The stock '68 12xxx layout

[tWreCK]

I've been reading through the various threads and build forums for a correct layout of a stock '68 era 12xxx plexi. Do the standard '68 specs differ from a '68 12xxx amp? I found a couple of layouts with different component values etc. Can someone point me to a '68 "stock" layout? I downloaded George's 12000 parts list but again it differs a little from what I've seen in the layouts.

[wdelaney72]

The only thing consistent about Marshalls from '67 - '69 is their INCONSISTENCY.

My understanding is 12xxx refers to late '67 early '68 but things were constantly changing. Components that varied around this time:
move to split cathode
.1uF to .022 PI caps
Tone stack 56k/250pF to 33k/500pF
V2 Cathode bypass cap. Values varied from none to 330uF. Marshall eventually settled on .68uF
Bright Cap on volume pot.

This was right after the change from Drake OT to Dagnall OT...perhaps they were optimizing for a slightly different OT....just speculating.

[tWreCK]

Thx for the reply.

Yeah, that's pretty much the same conclusion I've come to.

Unfortunately a lot of info has gone lost on the forum due to the '14 crash and recent move. Makes the search function kinda irrelevant since all the links/attachments/docs aren't working anymore. A great shame!

Nevertheless good info in your post - every little bit helps I found the following board layout (probably posted by George back when he offered kits) and it seems to be the "best" representation of a '68 era 12000-series I've found to date. What do you guys think?



BTW, are all resistors rated 1/2W except for those marked 2W?

[Strat78]

That's pretty much it right there. There are small things that you can interchange like 820R instead of 2.7k at V1B. A dual 15/15uf for the pre amp filtering intead of the 32/32. Where you have the bias tap (purple wire) combined with the feedback resistor, seems to influence the feel quite a bit. 32uf cans for the screens seems to be classic for the 12xxx. Several members are gearing up for some 12xxx builds, maybe we could get a couple of threads going with pics. It helps to follow the wire color codes when laying out your build, makes for less confusion when troubleshooting. Understanding the difference between the super bass and super lead spec is a good way to explore some tonal options for this era amp. Also, the physical layout of the 12xxx with only the P.I. filter cap on top, lay down P.T. and orientation of the O.T. seem to be equally important to the 12xxx identity. You could explore the Larry ground option or not.

[Tazin]

Thx for the reply.

Yeah, that's pretty much the same conclusion I've come to.

Unfortunately a lot of info has gone lost on the forum due to the '14 crash and recent move. Makes the search function kinda irrelevant since all the links/attachments/docs aren't working anymore. A great shame!

Nevertheless good info in your post - every little bit helps I found the following board layout (probably posted by George back when he offered kits) and it seems to be the "best" representation of a '68 era 12000-series I've found to date. What do you guys think?



BTW, are all resistors rated 1/2W except for those marked 2W?

The layout you posted is basically correct with the exception of the 2.7K resistor for the cathode of V1b which should be a 820 ohm. The 2.7K didn't show up until Aug. - Sept. of 1969. Also, the coupling cap for V1b should be 0.0022uF whereas the layout has 0.022uF. The resistors are 1/2 watt with the exception of the B+ dropping resistors (shown as 2 watt on the layout) which were 1 watt. Most people use the 2 watt resistors since the leads on current production 1 watt resistors are too short to span the distance between the turrets.

[tWreCK]

Thx for the info Tazin - that helps me a lot

How does the 820R vs 2k7 manifest itself tonewise?

[wdelaney72]

I agree on the .0022 output cab on V1b. That was there by '68.
Not sure I agree with V1b cap of 820 ohm. unless you are talking shared cathode...I think Superleads were split cathode for much of '68 which was 820omn/330uF on V1a, 2.7Kohm/.68uF on v1b.

[neikeel]

Tazin is right

0.0022uF on V1b coupler and 820R on V1a cathode with 250uF cap (NOT 330uF) although sonically you her little difference with 250 vs 330.

The 820R gives a bit more gain but also a little extra fizz. If you play in the clean zones you hear it, cranked there is a little more gain but not quite as much clarity, particularly with hum buckers. I run my 68 with 2k7 and did AB with similar amp wit 820R and you can hear the difference.

BTW my amp is actually a 10 series (dual drilled for Drake 2" OT and Dagnall 1 1/2' OT) but the spec is the sam as 12 series. Mine has grey RS 0.68uF cathode caps.

The rules about Bass and Lead amps starts to apply now although some early 10 series Superleads (JMPs) are very similar to the last BF 10 series Superleads (i.e. 250pF/56k tone stack 0.022uF couplers except PI with 0.1uF couplers and 27k/16ohm NFB etc.

Apart from Tazin's comments the above looks good (IMO 32/32 on the board is correct, I personally think the 16/16 were replacements, never seen a stock 22/22 either but happy to see absolute proof and believe if someone shows me!

[Tazin]

I've seen a lot of 12series Super Leads and the one's that are setup with the split-cathode preamp all had 820 ohm resistors. Like I mentioned before, the 2.7K resistor doesn't show up until Aug. - Sept. of 1969 as far as I have seen.
Regarding the preamp filter cap...99% of the one's I have seen use the 32/32 on the board. I've seen a few 20/20 (Syncro brand) on 12series Super Leads and also used on Super Bass and Super Tremolo amps of the same era. I've even seen a few 20/20 Hunts but mainly on '68 50w amps. I've never seen a 12series amp with a 15/15 for the preamp filter though.

[tWreCK]

Thx for all the info and discussion - I'm learning bits and pieces everyday

Question - I got a couple of NOS Erie 100uF 350V radials from a friend - would these work for the mains filtering or do I need higher voltage rated electrolytics?

[neikeel]

They will be fine:

350v + 350v in series = 700v handling

100uF + 100uF = 50uF

[tWreCK]

Thx Neil

Next question - what is the minimum voltage rating for the 10uF bias caps?

I wish the various layouts and BOM's would state the minimum voltage rating needed for all electrolytics. Being a first time builder I'm having a hard time finding this info. So here goes

mains: 2 x 100uF axial (350V - 500V)
screens: 2 x 32uF axial (350V ?)
PI filter: 32uF can (?)
preamp: 32uF + 32uF axial (?)
bias: 2 x 10uF axial (?)
V1A: 330uF axial (at least 25V)

I think that's all but if I've forgotten something feel free to add the info.

Forgive my noob questions but I'm afraid I have many more before I'm done..........

[neikeel]

Hope this helps (my notes in red)

mains: 2 x 100uF axial in series (350V - 500V)
screens: 2 x 32uF axial in series (350V-500v)
PI filter: 32uF or 32+16 or 50uF (must be rated 450v))
preamp: 32uF + 32uF axial (350v)
bias: 2 x 10uF axial (stock is 160v but 200v size look more correct)
V1A: 330uF axial (at least 25V) (no a 250uF WIMA is stock rated at 6v)ould be

All coupling caps should be 400v, although the cathode bypass caps on V1b and V2a are usually 160v but can be 120v (there are some 120v mustards knocking around at lower rating and a bit more affordable).
Modern Xicon resistors are 1w rated as modern 0.5w are too small. Original Iskras and Pihers are 0.5w.
Of the dropping resistors (10k and 8k2) these were originally 1w rating but in modern amps 2w flameproof metal oxide are used.

[tWreCK]

Thx for the clarifications and corrections Neil!!

That info helps me a lot That's why this board rocks! There are a few "other" forums out there that are nice but nowhere else does the spirit of sharing knowledge & passion flow so freely as here on the metro forum!

So the V1A 250uF-330uF axial can be anywhere from 6V upward (size restrictions not withstanding)?

Thx to all

[johnnyquest]

Hi,
I'm gathering parts for my 12000 series build and got curious about the PI cap, the 32uf can mounted on top with only the 2 connections. I've noticed the Billy Batz's 67-68 layout that shows a dual can connected to 3 leads without any value listed for the cap. I was wondering if there was an advantage in using a dual can setup (not using just one side) and if so, what values would work and what changes would I hear or should expect, if any? I know this is an older thread but this is where I ended up.

Thanx In Advance,
Johnny