I agree with neikeel. The rectifier diodes have been changed but by this date (October 1970) Marshall replaced the 'block' type rectifier with the typical four individual diodes style for the most part.
The Super PA amps can be very tough to figure out a build date since they tend to use components spanning a couple of years. I would guess this build to be from very late '68 through the first quarter of 1969.
The amp is an early 1967 build and has seen some changes. The mains transformer been changed out to a Mercury Magnetics (P4550-G2 or G1 ??).
The bias caps have been changed and the circuit updated with a trimmer pot.
The two 8K2 1-watt resistors are newer Allen Bradley's and originally it would...
I checked through my files and I believe the input & output jacks are original since other builds around this time frame used them too. Anyhow, here's an example of the typical 1 meg potentiometer used as substitutes during this time. I can't remember the brand name off hand (Ohmite?)...I've...
A side from the mains tx being changed I also noticed the following:
All three 1 Meg potentiometers.
The three power supply filter caps.
The one bias filter cap & the bias circuit diode. Also updated with a trimmer pot.
The rectifier diodes; along with being relocated due to a...
The 'S/ xxxxA' serial number setup was available for the first 2,900 amps. But, the 'S/A xxxxA' serial number setup was also being used at the same time starting around S/A2000A. So it is possible to see two amps with the same serial number but differing a prefix (e.g. S/ 2212A & S/A 2212A).
The amp looks to be from the end of September through the beginning of October 1970. Those big silver colored caps I haven't seen before so they may not be original...Usually around this time frame Marshall used WIMA caps as substitutes (normally the 0.1uF value) for the Mullards. The Mullard...
The OP contacted me via private message and I gave him my input. Anyhow, I'm thinking mid February 1969 based on the green HUNTS signal caps in combination with the white colored link wires. From what I've seen, Marshall started to have an issue with acquiring hookup wire in the colors they...
The general rule of thumb is the lower the AC voltage the higher the amperage...So, USA amps run a 3 or 4 amp mains fuse & a 1 or 1.5 amp H.T. fuse. England amps run a 2 amp mains fuse and a 1 amp H.T. fuse.
The stock bright cap is the tan colored Lemco 5K; labeled as such.
Position V2a should be a 1K resistor with no bypass cap.
NFB is a 100K resistor from the 4-ohm tap.
Early '73 amps used the 0.1uF/5K Presence setup and by mid year the 0.68uF/25K/4K7 seems to have been introduced. This later...
It's a Canadian version which is why it has the CSA label on the back and the dymo labels for the tubes. Date wise your looking at very late 1968 or early 1969. If your able to post pics of the electronics (turret board) then you might be able to narrow down the build date even further.