I purchased a ’69 plexi (pray for me)

jeremybusby

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Nice amp. I see this amp suffers from the same factory oversight/mistake as other late 1968 'transitional' amps of this style. The mistake centers around the two 10K/1watt B+ dropping resistors for the preamp. The way it's wired up currently has the B+ voltage not going through the last 10K/1watt resistor before heading over to the two 100K half-watt plate resistors for V1.

It looks like it may be bypassing the choke as well - presuming the two red wires are the input and output to the choke?
 

Matthews Guitars

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The voltages will tell THAT tale. The style they wired up the boards with changed a few times in the late 60s and that includes whether the HV jumper wires went one way or another to the 10K-10K stack. I got bitten by that variance myself. If you wire it wrong and bypass the choke, all your high voltages will be TOO high. Once I corrected that, every voltage was within spec as per the period correct print.

Basically some versions feed the totem pole from the top, others, from the bottom. Look for whether nor not the HV jumpers under the component layer are arranged in a V /\ or in parallel. //

Once I moved that jumper from bottom to top, or top to bottom depending on your point of view, everything returned to normal.

I got the amp I mentioned for 600 dollars at a guitar show. And it was RUNNING. It was also a KILLER amp. Literally. It could KILL you. The metal bases of the 6550 tubes were electrically HOT, with plate voltage on them. Touch them, get a heck of a shock! A potentially lethal one.

And let's not forget that it had 86 holes in it that weren't factory original. I had them welded up, reground flush, and had the chassis regalvanized.
 

GWWhitehead

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Doesnt a 68 have a laydown transformer and a plexi front that was changed to brushed aluminum in 69?!
 

ApathyForum

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The one I can see in the corner of one pic looks like a Fender, the number may tell you what it is (and then you will know spec)
Originally would be a Dagnall C1999 which is same spec as earlier Drake 354-114 (so 100ohms DCR, 3H and 250mA rating). I seriously doubt you will find an original but you can buy clones from Marstran or Metro. If (as it looks) yours is the 4H off a Bassman there will be no discernable tonal benefit.
You’re correct, it is a Fender. Number 022699. I’ll probably just leave it be. Thanks for the advice!
 

ApathyForum

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Doesnt a 68 have a laydown transformer and a plexi front that was changed to brushed aluminum in 69?!
Hi there. Yes, the power transformer is a replacement, which I mentioned in my post. I’m going to have it put back down 90 degrees.

This is a ‘68. I initially thought it was ‘69 but changed the info in the thread.
 

ApathyForum

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The voltages will tell THAT tale. The style they wired up the boards with changed a few times in the late 60s and that includes whether the HV jumper wires went one way or another to the 10K-10K stack. I got bitten by that variance myself. If you wire it wrong and bypass the choke, all your high voltages will be TOO high. Once I corrected that, every voltage was within spec as per the period correct print.

Basically some versions feed the totem pole from the top, others, from the bottom. Look for whether nor not the HV jumpers under the component layer are arranged in a V /\ or in parallel. //

Once I moved that jumper from bottom to top, or top to bottom depending on your point of view, everything returned to normal.

I got the amp I mentioned for 600 dollars at a guitar show. And it was RUNNING. It was also a KILLER amp. Literally. It could KILL you. The metal bases of the 6550 tubes were electrically HOT, with plate voltage on them. Touch them, get a heck of a shock! A potentially lethal one.

And let's not forget that it had 86 holes in it that weren't factory original. I had them welded up, reground flush, and had the chassis regalvanized.
Fantastic find, and wonderful that you were able to do the work yourself. 86 holes, though, seriously?!?
 

bobyoung53

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First of all, that amp looks killer the way it is, I'd leave it the way it is, it has character and I'm sure I'll be in the minority here but I'm a bass player who's been using Ampeg SVT's since about 1975 and they have a fan and in all this time I'm never heard of an SVT blowing a power transformer and the amps last forever and I attribute that at least partially to the fan which keeps everything cool inside the head. Since the hole is already cut why don't you just have someone fit a fan into it and know that the amp runs cool? I have a Major with no hole and would not cut a hole in it but if it did have one I'd just throw a fan in it. Just food for thought. You don't even notice the fan about about a few seconds especially when playing. You could get a relatively low RPM fan too if you're afraid of fan noise. With a fan you could tuck in all that tolox too.
 

ApathyForum

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First of all, that amp looks killer the way it is, I'd leave it the way it is, it has character and I'm sure I'll be in the minority here but I'm a bass player who's been using Ampeg SVT's since about 1975 and they have a fan and in all this time I'm never heard of an SVT blowing a power transformer and the amps last forever and I attribute that at least partially to the fan which keeps everything cool inside the head. Since the hole is already cut why don't you just have someone fit a fan into it and know that the amp runs cool? I have a Major with no hole and would not cut a hole in it but if it did have one I'd just throw a fan in it. Just food for thought. You don't even notice the fan about about a few seconds especially when playing. You could get a relatively low RPM fan too if you're afraid of fan noise. With a fan you could tuck in all that tolox too.
Thanks for your comments and input, I appreciate it. I haven't thought about doing that. I think I'll leave it the way it is for the moment. But if I start playing with a band and putting in some serious hours on this thing, I may reconsider.
 

guitronics

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you can always get a new Marshall plexi type head box. You can also get metal panel plates for front/back.

Depending on what replacement parts, you can get it all sorted out & looking nice & you'll be happy w/ it.

Maybe keep whatever old parts you swap out, for better aesthetics, if you do any...
I have a '69 50W Plexi, that needs new Front and Rear Chassis plates. Where can I find them?
 

Matthews Guitars

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I make repro metalface plates for all JMP 50, 100, and 200 watt Marshalls. I'm currently making my first batch of plexi panels as well,
but it will be a little while before I've got stock on as many types as I do the metal panels.
 

Ruben Toapanta

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Hello, I have one question. My father has Marshall JCM 900 Model 2500 SLX Head Black/Gold 5881, made in England, How much is it?, Is the same product of your post?
 

Dogs of Doom

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Hello, I have one question. My father has Marshall JCM 900 Model 2500 SLX Head Black/Gold 5881, made in England, How much is it?, Is the same product of your post?
Hi Ruben, :welcome:to the forum...

No, your amp is totally different series/model. His is a JMP series 1959 amp, from the '60s. Your amp came along around 1990 +/-...

Best bet, you'll have to start your own thread & ask about it. As far as "worth", people will want to see pictures. Being a new member, you might need to get some posts & time on the forum to be able to post images, links, etc.

:cheers:
 

Derrick111

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...new power tubes, anything except JJ. JJs tend to fail prematurely in Plexis, and can take the OT with them....

I like EH Mullard Reissue, and Tung Sol EL34B ( they can take all the power and voltage with very low failure rate, but not as aggressive sounding as the EH)

Really? A counter point - as a tech, I have found that JJs are about the most robust tubes currently avaiable. he Sugs are of good reputation as well, but I don't use them much. In contrast, EH tubes don't tend to be very robust at all and I have had lots of failures with Sovtek/EH tubes. Perhaps you meant the Reflektor made Mullards, not EH? I think there is no such thing as EH Mullard. I believe the New Sensor/Reflektor plant makes EH and purchased the mullard and Tung Sol name and makes them all. They are not however, the same tube at all. In fact, the Mullard seems closer to the Svetlana which Reflektor also makes. That came about after an ill fated partnership between New Sensor and Svetlana. Svetlana made nice EL-34s, but unfortunately no longer does. Anyhow, enough on all that. JJs are great tubes that are descendants of the wonderful Tesla EL-34s of the 80s/90s.
 

neikeel

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I agree with Derrick to a certain extent (as per my previous comment) although I see no relationship between the structure, sound etc between my 37 labelled Teslas and my JJs (I have Ls, std and II in different amps)
 

Derrick111

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I agree with Derrick to a certain extent (as per my previous comment) although I see no relationship between the structure, sound etc between my 37 labelled Teslas and my JJs (I have Ls, std and II in different amps)
The JJs seem to be an evolution... I thought I read something along those lines a while back as well. But we both know that doesn't mean it equates to the JJs and 80s Teslas sounding similar. I never looked closely at the plate structure, but the compressor bars on both have that oval crimp at the top that seems unique to that EL-34. Moot point though, as the point here really is the JJ EL-34's robustness and being an all around good tube.
 

Rej

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Can i ask those in the know what kind of master volume has been installed in the $2099 dollar '68? Reason i ask is that i have a pre phase inverter installed in my '69 50w small box [ also with cascade mod ] and it works great ......just wondering if the one on the '68 is similar? thanks
 

ApathyForum

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Can i ask those in the know what kind of master volume has been installed in the $2099 dollar '68? Reason i ask is that i have a pre phase inverter installed in my '69 50w small box [ also with cascade mod ] and it works great ......just wondering if the one on the '68 is similar? thanks
Yes, it’s a simple pre-phase inverter MV mod on the ‘68. It also works great on this amp, and I don’t plan on removing it anytime soon. It’s very fun to use.
 
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