Marshall plexi reissue OT dilemma

[Randall]

I have a 50 watt plexi reissue in for generally not sounding very good, lower output than it should have and raspy tone. I don't even know exactly what model it is, damned Marshall. All it says on the front is MKII, and the board says 1959slp, but other ones I have seen claim to be 1987X. I don't know.

It has a preamp tube with a weak side, and the outputs were biased hotter than Hades, but correcting those things, as well as subbing all tubes does not correct the underlying complaint. Voltages don't look out of line. It does it on both channels.

So,, I have been burned before thinking OTs usually either work or they don't, or almost don't. If this one is faulty, it will be the best functioning faulty OT I have ever had in front of me. An OT swap is all I can think of at this juncture, but all I have on hand are Fender replacements, one for 6V6 and one 6L6. What is stopping me from clipping one of them in to see what I get?

And if that is OK to do, and makes an improvement, what in the world do I use to replace the original, and where do I get one? The original is a Drake TXOT-00005, no longer made I believe. This is a $3K head, and I know this dude is going to be very concerned about what it gets replaced with if it comes to that. Impedance taps come to mind as an important issue, too, given the 3 way switch.

[Enzo]

I had a 50 watt Fender Bassman OT by my bench for years. I used it as a sub in just about anything. It is close enough for a go.no-go test. So what if impedances are off. It will tell you right now if the old one is frotzed. Worth a try.

Just my opinion, but don't decide what to do about it until the need arises. If the OT is bad, we deal with it. If not, then no point in wasting time on it.

[Leo_Gnardo]

An OT swap is all I can think of at this juncture, but all I have on hand are Fender replacements, one for 6V6 and one 6L6. What is stopping me from clipping one of them in to see what I get?

And if that is OK to do, and makes an improvement, what in the world do I use to replace the original, and where do I get one? The original is a Drake TXOT-00005, no longer made I believe. This is a $3K head, and I know this dude is going to be very concerned about what it gets replaced with if it comes to that. Impedance taps come to mind as an important issue, too, given the 3 way switch.

No problem clip-swapping in a different OT. I've done it scores of times. Most recently in a rare 100W Marshall 90's combo that showed me a near short on one side of the primary. Customer said "it never did get very loud..." Very suspicious for what's supposed to be a 100W amp. Only half the sine wave showed up on the scope, and not very large at that. Transformer in distress all right. Clipped in a Twin Reverb OT, got 80W right off the bat, good looking waveform, that proved it for me. Then the fun part - owner wants "the best possible part" and insists on a $303 Mercury OT instead of a JCM900 Hammond replacement at a mere $107. Now the fun part - shipment didn't arrive on time then was tracked just a bit off course. Way the hell across the Pacific in Guam. Finally made it here a couple days ago and I'll deal with the repair probably over the upcoming weekend.

Since your amp dude is "very concerned" he might want to lay out the dosh for a Mercury part. Let's hope it ships directly to you without any side excursions to exotic places. If it was me, I'd install a generic Marshall style 50W OT probably from Hammond or Heyboer and call it a day.

[nosaj]

Well if it is bad and he is that concerned about it. Tell him to get you one and you'll put it in. Put the ONUS on him if its really that important to him. Otherwise sell him a MM one. I'm sure itll sound better than a raspy OT.
nosaj

[Helmholtz]

I have a 50 watt plexi reissue in for generally not sounding very good, lower output than it should have and raspy tone. I don't even know exactly what model it is, damned Marshall. All it says on the front is MKII, and the board says 1959slp, but other ones I have seen claim to be 1987X. I don't know.

Being a 50W plexi reissue it would be the 1987X model. 1959SLP is the 100W plexi reissue. But as the circuits are essentially identical, the same PCB can be used for both models.

Did you test the OT?

[Randall]

Yes, I have confirmed it is a 1987X.

I clipped in a Fender 6L6 OT, the difference was subtle. I put the original back in. I found a bad cap on the presence pot. Now I'm back to not really being sure if it is better or not. I'm not too sure about this one.

[Helmholtz]

So start troubeshooting.

Measure DC voltages (especially at cathodes), trace and measure signal voltages, measure output power.

A scope would come in handy.

[Randall]

While writing here to you fine fellows about having found a loss of signal in one side of the PI, and being perplexed as to why, I answered my own question. Sometimes I will get to where I need to be by organizing my thoughts accurately by writing them out here. Just before I hit send, I thought, "you know, I better double check something". Sure enough, open PI plate resistor. I was focused on what I thought might be a OT or NFB issue, and overlooked it, thinking the loss originated elsewhere.

I am disappointed I let it get that far enough to clip in/out another OT.

[g1]

When you said in first post 'voltages don't look out of line', I kind of figured you meant all tube plate voltages in there too.

[Randall]

Yeah, I somehow missed that one. Customer kept talking about OT, which set me off wrong I suspect.

[Enzo]

I tell you what, after decades in a retail shop, the number one predicted failure is the transformers. I swear they get together on weekends and plan to come in and tell me they think it needs a transformer.

[The Dude]

I've even had customers tell me they're sure they have a bad output transformer....................... in a solid state amp.

[Enzo]

I don't doubt that for a second.

[glebert]

I've even had customers tell me they're sure they have a bad output transformer....................... in a solid state amp.

Isn't there a solid state Peavey with an OT? Musician 400G or something?

Also, transformer, transistor, who really can tell the difference?

[Randall]

This amp barks like a rabid dog now with both sides of the PI working. I think the older I get the more I overthink the wrong things and get distracted. Maybe at my age I should get used to it. I do the same thing working on my motorcycles sometimes. Where are my glasses....