Building again - authentic 18W this time!

[shakti]

After a two year hiatus, I'm at it again! This project has been long in the works, and was initially sparked by a very kind forum member hooking me up with a real vintage Radiospares output transformer. Major thanks is in order! I knew right then I had to turn this into something special, so I've taken my time. A couple of years later, I ended up with a pair of mid-60s alnico G12s, so then the heat was really on. I ordered a very authentic 2x12 combo cab from Jeff Swanson with EC Collins pinstripe. That inadvertently caused some problems, as the commercially available 18w chassis doesn't fit my combo cab built to exact specs...they're slightly too wide. So that derailed my project for another year and a half until, lo and behold...I happened upon a real vintage 18w chassis! Just about completely gutted, and still it cost me an arm and a leg. But the cool factor is priceless, and I figured with the luck I had already had in finding crucial tone-deciding parts at very affordable prices (close to gift in one case: THANK YOU THANK YOU THANK YOU N.K.), the karma balance was more important than my account balance, so I decided to go all in.

I am still hoping to find a vintage power transformer, but chances are a little slim on that one. Any leads are greatly appreciated though! I can actually get a Radiospares Hygrade transformer like those used in 1967 (open end, no bell cover), but this build, using alnicos in a 1966-styled cabinet, somewhat calls for the earlier, two-bolt power transformer with bell cover. To be fair, my chassis is probably a 1967, judging by the location of the board standoffs, the fuse holder and power transformer bolt pattern. I won't manage to be 100% accurate, but those things aside, the original 18w combos were a bit all over the place as far as components used (with even a lot of parts value variation). But I am gearing this towards a 1966 look, with Pihers and carbon comps, and mostly ceramic caps.

Here's where I'm at so far, only just started:

[shakti]

Trying to put some more photos up here, but edit function in Photobucket is a little erratic. Hold on...

[shakti]

[shakti]

[shakti]

I'd appreciate any discussion on this circuit. I know other forum members have built 18w amps, and there is a dedicated community/forum for 18w builds elsewhere, but there has never been much discussion on them around here.

One thing I am pondering is how much to stray from the original layout/circuit, especially with regards to grounding. These amps are notorious for issues with the tremolo especially, and grounding seems to be a big part of it. With my other Marshall clones I've used "Larry" grounding exclusively, but with this amp, being somewhat of a restoration as well, I am not sure how much I want to stray from the original scheme. The original used a ground buss across the back of all the pots (even running through the input jacks), and the grounds from the board were attached to that buss in a somewhat random pattern. The layout of the preamp tubes, with the phase inverter as the middle of the three preamp tubes, doesn't exactly help. On the original, the preamp filter cap (which supplies all the preamp tubes incl the phase inverter), is grounded along with the power tube cathodes, for instance. OTOH, I have read elsewhere that going to a star grounding scheme can actually cause even more problems with the tremolo channel, so I am a little unsure what to do.

One very common mod is to use shielded wire for the grids to the preamp tubes, and I think I will try that. I forgot how this is done...I use the shield to ground on the input jacks, but not by the preamp sockets, is that right?

I have nearly all the parts I need, except the pots, knobs and a tiny few bits and pieces. Most of it I can get from Valvestorm, but there are a few where I would love to find NOS parts, so if anyone can help me out with the following I would be very happy! Trades or cash paid...

1 x 2.7M Piher or NOS carbon comp 10%
2 x 68k Piher 5%
1 x 125R 5w Radiospares (for the power tube cathodes...but do I really need such a high wattage resistor??)
1 x 1.5k 5w(?) Radiospares (for the voltage drop/decoupling resistor between the two sections of the filter cap)

And lastly, the original can holder is a small diameter, and it is impossible to find a modern 16+32uF replacement that fits. I either need to change the cap holder to a larger diameter (and probably have to drill new holes which I don't want to), or find a NOS 16+32 that fits. Anyone who can help?

[DG45]

Really cool.

Not savvy enough to help other than to say that you have the shielded cable part right.

What's the original 16+32 can cap diameter?

[shakti]

Thanks! I believe the diameter of the original cap is about 25 mm / 1 inch. A bit less than most of the current stuff. I have an ARS 16+32 cap which, AFAIK, is the only current production cap with that value. But it's a bit heftier, I haven't measured but I am guessing around 35 mm. Slightly less than F&T, but still way too thick.

[DG45]

Yeah, 25mm is small. Cant help, sorry. But I'll be watching the thread. Good luck with it.

[neikeel]

About time too!!

Nice to see the project underway, impressed that you scored an original chassis. I used Pihers on my last build as well as the 67 open topped PT and my next 18w project will be similar but using mixed Iskras and Pihers and same trannies. I do not know of a skinny 32/16 can although the 32/32 axials exist that skinny.

I think shielded wire (grounded at jack, insulated at grid pin) is the way to go, make sure you use low capacitance wire to avoid clipping the highs on normal channel.

Ceramic couplers are a bit brighter and crisper than mustards. I use mine with the trem channel with tone turned all the way up at rehearsals and I suspect that is because of the mustards smoothing things out.

Grounding is critical - keep the trem section grounded to a lug on the ECC83 for trem (V3). I used a 2M2 rather than 2M7 and used this layout.
http://18watt.com/storage/18Watt_Layout_Original.pdf
This could be an option too:
http://18watt.com/storage/18watt_layout ... ents_b.pdf
Depends if you want it 'warts and all' original.
I have some RS 5w resistors tucked away somewhere - might have 70s RS logos tho'?

[shakti]

I have high hopes for this one, I have to admit that.

Yes, that is the layout I'm using Neil. Just to be clear - how far did you go in implementing the "improvements"? Just the shielded wire and grounding the shielded wire by V3? Or did you also ground the other parts of the trem channel by V3 (i.e. the cathode R/C, the 0.047 etc)?

I could definitely be interested in RS 5W resistors even with the "wrong" logo. Trying to keep it vintage looking as far as possible. Although it looks like I am going to have to stick with a new filter cap then...

[Colossal]

Beautiful, and you've even got silver bells in it.

[shakti]

I think shielded wire (grounded at jack, insulated at grid pin) is the way to go, make sure you use low capacitance wire to avoid clipping the highs on normal channel.

I always thought that was the way to do it, but on the "improved" diagram you linked above, the shield appears to be connected to ground also by the tube socket? On Hiwatts, typically shielded wire is used on the inputs, but then only with the shield connected to ground on the input jack, and insulated by the tube socket. A little unsure how to do it here...

And then on V3; the shield is connected to ground only by the tube socket, but insulated at the board/component end?

[neikeel]

I used the traditional grounding of shields on input grids to the jacks, insulated at socket.

On the trem I grounded the shield at the socket, insulated on the turrets.

IIRC I did not move components around or alter the 0.01uF ceramic disc on first trem cap or the trem pot, i did used 2M2 and not 2M7 as i did dot have a 2M7 and no source for one either!

[shakti]

Yep, looks like I am going to go along those lines myself. I will definitely use shielded cable for those trem Connections, but I am tempted to try regular cable for the normal channel input grids. Running from the jcks to the socket, it should be easy enough to change to a shielded cable if I get noise issues. I suppose the shielded cable is recommended because it is running without input grid resistors?

I think I'll try a couple little tweaks to the rest of the grounding layout, at least ground the pwer tube cathodes separately from the preamp filter cap. So basically ground the preamp cnnections along the buss on the back of the pots, and the pwer amp cnnections separately from those, moving closer to the PT and the mains erth.

I had also planned to use the stck values (1M pot, 1M and 2.7M resistors, 0.1uF cap) for the trem crcuit. I doubt I am going to use the trem much anyway. Just need to find a 2.7M Piher or carbon comp resistor, or failing that, a 2.2M. Anyone?

All the rest of the parts have been ordered, so this should hopefully be up and running in not so long!

[neikeel]

I use trem channel all the time (just with it switched off). I have some spare 2M2 Pihers.