50w plexi vs 2204 (I’m confused)

[neikeel]

Either circuit will be easily changeable. Marshall actually used the same boards for the 1987 and 2204 circuit for many years as their are only a couple different components between them. Most of the difference lies in the wiring of the input jacks because the 2204 routes the hi channel back through the lo input after it goes through the first half of v1. It then further amplifies the signal in the second half of v1 before heading to v2. Neils layout looks like you would need the four hole chassis (1987) to follow his layout so I would probably build the 1987 first. The loop and ppimv will work fine with it also if you decide to add one later.

Yes exactly.
That version there is designed to be a mod of a 1987 that someone can do easily.
If you were to go for much more gain in the front end there is an alternative lead dress version, slightly trickier and more invasive (if using a PCB version) but not difficult with a 4 holer and turret board.

[Explorerman]

Sounds great, yes looking at the two I might go with the stock 1987 to start with. Once that is up and running I might look to install the mods - loving the idea of being able to switch between the two! Before I go ordering parts, just wanting to knowto know:

-On the above 2 in 1 circuit, in the 1987 mode it still isn’t the stock 1987 circuit as it is missing the 320u/820ohm off v1 terminal 3 and is replaced with just a 10k, does this make any difference?

- some like to jumper the channels on the 1987, with removing the 1987 ch 2 inputs on the 2 in 1 circuit does the push pull pot switch option on this modded circuit alleviate the need for this option? Can I put the preamp gain pot on the back panel and keep the ch 2 high jack for jumpering in 1987 mode or is there no need for that?

- looking at the front of the 1987 amp, are the right most furtherest jacks the ch 2 -High treble? with top being high gain and bottom low gain? And the two jacks nearest the vol pot channel 1 - normal? with top being high gain and bottom being low gain? Which jacks are which on the 2 in 1 circuit? The schematics or layout diagrams don’t say which input jack is which.

- does the Valvestorm chassis require laydown style transformer? (I’m not sure what is meant by ‘laydown’) I’m assuming the heybore OT and PT on the Metroamp store will suit the Valvestorm chassis?

- I notice that the original circuit presence pot uses a 25k pot with a 4k7 in parallel (as well as other layouts), whereas the Metroamp uses a 5k presence pot and no 4k7 resistor.

Cheers

[danman]

The 320uf/820r is the cathode resistor and bypass cap for the normal channel. The large value bypass cap is the reason that the normal channel is so wooly and bassy and for that reason, most people never use it unless they are jumpering into it. The bright channel is where the famous Marshall sound is normally produced. It is biased slighly colder with the 2.7k and the lower value bypass cap (.68uf) rolls off a lot of extra bass which makes the sound brighter and tighter. The 10k is the cathode resistor from the 2204 circuit. It's higher value is what helps give the MV circuits their unique sound and is known as a "cold clipper stage".

In the 2 and 1m you will not be able to jumper like a standard 1987 because the normal channel is missing. You can only have the 1987 bright channel and the 2204 hi and lo channels.

On the 1987, you have two volumes. One is for the hi channel and one is for the low channel. These are the only volume controls for the entire circuit unless you add a ppimv. The 2 and 1 has the normal 1987 volume when in that mode and when in 2204 mode, you will have a gain control for preamp volume and the standard pre PI master volume. The master volume will also be active on the 1987 mode, but if you leave it on 10 and forget it's there, you will have the normal 1987 sound.

Older amps used a laydown style PT while the newer amps switched to the standard upright PT seen in most of todays amps. You need to buy the correct PT for the chassis you intend to use. Classictone makes both styles and also have a couple with multiple secondary voltage options. Marstran also makes great iron (Heyboer), or you can order the Metro spec stuff direct from Heyboer.

The 5k presence pot is the older style circuit which has a bit of scratchiness when adjusting. The 25k pot is the newer circuit. Most seem to prefer the older circuit but it's your choice.

[Explorerman]

No worries, thanks again for the detailed info. What you say about the v1 cathode bypass makes sense, I was contemplating whether a toggle connected to a 68k resistor switch could be used to internally mimic jumpering the bright high with the normal low when in 1987 mode as an added option, but like you said the v1 bypass cap is no longer there so would probably be a waste of time?

With the chassis and transformers, the VS checklist link to Heyboer ‘Metro-Spec 50W PT Drake 1202-118’ says it’s lay down mount but the Heyboer ‘Metro-Spec 50W OT Drake 784-139’ doesn’t specify if if is laydown mount. The 50w chassis on the VS 50w checklist also doesn’t mention if it is for laydown mount transformers. I’m guessing it should all be good?

[neikeel]

The 118 is laydown PT
The 139 is a standup PT (all the OTs for these are stand up)
If you go VS then it will be laydown.
I usually go with Marstran and have never been disappointed.
I had nice OT from ****** (more expensive) and have not wired up the 50w I have with the Classic Tone OT in it.
YMMV.
I also get the aluminium finishing ring for the laydown PT and clamp the PT down flush to the chassis, like the originals.

[Explorerman]

Cheers, yep should have paid more attention to the instructions, those tfmrs should match the chassis.

I think I've decided to start with the stock 1987, then make some mods to be able to switch to 2204.

I've tried to attach a marked up 1987 schematic that I've modified to match the Neil special from the layout above. From that I've changed for the final result:

- Kept the 4th hole with a 68K to CH2 1987 Hi, I'm thinking I'll put the PPIVM, 1987 bright vol, treb, mids, treb, bass and presence pots on the front panel, and with 2203 pre-amp gain and master vol on the pack panel - both are push-pull switched. So with both rear pot in I'll have an almost stock 1987 (minus CH2 Norm Hi), with the ability to plug into Ch1 hi and jumper Ch1 low to ch2 Hi. With both pots pulled, I'll have a 2204 via Ch1 Hi and Ch2 Low inputs.

- Ch2 Hi (1987) goes to a 68K resistor to a switch contact off the rear master vol pot onto v1 terminal 2, this switch opens the circuit from the Ch2 2203 low input to v1. The other contact off this pot switches in a 894 Ohm resistor (which gives the 1987 values when in parallel with the 10K) and 330u Cap onto the cathode bypass.

- Larry grounding - just not sure where the cable shields go in this method?

I might need to be careful and make some of the extra switch wiring with shielded cable.

Thoughts?

I haven't had any success trying to attach the schematic - I've reduce the size to 90K (at that res its barely readable) but says "ERROR: Sorry, the board attachment quota has been reached" any tips.??

[danman]

To post any image, you need to use and image hosting sight. The forum got hacked a few years back and this is one of the changes George made to protect against it happening again. If you can post your schematic, someone should be able to check it for any errors. With shield grounds, I usually just attach those to the easiest spot even with the Larry's grounding scheme.

[Explorerman]

Try this:

https://www.dropbox.com/s/duw4pdsojlnao ... 6.JPG?dl=0

Its drawn with both pot switches on back pushed in (pos 1) 1987 mode.

Edit: the 4th 1987 hi input grid stopper needs to be 34k I think, to mimic 2 x 68k in parallel between hi and low inputs.

[danman]

Yes...if using only one input, go with 33k on the single input. I'm having a hard time making out exactly what the switching will do in your schematic unfortunately.

[neikeel]

Yes...if using only one input, go with 33k on the single input. I'm having a hard time making out exactly what the switching will do in your schematic unfortunately.

https://www.dropbox.com/s/duw4pdsojlnao ... 6.JPG?dl=0

Yes you have x4 switches to operate. You might be able to get a 4-pole switch to do them all in one go?

It depends on how much you really want to be able to bridge channels (I dont usually so not a big deal for me) hence the 2 in 1, this is switched by the push-pull gain pot in one go. You have the lo 2204 input available but not to bridge option. Must confess that once I had it dialled in I did not put a PPIMV in as I mainly use the 2204 type input.

[Explorerman]

Yes, I did consider (and still am) a 4pdt switch when I started to realise how hard it is to find lugged (not pcb) 1M audio pots with 2pdt switch in a high quality pot as I plan to use CTS pots.

The schematic I attached earlier is simply a 1987 schematic with the Neil special mods added (minus the PPIMV and on a 50w instead of 100w) + 2 x extra switch contacts to add a 894R and 330uF to v1 cathode for a 1987 circuit (the R894 + 10K in parallel = R820 as per 1987 spec) and the second extra contact switches between the 2204 low and 1987 norm Hi inputs. Hope that makes sense.

So with both pot switches in, it’s mostly 1987 mode, with both switches out it’s a 2204.

Cheers.

[neikeel]

Yes, I did consider (and still am) a 4pdt switch when I started to realise how hard it is to find lugged (not pcb) 1M audio pots with 2pdt switch in a high quality pot as I plan to use CTS pots.
The schematic I attached earlier is simply a 1987 schematic with the Neil special mods added (minus the PPIMV and on a 50w instead of 100w) + 2 x extra switch contacts to add a 894R and 330uF to v1 cathode for a 1987 circuit (the R894 + 10K in parallel = R820 as per 1987 spec) and the second extra contact switches between the 2204 low and 1987 norm Hi inputs. Hope that makes sense.
So with both pot switches in, it’s mostly 1987 mode, with both switches out it’s a 2204.
Cheers.

Interesting. It will be cool to see if you can make it work as you want. Keep us posted.

[Explorerman]

Thanks, will do.

However I think I just found an issue with the 4th CHII Hi input (in 1987 mode) - see attached pic, I may require another switch contact to connect the 0.022uF (from Terminal 1 of V1) to the 470k Resistor (which on the original 1987 circuit is done through the CHII Vol pot) since the 2 in 1 circuit doesn't have the CHII vol pot, this circuit can be made via a switch. On the below schematic I've highlighted this in green - switch labeled "B1". Is this needed?

While I was at it, I added a switch contact labeled "B2" to change between the NFB 47K to 8 ohm (as per 1987) and 100K to 4 ohm (as per 2204).

Starting to wonder if all of these extra switch contacts and wiring might start to create some unwanted noise in the circuit.

https://www.dropbox.com/s/utdrufm4xe2tg ... m.jpg?dl=0

[neikeel]

Yes it probably will

If you want to go for higher gain sounds then I suggest keep it simple and build the neil special alternative layout for least noise.

[Explorerman]

Yes it probably will

If you want to go for higher gain sounds then I suggest keep it simple and build the neil special alternative layout for least noise.

Yep, I think you're right, the neil special will keep wiring minimised and simplified. It will also be very tight fitting 2 x pots, toggle switch and effects loop kit on the 50w backing plate, and a lot of crossing of wires, etc.

Cant seem to source quality DPDT 1MA pots, so might opt for:
- both CHII/Normal Vol and ChI/1987 Bright Vol pots in CTS 1MA SPDT to do the switching
- a SPDT switch on the back (mounted next to the impedance selector) to switch between NFB 47k/8ohm tap and 100k/4ohm tap.

This can be initially wired as per the stock 1987 circuit, with the connections made between the two pot switches normally closed contacts as well as the NFB switch. Can also drill and mount the effects loop kit and PPIVM pot, leaving these disconnected until I get the 1987 circuit working.

Once the basic 1987 circuit is working I can then connect the PPIVM and effects loop, then once that is all working it will only be a matter of swapping out the CHII Hi jack for a 1MA pot and swapping over a few wires to get the switchable 2204 part complete. Ok, I think I'm set...