Plexi Vintage 50W 1968 without screen grid resistors blowing tubes

[TAZIN]

Hmmm... how late did Marshall continue the shared cathode? Also, do we now when the 10XXX serial numbers begin to appear? Lastly, do you know anything about the use of these lamps? I've only seen them used a handful of times around 1968 or so (with red dye on solder intact).
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That style of Indicator light doesn't look familiar without me thumbing through my records. The one's used on some of the JTM black flag amps were kind of similar but without the bright metal bezel.

My best guess on the first 10XXX series for the 50w & 100w started around late February - March 1967. For the second 10XXX run with the '/A' prefix (i.e SL/A, SB/A, S/A, T/A, etc.,) I'm thinking late 1968 (December). Some models may have started slightly earlier or later depending on popularity and/or how much old stock still remained.

The shared cathode front end remained for the Bass and PA amps into the '70s as far as I know.

 

[Derrick111]

The shared cathode front end remained for the Bass and PA amps into the '70s as far as I know.

I have seen that too - such interesting info. Like I said once before, you should write a book! Sorry, what I was asking about was for lead amps like the 1959 and specifically the 50w 1987. How late do we see Marshall model 1987 continue with the shared cathode?

 

[TAZIN]

I have seen that too - such interesting info. Like I said once before, you should write a book! Sorry, what I was asking about was for lead amps like the 1959 and specifically the 50w 1987. How late do we see Marshall model 1987 continue with the shared cathode?

For the model 1959 I would say mid '68 which would be some of the 12 series amps.

On the 50w amps it's impossible to determine with any degree of accuracy. This is due to the fact that almost all of the amps have lost their Stock Number badge from the back panel of the cabinet. In addition, the Dymo label, which designates Lead, Bass, Organ, etc., which was adhered to the rear plexi or metal panel after the serial # are either MIA or were never there to begin with. Unfortunately, the Inspection sheet doesn't start to include any type of circuit identifier (i.e. USA LEAD, ENG BASS, etc.) until around October '70. There are some '70 amps from May thru October that have Lead, Bass, Organ written on the Inspection sheet but it's not consistent.

 

[Derrick111]

For the model 1959 I would say mid '68 which would be some of the 12 series amps.

On the 50w amps it's impossible to determine with any degree of accuracy. This is due to the fact that almost all of the amps have lost their Stock Number badge from the back panel of the cabinet. In addition, the Dymo label, which designates Lead, Bass, Organ, etc., which was adhered to the rear plexi or metal panel after the serial # are either MIA or were never there to begin with. Unfortunately, the Inspection sheet doesn't start to include any type of circuit identifier (i.e. USA LEAD, ENG BASS, etc.) until around October '70. There are some '70 amps from May thru October that have Lead, Bass, Organ written on the Inspection sheet but it's not consistent.

OK, so all of these questions were related to helping to pin down a more definitive date on what I believe to be a later '68 50w Lead small box plexi with round lip. It has features that seem consistent with mid-late '68 as well as one or two from the very beginning of '69 except these wouldn't make sense with the '68 features. The only dates I know of on the amp aside from the mustard caps which I did not check are the two large yellow filter cans on top of the chassis. They have date codes of 28 week of '68. That would be around the first week of July so it is pretty conceivable the amp was made in late fall or early winter '68 because there probably isn't usually more than a 3-5 month part date vs amp finish on date from what we usually see in amps and guitars unless there was a huge batch procured. We know there was not a huge batch of yellow can caps procured because they were not used all that long on these. So, given that, and that the amp came from the original owner who didn't do much with it, I would say there is a relatively decent chance that round lip cabinets were indeed used starting in later part of 1968 at least. Too bad we can't be more definitive for these things. I guess if we know when they began to add the date on the chassis inspection tag, that would also help narrow my amp's date down.

Derrick111 said,
The time frame on cabinet lip type sounds pretty close to what I've seen too, although I think I have seen the use of the round lip as early as the second half of sometime in 1968. Sure heads/chassis can be swapped, but too much of a coincidence that I have seen so many round lipped cabinets from sometime in the 2nd half of 1968 that all happened to be swapped...


1000 percent. In fact when I bought this I called the guy out because I myself associated the round lip flat ledge thing to be only for the metal panel.

Still very interested to know what the date codes are on your mustard signal caps. Dating our amps is like an investigation and quite interesting for amp geeking nerding-out.

 

[TAZIN]

I guess if we know when they began to add the date on the chassis inspection tag, that would also help narrow my amp's date down.

I believe the earliest authenticated Inspection tag date that I've seen was the 1st or 2nd week of February 1969.

Aside from the mustard cap date codes, also check the potentiometers and the bridge rectifier if equipped.

I've seen those yellow Dubilier Drylitics used in '69 amps.

 

[TAZIN]

The only dates I know of on the amp aside from the mustard caps which I did not check are the two large yellow filter cans on top of the chassis. They have date codes of 28 week of '68.

I did a quick check on the yellow Dubilier Drilitic filter caps and they seem to be used in some amps built between late February thru April '69. The latest amp I have that used them is from mid May '69. The 50+50uF/450v cans are dated 68 21, 68 23, 68 28, and March '67 while the 100uF/450v cans carry a 68 11 date.

 

[Derrick111]

I did a quick check on the yellow Dubilier Drilitic filter caps and they seem to be used in some amps built between late February thru April '69. The latest amp I have that used them is from mid May '69. The 50+50uF/450v cans are dated 68 21, 68 23, 68 28, and March '67 while the 100uF/450v cans carry a 68 11 date.

Interesting! This model 1987 plexi has two yellow Dubilier Drilitic 50+50uf @ 450v that are both marked 6828.

 

[jftman]

I contacted John Nau, he was on Vacation in Italy and is back at work.
He said the plate voltage is 404v.
I don't yet have the amp with me.
But I am pretty sure a lawyer in Georgia is buying the head