Marshall JTM45/100 550V!!!

[Smitty]

A good friend just brought over his brand spanking new Marshall Heritage JTM45/100. After unpacking it, setting it up and warming it up for five minutes, we cranked it up. It lasted almost two minutes before suffering a catastrophic tube failure.

Upon pulling the back cover the problem was obvious. The beam forming plate on one of the tubes had arced over to the plate leaving a 1/4" hole in both. I pulled the chassis. Fairly decent build quality. Pulled two tubes to check voltages and idle current.

550V on the plates and 548V on the screens. Idling Chinese KT66s at 29 watts! Yikes. No wonder it failed.

Not that it matters but I think the KT66 is a 500/400V tube. That's exceeding the screen limit by almost 40%.

Backing off the idle current to 35ma yielded the expected cold and grainy tone.

Any ideas why these guys decided to go with the English high voltage 850VCT tranny rather than the US 770VCT version? How do they expect current production tubes to live??? I can't even imagine how fast these would fry a quad of EL34s.

BTW screen resistors were 1K. Kind of high for KT66s if you asked me.

Ray is this a candidate for that choke input to the screen supply mod?

[Ray Ivers]

Smitty,

If the PT HV secondary it center-tapped - as you say it it is - then yes, that CT should supply something like 275V to whatever's connected to it.

Ray

[Steve Conner]

I went to the Marshall Amps roadshow a couple of months back, and their new prototype Hendrix signature stack died within about 20 seconds of the demo guy starting to wail through it. I'd be willing to bet it was a similar failure mode. :-P

I used to have an old 50w PA amp that ran EL34s at 550V on the plate and screen. I was none too happy with that so I started looking at ways of reducing the voltages, which eventally ended up being the "Toaster" amp.

If the beam plate arced to the plate, how is changing the screen voltage going to fix that? The beam plate is connected to the cathode. If it were me, I'd probably end up putting Svetlana KT88s or 6550s in the thing.

[Ray Ivers]

Steve,

If the beam plate arced to the plate, how is changing the screen voltage going to fix that?.

Unless the screen was getting white-hot and softening the metal, it won't - but lowering this kind of way-high screen voltage is still a good idea IMO, especially when an amp seems to be accumulating a history of ka-blam!-type tube failures after a relatively short time on the market, as this one seems to be doing.

I would think the purchasers of this amp would be adamantly EL34-only, but if not, the KT88 you mentioned (800Vp/600Vs) or KT90 (850Vp/650Vs) should work much more reliably than practically any EL34.

Ray

[Enzo]

Now I thought the EL34 was a true pentode, so it has a suppressor grid rather than a beam forming plate. No?

[Ray Ivers]

Enzo,

Yes, absolutely - the original EL34 had an actual suppressor grid winding, and FWIW that's always how I think of them being constructed (I added the "original" to cover my ass for the God-knows-how-many beam tetrodes that are kicking around with "EL34" stamped on the glass).

I also take back my "adamantly EL34-only" comment, as apparently this amp is an attempt to recreate the early JTM's w/KT66 outputs.

Ray

[Steve Conner]

Hey, Hendrix used 6550s in his Marshalls, so they must be good!

http://www.backstage-lounge.com/stor...storycode=5695

That article also mentions that he preferred the sound of amps with extra-high plate voltage. (So maybe Marshall's new Hendrix signature head that I saw self-destructing, is a JTM45/100 with Drake transformers that pump out even crazier plate voltage.)

http://namm.harmony-central.com/WNAM...ure-Stack.html

This link says it uses KT66s, even though Hendrix never did. If it's anything like the original poster's amp, I agree they're not going to last long! Maybe Marshall have a pile of tubes they need rid of ;-) If it were me, I'd change to a good brand of 6550 and get a kick from knowing I was using pretty much the same rig as Hendrix.

Ray: if it's got beam plates it doesn't deserve to be called an "EL34"

[Ray Ivers]

Steve,

Hey, Hendrix used 6550s in his Marshalls, so they must be good!

I like 'em too! IMO the GE's and old Tung-Sols get certain sounds that no other tube can quite duplicate.

If it were me, I'd change to a good brand of 6550 and get a kick from knowing I was using pretty much the same rig as Hendrix.

...with the extra added benefit of not squinting/cringing every time you switch the standby switch on.

Ray: if it's got beam plates it doesn't deserve to be called an "EL34"

I agree 100%. I have at least a dozen of them.

Ray

[Smitty]

Won't Accept 6550s

I've got a quad of current production Tung Sols laying around that I'd love to slap in there. The shark jaw retainers are riveted to the chassis.

I've been hearing that if you can get a quad of JJ E34Ls to bias up stable they'll live up there in that B+.

I emailed Marshall Technical Support before I started this post. I haven't heard back from them yet. I'll be sure to post their response or lack thereof.

[Satamax]

I've got a quad of current production Tung Sols laying around that I'd love to slap in there. The shark jaw retainers are riveted to the chassis.

I've been hearing that if you can get a quad of JJ E34Ls to bias up stable they'll live up there in that B+.

I emailed Marshall Technical Support before I started this post. I haven't heard back from them yet. I'll be sure to post their response or lack thereof.

Try KT90 then, same size as a EL34 or thereabouts or just push the sharkjaws down to check if it's working, and after modify accordingly!

http://cgi.ebay.fr/EI-ELITE-KT90-MAT...QQcmdZViewItem

[kg]

Try KT90 then, same size as a EL34 or thereabouts or just push the sharkjaws down to check if it's working, and after modify accordingly!

http://cgi.ebay.fr/EI-ELITE-KT90-MAT...QQcmdZViewItem

kt90s are crap tubes.

[Satamax]

kt90s are crap tubes.

Yeah, may be he should try some of that shitty stuff! http://search.ebay.fr/6P3S-E_W0QQflo...sPageNameZWD2S

Don't even bother taking readings if it doesn't redplate

[Jelle]

Hi guys,

I also have a 560V superlead with an 100w EL-34 OT and I really like the idea of replacing the el-34's with 6550's!
I have a couple of questions about putting 6550's in here:
-The 6550 data sheet tells me to reduce the 220 bias splitters to 47k. Should I do this? Hendrix amps needed service regularly so I think I know the answer...220K
-Should I change the NFB?
-What value of bias current should I go for, using the 1 ohm cathode method?
-Do you recommend 1.5k or 5.6k grid stopper resistors?
-What is the minimum screen grid filtering that you recommend? I use 32//32 right now.

Thanks,
Jelle

[Regis]

I was reading that Hendrix gear link Steve provided and was struck by this:

Hendrix’s Marshall of choice was the 100-watt Super Lead driving two 4x12 cabs, and his standard backline would quickly grow to three Super Leads and six 4x12s. He plugged his guitar into one amp, and linked it to the others by running a cable from an adjacent input (the Super Leads had four inputs) to the second amp’s input jack, and so on.

This was not the first time I heard of this but to put it in context, at my last band practice I used my rebuilt Bassman 100 with my 4x12 dimed with no attenuation, and it was real loud.

I like loud, but I can't imagine what three Super Lead stacks was like.

[Steve Conner]

At least they didn't have "11" faceplates back then