Power tube isolation ideas-

[AtomCap]

A rather perplexing project has been dumped into my lap.. For the sake of respect to the builder his name will be excluded from the discussion.

Any way its a $2795 basket-case.. Virtually un-serviceable!

So the problem is the power tubes are directly behind the speaker! Major design flaw...

The 5881's are literally 5/8" from the speaker magnet which is large (Fane) The tube sockets are not shock mounted. The fashion in which the amp is built -PTP/Component to component without turret boards etc, makes servicing difficult- its a rats nest inside! Moving a socket will disrupt just about every component in the power amp section, none of the components are isolated with jacketing etc- so any movement will result in shorts.. Need to be careful as they're not divulging a schematic or reference voltages etc.. All be it a Princeton Reverb sub vibrato and having a 6L6 power section..

Installing rubber bushings to isolate the sockets would be a major project.

The vibration is literally shaking the power tubes to self destruct, a new set of tubes, and a half hour of playing today turned a brand new set of JJ 5881's microphonic and popping to the touch.. The socket is tight- so its not a loose connection issue.. The amp chassis is mounted to the plywood cabinet in 8 locations, so lots of vibration coupling through the cabinet into the chassis between the mounts and also lots of surface area touching cab/chassis...

so big question is-

Has anyone tried isolating an entire chassis? Is it pointless?

Need to figure something out because as it sits, the way the amp is built
every month the customers going to need a set of power tubes- Or deal with microphonic power tube noise- which is a bullshit answer for a $2795 amp..

After a rather alarming chat with the builder he was rather apprehensive in acknowledging these blatant design flaws. Claiming they're not real concerns and that the massive tube isolators he includes with the amp should solve that issue..

Crock of bs..

anyway- if anyone has any ideas and doesn't mind sharing great.. I've seen allot of doozies but this takes the cake-

Thanks

[galtjunk]

Build it into a head cabinet.

[pdf64]

Yes, moving to a separate head cab seems the most effective way to stop the issue.
The vibration from the speaker is radiated acoustically via the air, as well as being conducted via mechanically coupled solids.
Use a mic (omni directional probably best) and voltmeter to get a feel for how strong the acoustic pressure is at any particular point.
Dense rubber jackets over the tubes may help to reduce the acoustic coupling, but the tubes would overheat.

[Stevem]

Do what many of even the low power amps of the 50's and early 60s did and mount the power tubes and the OT on another seperate Shock mounted chassis.

[M Fowler]

How about shorter power tubes, tube dampener rings and HiWatt type aluminum plate in front of the tubes to help with speaker waves crashing down on those 5881's.

[TUBEDUDE]

Have the user return it for a refund.

[rp]

Build it into a head cabinet.

Unless it's like a tweed then SOOL.

If it's like a Matchless, try and talk the builder into taking the empty combo cab back and swapping for a head, could be hard if he has to pay and have a head built - the faceplace and back might be upside down, got to deal with that too. If he offers both styles it should all be pretty easy.

Owner keeps the speaker and will have to find an ext. cab or pay for that part from the builder or the shop that makes the head. Get everything drop shipped. Seems the easiest to me, least investment, prolly less than your hours of labor, and holds the value of the amp.

[stretch2011]

I second TUBEDUDE.

[JMFahey]

Same here.

Or you'll be stuck with a full redesign-reconstruction-rebuilding-plain building nightmare.

You don't have a formerly good, now broken amp which by definition only needs repair, but full re engineering.

Who will pay for that?

And even so, success is not guaranteed.

Sorry.

[AtomCap]

The general consensus at the shop is to abandon the project and tell the customer to return the amp, this is our current plan...

upon further inspection - they mounted the speaker through a plywood baffle with 10/24 machine screws- so the screw heads are behind the grill cloth, there is also loctite threadlocker on the nuts on the speaker side, so the blown speaker cant be changed without either puncturing holes in the grill cloth or redoing the grill...

[Reeltarded]

I am not saying shit until you tell us who built it.

Speak up. That is robbery. Someone needs to find a burger flipping job. Not speaking is what the fuck is wrong with this World we live in.

I'm a mean bastard. I'll shout them down if they point hate at you. (and as soon as you tell me I am telling everyone, so.. )

[TUBEDUDE]

I agree with Miles. If not to shout down such bizarre engineering, certainly to prevent another poor soul from going through the same nightmare.

[gktamps]

I agree with Miles and Tubedude.

It's not like this was one stupid mistake, like the PRS Custom 50 amp I bought cheap from a fellow who couldn't figure out why the new amp he bought sounded like shit, with ghost notes, flubby bass, awful distortion... turned out whoever assembled it installed one of the big power section electrolytic caps backwards! Of course it blew up early in life, taking dropping resistors with it, the amp never sounding right until I repaired it.

Sounds like your attempts to work with the builder went as well as did mine; I had to listen to flapping lips as they did nothing to resolve the problem at their time and expense, or compensate me for mine.

Your situation is much worse - and sounds like the builder just doesn't want to admit he needs a major re-think about his approach to building an amp, or maybe his abilities.

Good luck - sounds like you've made the choice I would make. Hand it back to the unfortunate owner.

[Leo_Gnardo]

loctite threadlocker on the nuts on the speaker side, so the blown speaker cant be changed without either puncturing holes in the grill cloth or redoing the grill...

Loctite may be loosened by dropping some solvent on the threads & letting it work in. Acetone's a good choice, and it's a component of the Caig spray-cleaner I use, maybe you have some there. It's worth a try.

Heating the nut may also help loosening it, get a nice hot soldering iron on it & hope the bolt doesn't start "windmilling" along.

FWIW I build baffles with tee nuts friction fit tight & glue besides. On the tee nut externals, NOT on the threads! Why mister smartypants couldn't do this?????

Can't wait to find out who the offending so called amp goo roo is...

[didit]

Detailed photographs necessary to crowd in the appropriate re-design solutions.

Best .. Ian