GK 500MBFusion issues

[ArtechnikA]

At the risk of opening like an old Penthouse Forum letter...
I never imagined myself posting this but...

MB500Fusion owner, not quite a year old (IOW - maybe still under warranty, hafta go read the small print...) is crapping out on me. No volume, and what there is sounds like it's being played through a paper bag... Even with the Drive down, I get crackly distortion. 'A' channel, 'B' channel, same... Multiple basses, multiple cables - instrument and speaker - multiple cabs. The only constant is the amp.

My first inclination is to pop the top, reseat the tubes, and put deOxit on any ribbon cable connectors I find. Hate to do that if it would invalidate any warranty I might still have.

Open to suggestions on what to do next. Disinclined to send it back to the vendor (MF, on a super-duper per-incident discount special) but I will if I have to. Open the top and proceed with Plan A? Just send it off to GK?

I do have a backup with the rack rig, but I bought the 500 for rehearsals and acoustic gigs and it;s worked well - until recently. It is getting worse, if that helps.

TIA...

 

[Red Planet]

Sounds like a bad tube. I would pop the top.

 

[ArtechnikA]

Sounds like a bad tube. I would pop the top.

Thanks - I have a spare 12AX7. I suppose I'll have to go look up the rest of the preamp tube complement...

 

[MrLenny1]

GK engineer use to drop in on TB here.
Maybe you could get a tip on what to do.
Good luck.
That is a great amp, heard one pumping 2 Berg 1x12 cabs. Bass nirvana!

 

[wave rider]

Does the amp have an FX loop? If so, use an instrument cable to jumper between the in and out. Sometime they have a little corrosion.

Think the amp has a two year warantee.

@B-string knows the GK tech to email for other suggestions.

=wr=

 

[Red Planet]

Thanks - I have a spare 12AX7. I suppose I'll have to go look up the rest of the preamp tube complement...

There are three of them I would start there, and one of them can go bad. Also what wave rider suggested is also a very valid/common check, sometimes the switching contacts in FX loop jacks can get a little iffy. Stick a short cable from one to the other and see if problem persists.

And Bstring is the GK expert around here. Also the owner of the company can be found on occasion.

 

[B-string]

@ArtechnikA you can email [email protected] or PM @rag here on TB (Robert A Gallien, president, owner and engineer). There are three 12AX7/ECC83 (ect. lots of different numbers for that particular tube type) tubes.

 

[lakefx]

This sounds exactly like what happened to mine except that I bought mine used so no warranty. I pulled the tubes and hit them with DeOxit. I haven't had an issue since and that was about 6-7 months ago.

 

[ArtechnikA]

@ArtechnikA you can email [email protected] or PM @rag here on TB (Robert A Gallien, president, owner and engineer). There are three 12AX7/ECC83 (ect. lots of different numbers for that particular tube type) tubes.

Thanks for the contact information. GK's legendary support and accessibility were key factors in my choosing this amp in the first place.

I'll try the FX loop trick first - I knew that but was glad to be reminded of it. Then I'll R&R the tubes with a bit of anticorrosion fluid. If that doesn't do the trick I'll swap in my spare one position at a time. Thought I had heard that amps like these are not particularly sensitive to individual tube characteristic - IOW - no real payoff to tube rolling efforts. Am I remembering that wrong, or is that your impression as well?

Thanks for all the responses.

 

[B-string]

Thanks for the contact information. GK's legendary support and accessibility were key factors in my choosing this amp in the first place.

I'll try the FX loop trick first - I knew that but was glad to be reminded of it. Then I'll R&R the tubes with a bit of anticorrosion fluid. If that doesn't do the trick I'll swap in my spare one position at a time. Thought I had heard that amps like these are not particularly sensitive to individual tube characteristic - IOW - no real payoff to tube rolling efforts. Am I remembering that wrong, or is that your impression as well?

Thanks for all the responses.

I tried a couple different NOS and new production tubes in my Fusion 550 and the design is not sensitive to variations. Very little difference between the stock tube and a Telefunken as an example. So long as the tube is good you'll be hard pressed to hear any difference in tubes.

 

[4Mal]

This Amp is pretty new to be experiencing the dirty effects loop jack thing... iME that hits amps in the 5 yrs plus range... Not that it couldn't be, just unlikely. I would guess tube reseat. I have an early MB Fusion and it's been rock solid for me. I don't use the effects loop but I backline the amp many times over the summer while doing sound and providing back line.

Tube rolling... I actually like the stock sound. Never even crossed my mind... mb Fusion is the best GK yet.

 

[Killed_by_Death]

This sounds exactly like what happened to mine except that I bought mine used so no warranty.

Same for me, but I jacked up one of the screws trying to get it open. I didn't realize they're JIS screw heads and was using a standard Phillips head.
Jason at GK said they're not special, but that Phillips screwdriver sure did round out the head of that screw.

 

[ArtechnikA]

I pulled the tubes and hit them with DeOxit.

What did you use to loosen the flanged nuts that retain the tube assembly to the chassis?
My 3/16" nut driver will fit between the shields - but the nuts are bigger than 3/16" My 1/4" driver won't fit in the gap.
I think I'm looking for a short 1/4" nut driver with a really skinny shaft - ideas on sources for such a thing?

Glad I didn't decide to wait until the very last minute to do this but it's looking like I'll get to drag the rack rig out to Saturday's practice...

Jason at GK said they're not special, but that Phillips screwdriver sure did round out the head of that screw.

Thanks for the advisory - My normally trusty SnapOn screwdriver was too pointy for the job but I was able to find one that fit well.
And there's a lot of them - 3 on each end, 6 on the top.
For anyone thinking of doing this, in addition to having the right tools for the screws and the flanged nuts, be aware that the slide switches are mounted to the cover. The pigtails are long enough that you can lay the cover over flat to gain access, but don't just yank it straight up...

 

[lakefx]

What did you use to loosen the flanged nuts that retain the tube assembly to the chassis?
My 3/16" nut driver will fit between the shields - but the nuts are bigger than 3/16" My 1/4" driver won't fit in the gap.
I think I'm looking for a short 1/4" nut driver with a really skinny shaft - ideas on sources for such a thing?

I used a generic nut driver. I don't know the brand, some cheap chinese made crap that I've owned for years. It took some work to get it in there and I had to loosen the tubes a little to get some wiggle room.

 

[ArtechnikA]

Thanks. Probably your cheap Chinese crap tool is nicer and newer than mine that I think I bought in the 70's, which is built quite robust, probably in the interest of looking strong... And it has a tubular shaft which has proven useful on occasion but makes it a lot thicker than it needs to be for this application.

There's gotta be something out there...

 

[lakefx]

Mine looks something like these:

 

[Killed_by_Death]

FWIW, a bit screwdriver can function as a nut driver, but I can't recall the size. I think either 1/4" or 3/8".

 

[ArtechnikA]

Thanks - I've got some ideas of things to try tonight.

 

[ArtechnikA]

Success!
Well - insofar as I had it apart, it's back together, and it still makes bass noises. And not faint, crackly, distorted ones - yet.
Only time will tell of course.
So - for those of you following along at home - here's some stuff I learned...
1) I used a "power driver" type nut driver - the 1/4" hex shaft is _almost_ thin enough to fit. Probably a real high-quality nut driver like I expect they use at the factory would be perfect, but once I found something that would work I cancelled my trip to the hardware store and got on with it.
2) It's 7/32".
3) If you loosen the center hold-down cap, there will be room to work the nuts off. There's not room to remove it completely until the subassembly is unbolted from the chassis.
4) Once the tube board is loose, it's straightforward to remove the tin caps and tubes.

Each tube has 2 pigtail leads that connect the tube daughter board to the main board. Since the octal sockets were quite snug and there are spring-loaded hold-downs in the tin caps, I decided these little 0.1" headers were a far more likely source of bad connection, so I pulled each end of each lead and gave it a light touch of DeOxIt and reassembled. I did a 'tiger roll' with the tubes which were all JJ ECC83S - moved one end tube to the other end and shuffled the other 2 down one space.

"Reassemble in reverse order."

It's very tight/fiddly in there. If you're easily frustrated, this may not be the job for you, at least not all in one sitting. I had the most trouble with the tin caps, whose retaining tabs are easily bent. Remember to reinstall the outer 2 before placing the daughter board. There are 3 fat power wires that run through a slot under the daughter board - those must be pretty much vertically aligned before the board will fit back on right. If you're a bass player, you'll have the manual dexterity and finger strength to do the job, just be careful.

If this _doesn't_ prove to be a semi-permanent fix, I'll probably just replace all 3 tubes because it's fiddly and frustrating enough that I prefer not to do it too many more times. I'll then test the 3 originals, pitch the weakest, and keep the other two as spares for my various other 12AX7/ECC83 devices, of which there are a few.

 

[ArtechnikA]

Only time will tell of course.

It did.

At Wednesday's practice, a low-volume living-room affair, it started exhibiting the same behaviour.
Since it was available, I did the plug-in-the-FX-loop-jacks trick and it recovered.

Last night, it went out and stayed out. No amount of plug-jiggling or loop-jumpering brought any relief.
Plugged bass directly into effects return and got practice-usable volume.
Dead preamp.

Gotta figure out how to get it back to GK where it should still be under warranty.
I suppose it could yet be something with the effects send jack.
If I were the GK service tech I'd probably replace at least the send jack, probably both, and the tube daughter board with teeny ribbon cables as a unit.

I've got a DarkGlass VMTD I can use as a replacement preamp with EQ. That's a whole lot more portable than the 6U rack with the LH1000. Although that's going to be 'what there is' while the GK is off on its service...