Dating a 50 watt plexi head and 8x10 cab

Matthews Guitars

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I note it has no grid resistors on the power tubes. They should be added if the amp is to be run with modern power tubes.
 

sct13

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You may also want to replace that power cable with a grounded three prong … unhook that death cap and polarity switch … leave them in … but unhook them…
 

nightbird

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Grid resistors and 3 prong plug, great advice but not my amp. My friends brother, who now owns it is probably going to sell it. He doesn't play guitar.
 

nightbird

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Wow !! This is very close to mine… I have many of the same components… only mine was built for export to Canada and has all the Canadian safety “Features” like fuse placement… stuff like that… and had the OT replaced (period correct part) ….and yours is very clean!!

Sweet spot is “6” … “8” starts to get flubby…

Yours is late 68 …early part of 69…

Mine has some caps that (I was told) date to 65…
Is yours a PA?
 

sct13

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It probably is not … I thought PA amps had inputs in between and at the end… I’m not sure what internal differences are…

Come to think of it it’s not… something about a 68uf cathode follower …

Too bad … you should buy this… these scream…
 

sct13

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Looking at mine Tazin and Neikiel are right…

And there is no cathode follower on yours
 

neikeel

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The basic plexi PA amp is very similar to the Bass and Organ of this era (the PA has no cap on the pots or the mixers, bass has 500pF on mixer of ch1 but no pot cap, organ has 500pF mixer on ch1 and 100pF cap on vol 1 pot). The earlier lead amps are the same as the 68on organ. The lead amps from 68 had split cathode on v1, a different tone stack, extra 0.68uF bypass cap on V2a cathode and smaller pi output couplers.
They all have a cathode follower (maybe check your Marshall topology and nomenclature) as this is how the tone stack is fed.
 

nightbird

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Thanks for the info, neikeel, but most of that is over my head, So, if my friend decides to sell this, would it be advertised as a 50 watt , PA?
All the searching I've been doing shows PA amps have 8 inputs.
I guess I'm looking to see what model this is.
 

Seanxk

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Why they made two versions of the 50 watt Pa I have no idea, possibly they had just too many amps made 68/69.
 

Matthews Guitars

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Lots of companies make/made multiple versions of a given amp. Mesa alone made not less than five distinct versions of the Mark III amp, identified by a black dot or stripe, red stripe, blue stripe, green stripe, or purple stripe, that stripe being a mark on the chassis back face near the power cord grommet. Internally the differences between these five versions range from minor to major. Yet they're all Mark IIIs.

I'm not convinced that Marshall ever made 100 identical amps in the JMP era and earlier. Even looking at amparchives, it's uncommon to find two of the same type and year that are really identical.
 


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