1967 Marshall Plexi 50

[Vpvalmus]

Hey guys, I'm new to the forum and to vintage marshalls. I recently acquired this vintage small box 50 watter and not sure of the year. I think it is a late 1967. Here are my thoughts. It has the original transformers. All the mustard s and electrolytics have been changed. It appears to have all the original pots. I was able to get the date code off one and it and it was July 1967. Here are some of the reason I believe its a late 1967 other than the pot code. I have seen 1967 with 2 different size filter caps. In the pics you can see that one of the cap retainers has been changed, probably because the original cap retainer was smaller. There is no .68 caps. All large pots. It has the older ohm and voltage selectors. I have seen some post that the turret board mounts are different in older Marshalls, but I don't know the difference. I have ordered NOS late 1966 mustard caps (.022 and .1). Some strange points is the solid read turret board and the twin .47 electrolytic caps. Any comments I would greatly appreciate it. Once again, this is my first post on this forum and started collection vintage Marshall amps this year.

 

[Beryllium-9]

Welcome! Late '67 looks right. Maybe early '68. I have the same amp and the pots date 2/68. Great amp! A lot of great resources here and I am sure the others will chime in.

 

[Vpvalmus]

Welcome! Late '67 looks right. Maybe early '68. I have the same amp and the pots date 2/68. Great amp! A lot of great resources here and I am sure the others will chime in.

Thanks, Do you have any original mustards? If so, what are the date codes? I have found that the cap date codes are usually earlier than the pot codes. Also, I couldn't get the knobs off the pots and had to pull out the shaft of the pot to see the date code. The screws were frozen even with pentrating oil. Go figure. Thanks again, Vic

 

[Ned B]

The first shot appears to be a different amp. The one with the .68 Lemco.

 

[Vpvalmus]

Yes it’s is Sorry. I think that was a 68.

 

[StingRay85]

Nice looking amp. If I may ask, how do you know that your purchased late 66 mustard caps? They would surely be a nice addition

 

[Vpvalmus]

The mustards have a date code. The sixties would be a letter A, B, C or D. A is the first quarter and D being the last quarter. The next will be a number is the year and if it's D6, that would be late 1966. The third number or letter is some kind of production code. I believe the the 70s mustards may have a different date coding.

 

[Beryllium-9]

The mustards have a date code. The sixties would be a letter A, B, C or D. A is the first quarter and D being the last quarter. The next will be a number is the year and if it's D6, that would be late 1966. The third number or letter is some kind of production code. I believe the the 70s mustards may have a different date coding.

Mustard Caps ID help

I was wondering about these numbers on some old caps. I believe these to be Mullard caps? anyways are these numbers a date code? and does anyone know how to read them? I may be on the search for a cap and don't want it to look out of place more than needed. Amazing seeing some of these being...

www.marshallforum.com

 

[StingRay85]

The mustards have a date code. The sixties would be a letter A, B, C or D. A is the first quarter and D being the last quarter. The next will be a number is the year and if it's D6, that would be late 1966. The third number or letter is some kind of production code. I believe the the 70s mustards may have a different date coding.

I have now over 2000 mustards and still couldn't figure out the date codes I must confess. I have to admit I don't really care about the year they were made

 

[neikeel]

The solid board with spare turret near the bias feed dates it to 67. These were the transition from JTM 50 Black Flag with GZ34 rectifier so the filter cap mounts were different sizes, As you say one is newer.
This appears to be PA spec (no mixer bypass cap and 56k slope resistor.
Why it is so original except for the mustards and treble cap I have no idea except someones twisted idea of cap replacement to freshen it up with all new caps (eye roll!).
Marshall bought a big supply of D7W 0.022uF mustards - you see them all through 68 and early 69.
66 will be close enough.
Wonder what that odd loop is soldered to the treble cap lug
Correct preamp cap is 32/32uF 350v Hunts/Erie. Nearest is dual axial F&T 33/33.
Mains would be 32uF 500v and each of screens and PI would be 32 uF.
Really nice amp well worth some careful sympathetic resto work.

 

[Vpvalmus]

Thanks Neikeel. I have just started this year getting into vintage Marshalls and I'm trying to learn the circuits. I appears you know a lot about them. If you don't mind, I have a few more questions.
What is PA specs? Is that the .68 cap the mixer bypass cap?
Do you see anything else that maybe incorrect?
The filter caps are JJ 50 + 50. Should those be 32 + 32?
The 2 47uF caps in the middle should those be 32 and 32 uF?
If so, what difference would it make?
Yes, the loop soldered is strange.
Also, I found some late 66 mustards .022 and .1, 6 each for $300 shipped. I guess that's a good deal.
I really appreciate the info.

 

[neikeel]

The presence cap should be 0.1uF (you can use 400v as original although you occasionally see 0.1uF 160v.
These are much sweeter if you lower the filtering so imo use a 50uF (only half of the can you have now) and get 32/32 can for screens/pi and get that F&T dual axial for preamp.

 

[sharbono]

That is killer! I have a ‘67 JMP50 . . . nothing sounds like these amps, they are so immediate.

 

[arthur.lowery]

Hi
What's the Serial Number, please?
Looks better than mine (10348), because at one stage (probably late 60's, the mains transformer blew and they replaced the GZ34 with solid-state rectifiers.
Here's the turret board for comparison.
Cheers
Arthur

 

[Vpvalmus]

It’s 10942. So yours is probably a little older. So others chimed in and said it was a transition from the black flag. Did you add screen resistors? I found some nos mustards from late 66 and switched out the non mustard ones.

 

[stickyfinger]

Lets see the Mustards. If the date code has P in them I'd assume 70s

 

[arthur.lowery]

Lets see the Mustards. If the date code has P in them I'd assume 70s

The caps to the grids of the output tubes: 2 are original; the one's marked 100n are 1980's or earlier (I added them for the Master Volume so there is no DC on the pot track). I might change them all to 220nF, and that would be closer to the original 0.1 uF. (That is, 2 in series have 1/2 the value. ). That said, Marshall reduced these grid caps to 33nF later on (and maybe 22 nF), and I have the equivalent of 50nF now. Larger values can cause low-frequency instabilities, like motorboating.
Arthur

 

[Vpvalmus]

Thanks for the info. Here are the pics of the NOS mustards. They do have P after the date code. I looked at my 70s amps and the mustards had an S after the date code. Let me know you thoughts. Either way, I think the amp is better with the mustards v. the previous caps.

 

[stickyfinger]

I'd say 76. Don't ever recall seeing P in a 60s amp with mustards though I have seen some 76. 77, forget, amps with P dated mustards.
Regardless amp looks correct now, hope you are enjoying it!

 

[StingRay85]

Is there someone a complete guide on how to do interpretation of the mustard caps? I would be highly interested in that