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Stabilizing a Marshall Major?

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4/25/2003 9:02 PM

Stabilizing a Marshall Major?

I've ended up with a Marshall Major in my possession, and I was thinking of using it as a bass head. However, I've heard it mentioned a few times lately that they are unstable tube eaters. I'm wondering what the causes of their instability and hunger for tubes are, and if there's anyway to mitigate them. Nothing in the schematic jumps out at me. The voltage is high, sure, but it doesn't seem murderously high for a 6550/KT88 (646vdc in mine with no tubes in place, and the GE book lists 6550's max plate voltage at 660. SVT's are much higher than that anyway). Is it the screen voltage that's too much?
Thanks to anyone with Major experience that can offer any insight.

4/25/2003 11:03 PM

Tim,

I've heard the term 'unstable' used about them as well. I've been able to induce a full-power HF oscillation by connecting the speaker ground to the 'scope ground - the only amp I've ever seen do that with my test setup.

I don't use 6550's in any of mine, although NOS Tung-Sols work well. I think the Svetlana, Valve Arts, and JJ KT88's are all pretty good, and reasonably priced. The UL OT draws output power from the screen grids, the stock screen resistors are pretty small (250 ohms - always check 'em all for drifts and opens) and the screen voltage tracks the plate voltage swings, so these amps are a bit harder on screens at high power outputs than the average bear.

Try disconnecting the feedback wire (it's normally the purple wire from the impedance selector); the presence control won't be as effective as before but the other tone controls will work better, and it may improve the stability situation as well.

Ray

4/26/2003 6:14 AM

Thanks Ray. I was wondering about those 250-ohm screen resistors. Would it do any good to increase the resistance there?
Sounds like you must have a few of these monsters. Have you tried the all the KT88's you mention in a Major? I spoke to Lord Valve yesterday, and he recommended the Electro-Harmonix KT88EH, so I'm gonna give those a shot.
I don't know if mine actually acts unstable. It came to me without tubes, and I had to rebuild the output section, so I haven't heard it yet. I'm surprised that disconnecting the negative feedback might decrease the instability; I figured that would have the opposite effect. Glad to hear it though, I like going feedback-less.
By the way, what's the difference between KT88's and 6550's? The GE book lists 6550's as being the American equivalent of a KT88, but that doesn't quite seem to be the case.
Tim

4/26/2003 4:56 AM

Tim,

If you're going with KT88's I would leave the 250 ohm screen resistors if they're all in good working order. Obviously the 'standard' replacement with 1K 5W resistors is an option as well.

I'm not familiar with the EH KT88's, and I still have yet to get around to testing the Svets, JJ's, Valve Arts and Tesla/Vrsovice KT88's in a Major as I promised the guys over at the Weber tube board, so I can't really give a recommendation right now (my Majors all have GEC's). FWIW, I do know Svetlana KT88's work great in an old SVT.

Did rebuilding your output section include replacing the OT, as it often does in these things? Now you'll have a chance to experience first-hand the whole UL/no-NFB thing for yourself - I'm sure you'll enjoy it.

Real KT88's (as opposed to the early Chinese "KT88's") have higher plate voltage, screen voltage, plate dissipation, and maximum cathode current ratings than a 6550. I don't consider the two tubes equivalent, even though a lot of tube substitution manuals do.

Ray

4/25/2003 11:37 PM
LFOscalator	The Major was originally tubed with genelex KT-88's and not 6550's. The KT-88 is really a heftier tube. Bottom line is to get some full spec KT-88's. LFO

4/27/2003 5:10 AM

Thanks for the info guys. Fortunately I didn't have to replace the OT. The thing came to me after someone had replaced 3 of the 4 octal sockets with the shoddiest work I've ever seen: they had wired the sockets as though the pins were numbered counterclockwise rather than clockwise. Even taking that into consideration they were one pin off. On top of all that, the soldering was amateurish and the sockets they used didn't accomodate the wide base of a KT88. Helluva amp for someone so clueless to be working on. The one fortunate thing is that apparenly they didn't have a set of KT88's to install. God only knows what would be left of Albuquerque today if they had.

My KT88-EH's came today, so I'll be firing the thing up soon.

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