Marshallesque guitar amp

[Beer of the Bass]

After building myself a guitar amp several years ago (which was based on the Fender Princeton Reverb) and tinkering with a couple of other amps in between, I've found that putting together valve amps is something I really enjoy. I wanted to get a couple more builds under my belt to see if I can up the standard of my work, but parts can be costly, so when a friend asked if I would build an amp for him (in an informal sort of way) it seemed like a good idea.
After quite a bit of discussion, we settled on the idea of a 50 watt amp based on the old Marshall plexi but with added reverb, and it's going to be in a 1x12" combo cabinet. The reverb is based on the "1-tube reverb" circuit found on the Hoffman amps forum, which is less full-on surfy than the classic Fender reverb but reputedly works well in this style of amp. There will be a couple of small tweaks from the standard 50-watt plexi circuit, which I'll detail as I go along. My parts choice is an attempt to find the middle ground of decent quality, dependable stuff without either going too cheap and compromising quality or falling for the whole boutique snake-oil thing.
Once I'd got most of the parts together, I started assembling the board which the smaller parts are mounted on. Marshall would have used turrets for the wiring, but I'm used to working with eyelets so I used those, mounted on FR4 fibreglass material. There are still a couple of resistors missing here which I managed to leave off my initial parts order.



While waiting for some more parts to arrive, I got my chassis prepared with the necessary holes. I'm not much of a metalworker, so I bought a ready-made blank chassis rather than folding my own as some builders do. I did pick up a couple of old chassis punches on eBay which do a much better job of the 3/4" and 1" holes for the valves than drilling would. The hole for the IEC mains connector is always a bit tricky without specialised tools. I drilled out the corners, chomped out most of the material with a hand nibbler and then filed it out to the correct size. This left a few scuff marks around the hole, but thankfully those are on the back panel.




Next up came fitting the valve sockets and doing the heater wiring between them. The layout of this affects how quietly the amp runs, so I've tried to be careful about keeping them twisted together and routed correctly. I have deviated a little from old style Marshall construction here - I've avoided looping wires around the preamp sockets as I've read that this can induce hum.



Here's the chassis with the transformers mounted on top. I've used Dagnall transformers from Modulus Amplification as they were a reasonably priced option but are good enough for Marshall to use in their expensive handwired reissues. The reverb transformer is the exception to this - it's an unbranded generic one. From left to right are the reverb transformer, power supply choke, output transformer and power transformer. With all of that iron, it weighs a bit!



At the moment I'm in the process of doing the off-board wiring underneath the chassis, so there will be more to follow...

[ColinB]

That's a really pro job you're doing. I built a 5E3 many years ago and it's still going strong even though I've kept on 'tweaking' it. Bassman output tranny so I could run an extension cab, mods to run 6L6s, the two pre-amp channels in series rather than parallel. My next tweak will be the Ampmaker VCB/VVR variable power control ([url="http://www.ampmaker.com/store/VCB-VVR-kit-for-cathode-biased-amplifiers-ZK02.html"]here[/url]).

I'll be following this build with interest.

[Bottle]

Firstly, wow, nice lead dress!

Secondly, I did a valve amp build last year [url="http://basschat.co.uk/topic/257258-diy-valve-amp-kits"]here[/url] so I wouldn't worry about a guitar amp build in a bass forum my kit was bought from [url="http://www.ampmaker.com/store/home.php"]Amp Maker[/url] and is a 4W Fender Tweed Champ clone. Sounds amazing

Good luck with the rest of the build. Shall be keeping an eye on this thread with interest!

ATB,
Ian

[Beer of the Bass]

[quote name='ColinB' timestamp='1465463001' post='3068372']
That's a really pro job you're doing. I built a 5E3 many years ago and it's still going strong even though I've kept on 'tweaking' it. Bassman output tranny so I could run an extension cab, mods to run 6L6s, the two pre-amp channels in series rather than parallel. My next tweak will be the Ampmaker VCB/VVR variable power control ([url="http://www.ampmaker.com/store/VCB-VVR-kit-for-cathode-biased-amplifiers-ZK02.html"]here[/url]).

I'll be following this build with interest.
[/quote]

Cheers! I'd looked at the VVR kits, and if I ever do a cathode biased amp I'll probably give one a go. This one is going to have a post-phase inverter master volume to tame the volume a little, as it seemed like the simplest way to do that.

[quote name='Bottle' timestamp='1465472909' post='3068528']
Firstly, wow, nice lead dress!

Secondly, I did a valve amp build last year [url="http://basschat.co.uk/topic/257258-diy-valve-amp-kits"]here[/url] so I wouldn't worry about a guitar amp build in a bass forum my kit was bought from [url="http://www.ampmaker.com/store/home.php"]Amp Maker[/url] and is a 4W Fender Tweed Champ clone. Sounds amazing

Good luck with the rest of the build. Shall be keeping an eye on this thread with interest!
[/quote]

Again, cheers! I'm trying to put some effort into the lead dress - so far I'm not managing the full Hiwatt/Burman style neatness, but I'm sure it's coming out a bit tidier than my last effort. I'm also discovering that cable ties are my friends. I'll take another pic tomorrow - I've got most of the wiring around the transformers, power/standby switches and can caps done, but not quite ready to put the board in yet.

[Twincam]

Nice work. The iec power connector doesn't look too bad considering I bet it's a right pain in the arse to do. Is there not a way these can be cut out with a dremel and small cutting wheel?

[Bottle]

[quote name='Twincam' timestamp='1465551510' post='3069131']
Nice work. The iec power connector doesn't look too bad considering I bet it's a right pain in the arse to do. Is there not a way these can be cut out with a dremel and small cutting wheel?
[/quote]
Hole punch would be neater. Laser cut the neatest. For a one-off though a bit of elbow grease is the quickest

[Beer of the Bass]

Here's an in-progress shot of the chassis wiring - I've still got quite a bit more to do though. The overly long and coiled wires are the ones I haven't connected yet. As the reverb circuit is an add-on to this amp design and might need some tweaking once built, I've kept it off the main board and built it using a little strip of eyelets next to the valve socket.

[Billy Apple]

Subscribed

[Beer of the Bass]

[quote name='Twincam' timestamp='1465551510' post='3069131']
Nice work. The iec power connector doesn't look too bad considering I bet it's a right pain in the arse to do. Is there not a way these can be cut out with a dremel and small cutting wheel?
[/quote]
[quote name='Bottle' timestamp='1465559272' post='3069249']
Hole punch would be neater. Laser cut the neatest. For a one-off though a bit of elbow grease is the quickest
[/quote]

A dremel would probably do it, but I tend to go with hand tools first. If I ever do more than one-offs, I'd fork out for a proper rectangular chassis punch. The round ones I picked up for the valve sockets were a joy to use, compared to trying to drill the same size hole cleanly and then get rid of the burr.

[Bottle]

[quote name='Beer of the Bass' timestamp='1465564491' post='3069320']
A dremel would probably do it, but I tend to go with hand tools first. If I ever do more than one-offs, I'd fork out for a proper rectangular chassis punch. The round ones I picked up for the valve sockets were a joy to use, compared to trying to drill the same size hole cleanly and then get rid of the burr.
[/quote]
Yep, a rectangular chassis punch is the way to go (no faffing with the chamfered corners then). If you are intending making lots and lots of chassis (i.e more than five or ten) see if it's quicker to get a local metalworkers to quote you for a job lot of completed metalwork. It needn't cost a packet as it's mostly regular holes, cutouts and 90[sup]o[/sup] bends.

[Beer of the Bass]

Nearly there now - I've got the amp up and running in it's basic state without the reverb circuit connected. The reverb itself needs a bit of tweaking, but everything else is good. The voltages check out as expected and I've set the bias. I had one obvious error in my wiring; I had connected the output transformer primary with the wrong polarity. This design uses negative feedback around the power stage, so if the phase is accidentally reversed you get positive feedback, making it into a terrifyingly loud oscillator. That was soon sorted though.
As it turns out, my lead dress still isn't up there with some of the fancier stuff out there, but it isn't causing any problems either so I can live with that. There are a few tweaks from the original design - rather than having four input jacks and having to use a patch cable to jumper the two channels, I've used three jacks for bright/normal/both. I have a couple of added toggle switches - one switches between different value bypass caps on the second preamp stage. Adding a bypass capacitor gives a little extra gain and the value determines what frequency that starts at, so my switch selects between a full-range boost, a boost with the bass rolled off or no boost in the centre position. The other selects a resistor and capacitor in the tone stack, switching between the value used in earlier Marshalls or the values they changed to a little later, which lets more upper mids through and sounds more aggressive and crunchy.







As to the sound, it's a lot of fun to play through. It's a very different style of amp to my own little Fender Princeton Reverb copy, and really crunches up nicely. The Master volume works well to bring it down to reasonable levels, but I could do with checking it out somewhere I can play loud. It seems to have a bit more dominant low end than I was expecting, but then I am testing it through a ported bass cab rather than a proper guitar cab, so it could just be down to that.
Next up will be fiddling about with the reverb some more and doing a faceplate and combo cabinet for it...

[Beer of the Bass]

I've now got the reverb working. My reverb issue turned out to be that I had connected the reverb circuit to a point on the power supply where the voltage was slightly over the 300v maximum that an ECC83 can handle and it killed the ECC83 and blew the HT fuse as soon as I switched it on with the valve in place. Thank goodness for fuses! Moving the B+ connection for the reverb one step further down the chain sorted that, and it's working nicely now. I was worried that this stripped-down reverb circuit might sound anemic compared to a blackface Fender style reverb, but it sounds good. It has less depth available than a Fender reverb, but that just means the knob lives in the upper half of its range rather than in the bottom quarter. It doesn't go full-on surf, but it's not that style of amp anyway.
I'm taking it along to a jam tonight with the friend I'm building it for, so we can find out what he thinks of it before I start on building the combo cab for it.

[quote name='Meddle' timestamp='1466886243' post='3079597']
Have you tried a bass through it? Instant Jack Bruce?
[/quote]

I've just given it a try, and it does exactly that. It doesn't have much headroom in the preamp and breaks up pretty early. The bright channel can bring out more top end than the old Jack Bruce sound, even with flatwounds, but it really leans towards dirty rock bass. I can imagine it would be spectacular as the treble side of a Ric-o-sound style setup...

[Passinwind]

Very nice build! Looking forward t seeing how you do the front panel, as that often tends to make or break the overall perceived build quality.

I have tried really hard to get away from using IEC inlets these days. The Neutrik Powercon is my current favorite, for a variety of reasons.

[Beer of the Bass]

[quote name='Passinwind' timestamp='1467559980' post='3084540']
Very nice build! Looking forward t seeing how you do the front panel, as that often tends to make or break the overall perceived build quality.

I have tried really hard to get away from using IEC inlets these days. The Neutrik Powercon is my current favorite, for a variety of reasons.
[/quote]

The Powercon does look convenient for mounting in round holes, and locking too. I might have a look at those some time.
I'm going to do the faceplate in thin perspex, using laser printed decal film for the lettering and painted from the back. It seems a bit fiddly to do, but I've seen it work well.
It turns out I have a little more troubleshooting to do on the amp after trying it at volume last night. It really sounds good, but it just doesn't get as loud as a 50-watt Marshall-style amp should. Plugged into a 4x12" it gets up to earplugs-loud, but from past experience I'd expect leave-the-room levels of loudness. So I'm going to go through methodically to check component values and connections against the layout, recheck the voltages and bias, and if I still haven't figured out what's going on I'll have a look at it with an oscilloscope and signal generator. I'd imagine it will be something simple, once I find it...

[Norris]

Have you got the correct date set on the flux capacitor?

Lovely work btw

[Bottle]

Lookin' good, ace!

Re: PowerConns. Never used them myself. IIRC they come in two flavours - Inlet and Outlet. One is blue and the other grey (not sure which is which, only that you have to specify the right one when ordering etc).

Seem to recall they use the same size cutout as the Speakon and XLR bulkhead sockets.

I can understand the appeal of locking mains connectors but I've always opted for a standard IEC - more likely to have a spare at a venue should you mislay/forget the power cord and also I'm not a big fan of a locking connector on a lead where if someone tripped over it, they'd pull your amp down with them.

Anyway, great build thread

Ian

Edited July 4, 2016 by Bottle

[Beer of the Bass]

While the weather's nice, I've been doing some back garden carpentry. I used up most of my budget on the amp itself, so I'm using re-claimed 18mm plywood which was part of the stage in a short-term arts venue (Edinburgh is handy for that sort of salvage). It's not the nicest looking plywood or the finest bit of woodworking I've done, but the tolex should hide a multitude of sins!
I'm building the combo cab in a similar manner to the old Marshall combos - chassis mounted on the back panel with controls at the top, and top vents to keep things a bit cooler. The speaker will be an ET90 from Warehouse Guitar Speakers - it's supposed to have a Celestion type voicing, is rated for 90 watts and I've seen comments that it makes for a big sounding 1x12" combo so it seemed like a good choice for this amp.




I've also got to the bottom of my low volume issue with the amp. After spending a few hours measuring voltages and poking around, I tracked it down to a bad solder joint which was throwing out the biasing of the second preamp stage. I'm sure I learn more from figuring out mistakes than I do from the actual building...

Edited July 14, 2016 by Beer of the Bass

[Beer of the Bass]

It's very close to being done now, all I have left is to make a nice looking overlay for the control panel and a nameplate for the front. I had a go at this, but wasn't very happy with the first attempt. The Sharpie labelling is definitely a temporary measure, but otherwise it's ready to gig and I may as well get it to the friend I'm building it for so that he can be playing it in the meantime. I'm playing bass with him for some gigs over the next few weeks, so it'll be interesting to hear what he does with it.
This was my first go at tolex. It's definitely more hassle than Tuff Cab, but I think it turned out OK. Tolex hides small imperfections in the ply a lot better than Tuff Cab does, which I was glad of. I used the water based tolex glue from TubeTown where I bought some of the parts, which was quite pleasant to use as it gives you a little working time before it grabs hard and it has very little odour other than a faint rubbery whiff.







I've used aluminium mesh for the vents in the top, and the reverb tank is in a little pouch made of leftover tolex strapped inside the bottom of the cabinet. I have a feeling I may end up adding side handles as it weighs just sort of 25Kg. Still, my friend wanted this much power and with decent transformers and a stout speaker it all adds up. It sounds great though, it almost makes me want to do another amp in the same style for myself (though I'd go for less power). I'd been testing it through a bass cab, but it's more fun with the WGS ET90 speaker I've put in the combo cab - the bass is less pronounced but it's more clear and open sounding. As well as the expected crunch, the cleans are very nice too.

[Winny Pooh]

Excellent work and very tidy, the switchable bypass capsare a great mod.

[Beer of the Bass]

I've now got a slightly nicer looking front panel on there.


It's 1mm clear polycarbonate, and the text is printed on clear decal paper with black blocks around the letters. I then applied the decals to the back of the panel, filled in the text with white acrylic paint and sprayed black over everything. It's not absolutely perfect if inspected closely - the text is a bit fuzzy and it seems that the decals crept slightly in places when the black spray paint was applied. But I reckon it's smart enough looking and doesn't look too obviously bodged. It's nicer than the marker scribblings, anyway!

[ColinB]

That's lovely - very impressive.

[Beer of the Bass]

[quote name='ColinB' timestamp='1470402697' post='3105771']
That's lovely - very impressive.
[/quote]

Thanks! I'm now trying to figure out if I can justify building another amp for myself, which I guess is the issue with any project...

[rk7]

Lovely work! Very impressive outcome.

RK

[scojack]

[quote name='rk7' timestamp='1470722429' post='3107850']
Lovely work! Very impressive outcome.

RK
[/quote]

Must agree, fantastic stuff.

[Beer of the Bass]

This one is off with my friend now, but I decided I was unable to resist the temptation to do a smaller version for myself. So far I've picked up the chassis and transformers:



It's going to be essentially the same circuit but adapted for 6v6 power valves instead of EL34s, which should give me somewhere around 15-18 watts. The smaller transformers should make it a bit lighter too. It's going to go into a 1x12" combo cabinet, but somewhat smaller than the 50 watt version. It's going to be a slower build than the bigger one because I'm picking up the parts in dribs and drabs as my finances allow. I might not post updates at every step of this one, but I'll put up some pics when it's done...