a carpeted floor. Position the amp so the four head screws are towards you and the feet are facing away.
Remove the power tubes and set aside.
Take a picture with a digital camera (smart phone) of the reverb wire connections so you have reference how they go back in. You may be able to remove the reverb cables while the chassis is secure in the shell at this time. Leaving them attached will make things complicated.
loosen the tension screw, it will wobble around a bit but do not worry about that. The loosen all 4 chassis screws but do not remove them.
Remove the bottom two screws first, these are the ones in the front of the amp. Then choose one of the rear screws to remove. Since I am right handed I usually choose the screw on the left to remove first. Then I can use my left hand to support the chassis while removing the last screw (on the right).
Then reach in with both hands and grab the larger transformers. You will need to move the chassis away from the top panel a bit before sliding it out. The screws that secure the handle may interfere with just sliding it out as well as the chassis rubbing up against the protective foil shield that is on that surface.
Once you get the chassis out, place it on a flat surface with the tubes facing up. Remove the shields and then replace the tubes.
I find it easier to remove the amp chassis but depends on the amp in question. Some tubes are hidden behind the transformers which makes it difficult to gain access. However, that is not the case here.
Most important if removing the preamp tubes and not having the chassis out of the head shell, remember to note how the tubes came out. Preamp tubes will have 9 pins with a gap between two of the pins. That gap may be facing the front, or it could be indexed. Getting the preamp tubes back in is the tricky part as you have to do it by feel when the chassis is in the head shell. The tube shields can be tricky too but not impossible. Push them into the chassis and rotate to lock in place. There are nubs on the sides of the shields and the hole they go into also has cutouts for those nubs. The spring is inside of the shield, and they will be under tension when removing them. Getting them back onto the preamp tubes, take care to get the spring over the glass wick or seal. That can be tricky if the spring leans to one side after removal.
Assembly will be the opposite of the removal process. Good luck with that. In most cases, it is not necessary to remove the chassis from the head shell to replace preamp tubes. In some cases, it may not be possible to do that, it does not appear that is the issue here. There should be plenty of room to access the preamp tubes once you have oriented the head in a position that makes access easier.
Make the attempt to remove the shields and preamp tubes without removing the chassis from the head shell. It may be tight as I am not sure how much room there is based on the images I have seen.
The 25W models do present a challenge due to their size.
If you opt to remove the chassis out of the head, be aware there is a chassis tension screw that places tension on the chassis after it is secured into the head shell. this is to prevent vibrations. Losen that screw first before removing the 4 screws at the top. If you fail to do this, you will end up tearing the foil shielding that is on the wood surface on the top of the amp. I captured an image from MESA's website and circled the screw.
The easiest method is to place the amp face down on a soft surface like a carpeted floor. Position the amp so the four head screws are towards you and the feet are facing away.
Remove the power tubes and set aside.
Take a picture with a digital camera (smart phone) of the reverb wire connections so you have reference how they go back in. You may be able to remove the reverb cables while the chassis is secure in the shell at this time. Leaving them attached will make things complicated.
loosen the tension screw, it will wobble around a bit but do not worry about that. The loosen all 4 chassis screws but do not remove them.
Remove the bottom two screws first, these are the ones in the front of the amp. Then choose one of the rear screws to remove. Since I am right handed I usually choose the screw on the left to remove first. Then I can use my left hand to support the chassis while removing the last screw (on the right).
Then reach in with both hands and grab the larger transformers. You will need to move the chassis away from the top panel a bit before sliding it out. The screws that secure the handle may interfere with just sliding it out as well as the chassis rubbing up against the protective foil shield that is on that surface.
Once you get the chassis out, place it on a flat surface with the tubes facing up. Remove the shields and then replace the tubes.
I find it easier to remove the amp chassis but depends on the amp in question. Some tubes are hidden behind the transformers which makes it difficult to gain access. However, that is not the case here.
Most important if removing the preamp tubes and not having the chassis out of the head shell, remember to note how the tubes came out. Preamp tubes will have 9 pins with a gap between two of the pins. That gap may be facing the front, or it could be indexed. Getting the preamp tubes back in is the tricky part as you have to do it by feel when the chassis is in the head shell. The tube shields can be tricky too but not impossible. Push them into the chassis and rotate to lock in place. There are nubs on the sides of the shields and the hole they go into also has cutouts for those nubs. The spring is inside of the shield, and they will be under tension when removing them. Getting them back onto the preamp tubes, take care to get the spring over the glass wick or seal. That can be tricky if the spring leans to one side after removal.
Assembly will be the opposite of the removal process. Good luck with that. In most cases, it is not necessary to remove the chassis from the head shell to replace preamp tubes. In some cases, it may not be possible to do that, it does not appear that is the issue here. There should be plenty of room to access the preamp tubes once you have oriented the head in a position that makes access easier.
