68 Plexi refurbish

[Strat78]

This is a late 68 plexi that was part of a small batch of amps that had a slightly different layout. I've seen about 4 of these elsewhere on the net that have the same layout and use the same components. This one has a 10k serial number but the other ones I've seen have 12k serials, so it looks like they just grabbed whatever back panels were handy. The layout is similar to a typical 12 series but they all have the pre-amp filter can on top of the chassis instead of on the tag board. All the couplers are 600v Dubiler except the .0022uf mustard (still has the read lacquer so it must be stock). The hookup wire comes up through the tag board instead of soldered from the bottom of the board. The choke and OT are hooked to one side of the standby switch, the other side goes to the HT fuse. The HV secondaries go directly to the diodes. I learned the hard way that the standby switch needed to be 'on' to drain all the caps through v1, (ouch, those mains caps pack a punch!). I wonder if there is an advantage to hooking up the standby switch in this way?

Anyway, StarGuitar bought this amp in the Spring and sent it to me to refurbish. The amp had been kept (by Allan Holdswrth's manager) in storage for the last decade. Here is a short thread explaining its history:
http://forum.metroamp.com/viewtopic.php?f=3&t=43143
To my surprise the mains and screens formed beautifully, but the PI and pre-amp filter caps were leaky. The inputs and potentiometers needed cleaning and of course the pre-amp board needed to be put back to right. The old mod was pretty cool sounding but the amp is now back to a former glory as it should be. I kept notes on the mod and will try it on another build to give it a fare shake. We have .68uf mustards for the bypass caps, but I went with the philips for now. 470k Iskra mixers would be cool, but I like the CC's in this area of the circuit. Ceramic doggone in there! A 100% accurate restoration can be done piecemeal as the parts are available, but for now I'm just going for stability and tone. I tried to leave as much of the original amp intact as possible. I only lost the original lacquered solder joints on the PI and pre amp filter caps, everything ells was savable, except where the old mods had been done. All potentiometers are original 'AB' except the presence and vol #1. Vol #1 is scratchy and dirty so its coming out. Actually vol#1 is a 100k! StarGuitar sent a stash of Sylvania 6ca7, so I'll start sorting out the good ones. I have some cheep JJ's in there now, still the tone is pretty powerful:
https://soundcloud.com/iskra68/68plexi

[VelvetGeorge]

Beautiful amp. Transitional with the preamp cap on top. Only seen a few of these.

What are the date codes on the pots?

George

[neikeel]

Excellent stuff? Wonder what the late 68/69 cut off is - presume it has an intact QA label with inspection date.

That way of wiring the standby is common for this period (typically a piece of reddish brown lacquered cloth covering the wire). Suspect it gives the switch a hard time (a dual pole ac standby to the diodes -as per later amps- would get much less hammer).

What is the serial as late 68 would normally be a 12 series (I think SL/A 10 **** more likely?). I have a 10 series but that is grey 0.68uF caps and mustards (same as the one George has or had recently).

How does it compare with your clones?

I have a spare contemporaneous 1 meg AB pot if you want (with a 5k Lemco still attached!).

[emmjaydubya]

Nice work Phil! What an opportunity to work on one of these, great thread.

[Strat78]

Solder obscured part of it, but does xG J8 indicate the year?

[VelvetGeorge]

J8 should indicate October '68.

george

[Strat78]

Here is the tag, I can't make out what the date says maybe you guys can decipher the writing. Tazin mentioned the PT was a second series T2562 PT, what is the story about those? This week I'll A/B my clone with this amp.

[Tazin]

No date written on the tag sheet; just initials, which is typical for anything in the '68 thru very early '69 zone. The second series Dagnall T2562 PT is one that has the H.T. coils internally connected; hence there is only the yellow wire present. The first series T2562 did not have the coils internally connected so you have two wires (yellow and gray) exiting the PT. The second series PT seems to start at the tail end of 1968.

I may have a 5K Linear pot I can part with if your interested.

[rgorke]

Well done indeed!!! What did the mod do?

[vanhalen5150]

Great thread! I've been sitting on a T2562 tranny I had rewired for a while.

Those mains caps look like trash cans!

[Star*Guitar]

There are a few examples of this type of amp on amp archives. I found another example of one on fleabay. I saved the pics from that.

Tarzin said this about these amps in another post

These are those weird transition amps with only two top mounted filter caps. To me, all the indicators point towards very late 1968 (December) or January 1969. What's interesting is that out of the half dozen or so of these amps I've seen there all USA export amps and there all identical in make up which includes the following:
Blue & white (blue & Yellow) Dubilier caps.
Earlier red colored RS 0.005uF volume pot cap for the Super Leads.
Hunts 50uF/450v single element phase inverter filter cap.
A pair of TCC 32uF + 32uF/450v filter caps for the screens filtering....With the exception of one amp which has the Daly 32uF/450v caps.
A pair of the odd TCC 100uF/350v filter caps for the mains filtering.
The brown colored RS 47pF PI balancing cap.
The second series T2562 PT.
The earlier style lead dress for the main tag board...(end of quote)

I noticed that all of them have the Red Lemco 500pf for the bright channel cap.









Notice the 12 series serial number. All of the examples of these amps that I have seen had a 12 series serial number with the exception of a 50 watter on amp archives. The cab on my amp has been recovered. I'm wondering if the back panel is original to my amp. Because it is coming up off the chassis. Also notice the lower serial number, Ed's amp is 12301. Does that means that Ed's amp was made later in 1968 then these?

I appreciate Phil working on this. I am a novice and plan to do a build but for sure wouldn't work on an amp this valuable. Reforming the caps alone scares the hell out of me. lol

[Tazin]

From what I've seen most of the amps of this style were in the SL/A104XX range. A couple of others were in the upper SL/A103XX range, and a couple in the SL/12XXX range. The Super Bass amps with this transition style were in the SB/A103XX range.

Ed's amp appears to be made earlier than this style. Remember, serial numbers are not necessarily an accurate gauge to determine the year of an amps' manufacturing date nor does it reliably determine which amp was manufactured before another. The features of each amp (i.e. component type, wire type and color, lead dress style, transformer type, pot dates, cap dates, etc,.) are what you need to use in order to get an idea of the amps' manufacture date

[Star*Guitar]

While I am still sourcing original parts, I figured I would do the same for the amp cabinet. It currently has an ugly re-tolex job that has to go. Sure wish it had the original tolex, rippped and all. I wouldn't have touched it. I think their pretty. lol, While I was checking out "antique electrical supply awhile back, I found a green levant that looked pretty close to what it should be. I don't see that available anymore. Truth is I am not sure what the original tolex is. Here is the link.

https://www.tubesandmore.com/products/g ... 3DMarshall

Can anyone point me in the right direction? What about the correct white Marshall logo that will fit the original holes?

Are they any other sources for this? I have to buy the rubber feet for the bottom of the amp too. I suppose that any rubber feet would work in this instance.

[neikeel]

While I am still sourcing original parts, I figured I would do the same for the amp cabinet. It currently has an ugly re-tolex job that has to go. Sure wish it had the original tolex, rippped and all. I wouldn't have touched it. I think their pretty. lol, While I was checking out "antique electrical supply awhile back, I found a green levant that looked pretty close to what it should be. I don't see that available anymore. Truth is I am not sure what the original tolex is. Here is the link.

https://www.tubesandmore.com/products/g ... 3DMarshall

Can anyone point me in the right direction? What about the correct white Marshall logo that will fit the original holes?

Are they any other sources for this? I have to buy the rubber feet for the bottom of the amp too. I suppose that any rubber feet would work in this instance.

My 68 is definitely black, my 67s both green/black. Then I have a 69 speaker cab which is green/black (but pretty faded overall.
I had to recover the one I sent to George and the colours were not far off using levant I bought direct from Marshall, none of the green stuff I have seen has been close enough to the originals I have. The recover just needed to sit out in the sun for a summer and slide around in the back of a van for a while...............tell Phil the pot and cap are in the post

[Star*Guitar]

Thank you sooooooo much Neikeel. Really, for everything..