From-scratch amp build: 18-watt JCM-800

[Lumberjack]

Greetings lads,

I'm finally diving in on a long-term goal of being able to design and build tube amps, and this will be my first from-scratch effort.  I figured I'd better start off by imitating/modifying some popular circuits before trying to design my own stuff, and this particular circuit was designed by Trinity amps.  It's a mash-up of Marshall circuitry, and is somewhat like running a JCM-800 pre-amp into an 18-watt el-84 power section.  I have some plans to tailor the pre-amp to taste, particularly the low end, and maybe some re-voicing of the highs at the power stage depending on how it responds to some extra gain I have in mind for the pre-amp, but I think it will stay reasonably close to the general framework of the Marshall designs.  

I've been putting this off for about 12 years because every time I peeked into the world of amp building, it looked so mystifying and complicated!  Before I started reading about amp design I thought I had no idea what I was doing, and now that I've been doing some reading, I can confirm that I definitely know I have no idea what I'm doing.  But I can only learn so much by reading without hands-on experience, so I figure it's high time I start electrocuting myself .  I'm going to do lots of pictures, sound clips, troubleshooting, mistake-revealing, explanations, etc. in an effort to share as much of the unvarnished first-time-build experience as possible, with the hope that it might be helpful/enticing to anyone else who - like me - has spent far too many years on the fence.  
 

Here goes!

 

Most of the parts came in a few days ago.  I'm still waiting on tubes and some other parts, but I was too excited to wait, so I got started.  

Mounted some of the hardware: tube sockets, front panel, IEC socket, fuse and fuse holder, switches, can capacitor, pots, grounding lugs, indicator lamp, input and output jacks, transformer grommets, etc.  

I later realized that having the output jacks and impedance selector switch poking into the chassis directly over the tube sockets was dumb because I'm going to be doing a lot of soldering right underneath, so I took 'em out.   

Got a small start on the soldering; heater leads twisted and wired in.

[MiKro]

Just remember that Tube amps have HIGH VOLTAGES. They can kill you.

I'm sure you know the warnings though.

Other than that it is looking good.

MK

[Lumberjack]

32 minutes ago, MiKro said:

Just remember that Tube amps have HIGH VOLTAGES. They can kill you.

This will be the 378th time I’ve read that in the last few months - I was expecting to hear it again though! One of the books I’ve been reading had a fairly detailed list of instructions for what to do when you get a voltage so high through your arm that you can’t unclench your hand to move it off ... it was written in a way that makes me think the author had some experience in the matter, which I’ll be trying desperately to avoid.

 I remember the first time I modded an amp some years back I was literally sweating from nervousness around the filter capacitors. Fear not, I have a healthy respect for the electron. 

[MiKro]

7 minutes ago, Lumberjack said:

This will be the 378th time I’ve read that in the last few months - I was expecting to hear it again though! One of the books I’ve been reading had a fairly detailed list of instructions for what to do when you get a voltage so high through your arm that you can’t unclench your hand to move it off ... it was written in a way that makes me think the author had some experience in the matter, which I’ll be trying desperately to avoid.

 I remember the first time I modded an amp some years back I was literally sweating from nervousness around the filter capacitors. Fear not, I have a healthy respect for the electron. 

At least you already have that Healthy Respect ingrained as you move forward.

Mike

[curtisa]

Back on the bad old days before I got into guitar making I was heavily into DIY tube amps and pro audio stuff. Great to see something like this popping up here for a change of pace, and to stoke fond memories of firing up something for the first time without letting the brown smoke out

[mistermikev]

noice... on my bucketlist of things to try... but have held off because I figured eventually I'd fry myself!!  jealous.  you got balls for diving right into a fairly complicated circuit.  cudos.  looking forward to checking in on it from time to time.  looks like a very nice clean start.

[Lumberjack]

21 hours ago, curtisa said:

Back on the bad old days before I got into guitar making I was heavily into DIY tube amps and pro audio stuff. Great to see something like this popping up here for a change of pace, and to stoke fond memories of firing up something for the first time without letting the brown smoke out

Glad to rekindle some happy memories!  Hopefully my memories about my first build will be happy as well... (fingers crossed)

19 hours ago, mistermikev said:

noice... on my bucketlist of things to try... but have held off because I figured eventually I'd fry myself!!  jealous.  you got balls for diving right into a fairly complicated circuit.  cudos.  looking forward to checking in on it from time to time.  looks like a very nice clean start.

Thanks man!  I know this circuit is a little more involved than the ideal starter, e.g. a champ or tweed clone, but the instructions and support from Trinity (and their community forum) looked great, so I pulled the trigger.  Also, at the end of the day I'm a gainy sort of player (grew up on 70s/80s rock and heavy metal) and I wanted to build something I would actually use in more than a novelty sort of way.

 

Got the transformer mounted and wired up last night, I'll have some more pics up later.  Off to do some power supply voltage testing, hoping for a smoke-free experience.

[mistermikev]

7 minutes ago, Lumberjack said:

Thanks man!  I know this circuit is a little more involved than the ideal starter, e.g. a champ or tweed clone, but the instructions and support from Trinity (and their community forum) looked great, so I pulled the trigger.  Also, at the end of the day I'm a gainy sort of player (grew up on 70s/80s rock and heavy metal) and I wanted to build something I would actually use in more than a novelty sort of way.

right on, I could see that, I guess if it's worth risking a lightening ride then you might as well build something decent!  I'm sure with the docs they have it should make things pretty safe/do-able.  I have played a couple amp kit builds and they sounded great so looking fwd to sound clips!!

[Lumberjack]

Mounted the transformers, and got the power transformer wired up:

Dry fit the board, got the tube, control, and ground leads to add before solder.  

 

I've made all kinds of mistakes; everything from soldering a lug before all the leads that belong there were actually there (not just once either...) to reversing the IEC neutral and hot leads, some of which are actually in these pictures from before I caught them.  Before I did any voltage testing I ran through the circuit from start to finish checking the joints and leads, which is when I was able to track down those errors.  After my first run through and fixing the errors I found, I ran through it two more times and couldn't find any, so I plugged in and did my voltage testing; happily everything is getting the voltage should! 

Question for anyone with experience (or even an opinion, actually): I shared the images of the heater wiring on a different forum and someone noted that I had twisted the heater leads of the same type once they split off from the main line to the tube, i.e. in this post the pictures show the el84 sockets with red at pin 5 and black at pin 4, but between the pins and the main heater line I twisted wires of the *same* polarity, whereas the twister heater line is red/black = reverse polarity at about 6.3v and -6.3v.  The commenter pointed out that the reason builders twist those heater wires is to cancel hum, which works because their EM fields oppose eachother.  However, with the twisted wires of the same polarity the potential noise picked up by that portion would be additive, not suppressive, due to the fields being the same polarity.  

Is that 3/4" or so bit of twist worth worrying about?  Should I straighten them out?  This amp will be fairly high gain, so extraneous noise is definitely on my mind.  The fellow who brought it up wasn't able to provide any sources to read or even an anecdote from his personal experience, it was purely a theoretical thing, but it still got me thinking...

[curtisa]

5 hours ago, Lumberjack said:

Is that 3/4" or so bit of twist worth worrying about?  Should I straighten them out?

I wouldn't change it unless you find you have noise issues once it's operational. I suspect as it currently stands such a small percentage of swapped twisting of the heater wires compared to the overall run will amount to no real loss in noise performance.

A perfectly twisted heater pair all the way to the socket will not result in perfect cancellation of induced hum anyway. It's easy to fall down the rabbit hole of surgically-clean wiring and layout while making tube amps, but in many cases you end up splitting hairs to chase noise performance improvements that simply can't be obtained from the circuit. If you were building the same circuit using a PCB for example, you simply wouldn't be able to 'twist' the copper traces for the heaters to cancel induced hum, and there are plenty of examples of PCB-based tube amps out there that exhibit completely acceptable levels of heater hum.

[Lumberjack]

@curtisa Ok great, I had a sneaking suspicion that there might be a lot of these sorts of debates/disagreements in the amp world, similar to the stuff you get with people comparing IC chips and such in pedals.  Not that it doesn't matter, just that it might be smaller 'taters in the grand scheme of things.  Either way I'm curious to see if it has an audible effect, if I get any excess hum I may look to straightening those leads out first.  Another question for you; I wired the jacks with the recommended shielded cable, but wasn't sure whether to twist them or not?  I couldn't see the advantage to doing that technically since the current is so miniscule, and it's shielded, but I kinda wanted to do it for aesthetic consistency, and wasn't sure if that would be good or not.  Most pictures I see have no twisting on the inputs.  

In the meantime, I made some more headway: 

Turret board leads added and board mounted.

Leads dressed, jacks wired, output transformer leads braded, etc. 

 

Still have the control wiring to do and looots of clean up; excess leads poking out of connections, dressing, etc.  All in all I'm shocked at how quickly this is going.  I'll be busy tomorrow so I'm not sure if I would be able to finish this weekend, but I'm guessing I'm about 3ish hours of work away from tube testing.  Mentally I'm still operating on a guitar-building time table, which take me at least 2 weeks of full time, all day work to do from start to finish, so this feels like light speed.  Granted, I'm anticipating plenty of head scratching and troubleshooting before it's stable and I have yet to build its head shell, but still, I didn't think it would move along this quick.  

[mattharris75]

That's super cool looking, I hope your top is lexan or some other clear material to show off what you've got going on in there!

[curtisa]

1 hour ago, Lumberjack said:

Another question for you; I wired the jacks with the recommended shielded cable, but wasn't sure whether to twist them or not?  I couldn't see the advantage to doing that technically since the current is so miniscule, and it's shielded, but I kinda wanted to do it for aesthetic consistency, and wasn't sure if that would be good or not

It won't aide anything by doing so, other than making it look similar to the other wiring within the amp. Although I would recommend being a bit careful if you do decide to twist the shielded cable, as being a bit more substantial than the single-core wire used elsewhere it may not like being twisted tightly. A couple of small wire ties to bunch them together would be all that I'd do.

Twisting of the wiring to minimise noise only makes technical sense in a handful of situations in a tube amp, heater wiring being one of them, and even then only for a specific kind of heater supply (a 3.15-0-3.15 winding on the transformer, as you appear to have). In that situation you're attempting to minimise noise being radiated outwards to other parts of the circuit. On the other hand, the integral shielding in the cable used on the input jacks is providing some protection from the opposute side of the coin, against external noise getting in.

It's certainly looking pretty neat and tidy. It'll be done before you know it at this rate

[Lumberjack]

Confession: the amp has been done for weeks but I tore it all apart and have been designing my own ever since, here's where we stand so far!
 

 

This has been insanely fun.  Can't believe I put off this hobby for so many years, I've been enjoying it like I never imagined!  Very satisfying to finally get my hands in there and work towards the sound in my head, although to be honest I'm still a long way from reaching that goal.  Still have to settle on a bunch of component values, amp features, layout, parts, etc. before I lock in to a headshell design.  I'll be sure to share all that stuff as it happens!

[Lumberjack]

Been forgetting to update this; the amp has changed a lot since I posted the video above.  I decided to move this into "amp I'm actually gunna play live with" territory, and my buddies and I mostly play covers from the 60s/70s/80s/90s, stuff like Zeppelin, ZZ Top, Bon Jovi, AC/DC, etc. so that's what the changes are aimed at. OD channel is warmer and more "classic" sounding, lower gain, with an effects loop as well.  I also added a clean channel based on the Deluxe Reverb, but with independent pre-phase-inverter gain and master, along with a framus-style mid control and a tilt-shift EQ knob that covers bass and treble.   The pre-amp circuit is currently structured somewhat like this: 

I'll sound demo all the changes once the whole thing is finished, in the meantime I've been building a simple pine headshell for it to live in.  

Very simple/crude dowel joinery.  Just kinda hacking this thing together since it will all be covered and I don't really care what it looks like underneath.  Going to do tolex, leather corners, cane grill, etc.  

[Lumberjack]

Got some tolex on there, cane grill cloth, leather corners and handles.  Tolexing, I'm discovering, is quite a struggle.  This was far from perfect, lots of learning to do.  Still have to make face plates for the chassis and a logo.  

[Bizman62]

Your tolexing looks perfect to me, the front mesh shows more obvious unevenness.

[Lumberjack]

25 minutes ago, Bizman62 said:

Your tolexing looks perfect to me, the front mesh shows more obvious unevenness.

Yeah, it scoots down on the right hand side.  I think I'm going to end up prying up the staples and straightening that out, it's too bothersome to leave it be.  

[Bizman62]

That type of mesh isn't the easiest to get straight as the thread is pretty thick and shows any curves and scoots. Same goes with the silverface mesh, all square lines.

I wonder if there's any trick to stretch it straight? Maybe applying the methods used by artists to wrap a canvas on a frame? Maybe soaking the cloth so it can be stretched on the backing board - shrinking while drying should take care of any wrinkles?