One small step at a time OR my 18 watt Marshall build

ezietsman · 2011-04-06T21:41:41+00:00

So I've decided to build an amp. A twin channel 18 watt amp based loosely on the 1974x Marshall from the 60s. The first channel will be more or less the same...

One small step at a time OR my 18 watt Marshall build

singemonkey

Freakin' exciting. Can't wait to see the progress.

TomCat

Nice one......its alway exciting to build something yourself....the key is to exercise patience and not try to rush it.

ezietsman

Layout, work (very much) in progress. Hopefully I can upload the file and it will keep updating itself.

guidothepimmp

EZ?

What sort of voicing is this ampmeant to have?

ezietsman

guidothepimmp wrote: EZ?

What sort of voicing is this ampmeant to have?

The lite iib channel is more or less a straight copy of the Marshall 1974x's Normal Channel. Crunchy old school overdrive when cranked, all coming from the EL84s, no preamp drive, think early ZZ Top. The superlite channel is more like a 70s plexi due to its increased preamp gain levels, think early AC/DC / Maiden. That sort of thing. No modern tight metal chugga's to be had here (not without some simple mods).

guidothepimmp

nice.. look forward to hearing this..

ezietsman

An almost completed layout. Gurus' advice, comments will be appreciated ?

and a link to the fullsize version
http://crashtestastronomer.files.wordpress.com/2011/04/ewald_18watt_layout2.png

and a link to the schematic
http://rh-tech.org/public/18-Watt/18_Watt_LiteIIb_Superlite_VVR_PA_PI_162.gif

Gearhead

Not being a guru, I can only note that I would connect the v1 grids to the inputs ?
But seriously: the input circuit has some variants depending on what you want to achieve. Mesa has even done the 'tie guitar to grid and nothing else' thing...

Karel-Mars

What a good looking heap of components!

I checked the layout briefly. When following the +B rail things are not so clear. I guess the VVR gets inserted as per supplier's instructions. The VVR and the 2K2ohm 2W should be between the + terminals of the large 32+32uF filter cap. (Leave space on the chassis to fit a 20H 50mA choke later. It does improve the sound.) First + on 32+32 cap goes to centre tap on OPT (connection after VVR). The stated voltage on the schematic of 347V is a bit high for EL84's. They like 310V better (see AC30 voltages). I guess the VVR will fix this?

BTW I would still put a cap after the VVR where the OPT centre tap gets connected. Even a polyprop of any value (0.022/0.047/0.1etc) would be good.
Where are you going to put the two diode power supply splitter? It might have to be fitted to the filter cap I guess?

Good luck with the project.

ezietsman

Thanks guys. The layout is now almost complete, bar a few mistakes and the output jacks.

Here is the current version:
https://picasaweb.google.com/lh/photo/3SF7dJyAtpYeTGilcEcsBA?feat=directlink

Getting closer to starting day (on the turretboard) so any comments on mistakes, layout, grounding etc will now be most usefull.

Tonight the chassis is going to get populated and I'm going to wire up the heaters and maybe some other things, depending on how I feel ?

btw. I have opened a wiki build-an-18-watt.wikispaces.com where I'll be documenting the build, anyone can make comments on there.

ezietsman

Gurus!

Here's the complete layout. It REALLY needs checking now. I'd like to start ASAP but I'd like to start when the layout is at V1.0 which will be when some experts have checked it against the schematic and told me they didn't detect any mistakes. ?

https://picasaweb.google.com/lh/photo/A_jb6zfiZsDK5MUSVCUlZQ?feat=directlink

Obviously I'll go through it multiple times too but I may miss some mistakes just because I made the thing...

FenderBender

Do you have a link to a hi-res pic perhaps?

EDIT: My bad! Just found out how to use Picasa ?

FenderBender

My advice would be to produce a precise schematic of your circuit before finalising the layout. This promotes easier and error free conversion to a layout. When building, I always use my layout and schematic to ensure that I make no mistakes.

There are one or two connections that appear different to the original schem you worked off, but it's hard to trace the layout to a schem that doesn't represent the circuit exactly, so it's not easy going.

As for the heater wires, I'd choose a twisted pair over twinflex. The twisted pair will radiate far less hum.

ezietsman

Thanks Graeme.

I tried reproducing the schematic faithfully except for a few places, I've been advised to use 22uF caps instead of the 16uF in the schematic, also the cathode bypass cap I'm using is 150 Ohm not 130 Ohm. I have also changed the output jacks to be three separate ones instead of having a selector switch. Anything else not clear or different is likely a mistake I made and I'd like to know where those are ???

Gearhead

Cathode biasing resistance of 150ohm is cooler than the 130 that everyone seemed to be using, does not sound as good but your tubes last 100 times longer.

FenderBender

EZ: Take a closer look at the circuit near the standby switch, one of the large caps and the VVR. It looks as if the cap is on the wrong side of the switch, but it's difficult for me to follow the layout without an exact schem.

ezietsman

Graeme Parfett wrote: EZ: Take a closer look at the circuit near the standby switch, one of the large caps and the VVR. It looks as if the cap is on the wrong side of the switch, but it's difficult for me to follow the layout without an exact schem.

Oh right, I moved the standby switch to before the 32/32 Cap can, instead of between its + poles. Is that bad?

FenderBender

I think it might have something to do with ensuring that a pre-stabilised and smoothed DC voltage is applied to the Drain of the VVR FET when the standby switch is engaged, rather than full wave DC pulses that will only be smoothed once the cap has charged.
I would stick to the original schem just to be sure.

ezietsman

Thanks Graeme, Will change it, I need to make some mods to the layout as I can fit the VVR onto the turretboard too.

Here's some progress pics:

Tube sockets and transformers bolted onto the chassis

Rear plate, speaker and power jacks

Business end of the power transformer

Heater wiring all done. Could be neater/better I guess.

Gearhead

Be wary of those heater wires around the sockets. If you can and if there is opposing current in the two wires, twist them as well. This is the first place you need it - close to your signal.