Before pics

[loverocker]

Not sure if any of you guys are interested in seeing these, but I thought I'd take some snaps of this 15W WEM before taking it apart to make the 18Wer:

http://www.firstmail.com/18w/top1.jpg - The control panel, laid out with offset pots. Volume on the right is marked "Microphone". The inputs and Tremolo F/S jacks are on the back panel. Real ugly compared to the Marshall.

http://www.firstmail.com/18w/logomains.jpg - The logo and the um... slightly dangerous mains selector. The selector knob simply screws in to connect the relevant wires, leaving the other two holes open!

http://www.firstmail.com/18w/ptotfc.jpg - The PT, OT and filter cap. I'm a novice at this stuff, but from a quick search I think the two pairs of Cs and Rs on the OT are some kind of conjunctive filter - they're connected between the primary's outer tags and CT tag. Another thing, the extra R (100r) connects to one of the secondary taps and runs to the preamp - some sort of NFB?

http://www.firstmail.com/18w/mainpcb.jpg - Cheap and nasty PCB, but still working after all these years! The big blue Hunts cap is the 16uF filter cap, rated at 275V (close!)

http://www.firstmail.com/18w/smallpcb.jpg - V1 and V2 sit on a sub-board, with PCB-mount sockets. OK for an accurate 18Wer, but not for my plans on using all the triodes separately for Lead boost (i.e. with split cathodes)

http://www.firstmail.com/18w/sockets.jpg - Six sockets plus an extra pre-cut hole (for the mythical reverb?)

http://www.firstmail.com/18w/ptfuses.jpg - two internal fuses (for each of Live and Neutral), plus the EZ81/PT connections.

Niggles discovered so far:
* chassis is far too small to get the 18Wer's board in. I've got some ready made turret board that I'll use instead
* pots seem to be diffierent values, so I'll need to replace them
* filter cap is 50+50uF - too stiff for an 18er?

Desoldering starts tomorrow...

[bubba2]

It's old, It's part of history, It's beautiful, Let it live.
Build a 18watter from scratch and have 2 amps worth owning.

[loverocker]

Well, I hear you, but the cab needed a lot of work anyway, and it's already unoriginal (knobs changed, for starters). Had it been a '60s point-to-point WEM Dominator, don't worry, I'd have left it as it was. But then I wouldn't have got it for $70.

Part of the attraction of this beastie was that the chassis holes, PT, OT, filter cap, sockets, heater wires, etc etc are already in place - my weakness is the hardware side anyway. An 18Wer from scratch with someone else doing the cab and hardware would cost, what, about $500?

Be happy for it - it's getting a new lease of life

[zaphod_phil]

So this isn't a Mk III Dominator then..?

[loverocker]

Zaphod - no it's one of several other WEMs that had the same circuit. They had names like Custom 15, Mersey somethingorother, etc. They go for cheap - less than £100 (unless they're put on Ebay).

Death - I don't understand. I'd have gotten peanuts for the chassis (£20/$30 maximum I reckon), and I'd then have to spend more to get an equivalent chassis to the one I'd just sold!

[zaphod_phil]

Zaphod - no it's one of several other WEMs that had the same circuit. They had names like Custom 15, Mersey somethingorother, etc. They go for cheap - less than £100 (unless they're put on Ebay).

I was once bidding on eBay for one of those Mersey Specials (aka WEM Custom 15 - www.valveamps.com/custom15/custom15.htm ), which was going quite cheap. Unfortunately, for some reason the guy terminated the auction. The cab and chassis looked uncanily like a Marshall 18W - far more than any WEM Dominator does. I was thinking of doing something along similar lines to you - ie turn in into an 18watter.

The 50u+50u cap isn't a problem stiffness wise, but the value of the first cap is higher than usually recommended for use with a valve rectifier. However, as this cap will be pretty old, you would be much safer replacing it. See the note about these caps on www.valveamps.com/scout.htm

[loverocker]

Zaphod - thanks for the advice on the 50+50 filter caps. I'll change it then, as I've got to do some ordering of a few bits. Why is it that no matter how big your bits box gets, there's always something else you need?

For the cathode cap on the power valves, I'm opting for a middle value from the range mentioned on the 18W schematic in the Downloads section - 100uF. Any other recommendations, anyone?

Those WEMs all look alike, don't they? When you see one up close, you can see that Marshall upped the build quality when 'borrowing' the circuit.

[zaphod_phil]

Go big on the cathode cap for a stronger low-end response - say around 220uF to start with. Also, if you want to keep your EL84s closer to max anode dissipation specs, use 150 ohms for your cathode resistor, rather than the more usual 125 ohm.

Yep, these WEM amps do look a lot the same inside. Glad that Marshall did provide some more value, rather than just ripping Watkins' design off!

[loverocker]

OK, I'll try 220uF and 150r to start then. Thanks Z