6V6 JCM 800 build: re-tasking old build

[Amped]

I had an amp that I'd built a few years ago but was never very happy with it.  It was a chassis and transformer kit for a Marshall 18W TMB that I'd used to build something else, but it was an early build, I used the wrong glue for the Tolex, it didn't have faceplates, was a bit noisy, I'd made a bit of a mess of it.  I came across the Rob Robinette JCM 800 Micro page a while ago, and the Thor amp that used his design had pretty much the same transformers as was in the 18W TMB.  After a lot of procrastination, I thought I'd give it a go and try get some life back into the old amp.  Glad I did.

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I started - as I always do with working out what tweaks I wanted and mock-ups in DIYLC.  Ony tweaks I wanted over the Thor design (which has a few already built-in) was:
1) Changing the V1B cathode switch to be 10K and 39K with no disconnecting the Cathode resistor, did this with a 39K on the center pole, and a 15K switched in parallel for ~10K.
2) I wanted a switch for V1 instead of 2 input jacks, (V1B only,or ViA=>V1B cascade).  Designed something I hoped would work (spoiler-it did).
3) Heater elevation - pulled 70V off the B+ rail for heaters.
4) Redesigned Bias circuit.  To suit what I had and what I know, I changed how this works.  Also made it a full wave rectifier to see how this works (spoiler again - it works well thank goodness, trying new things is scary).   

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Then the cabinet, can I save it?  I started by stripping off the old tolex.  I'll never bother doing this again, even though the glue was terrible, getting it off - and then the adhesive off the wood was a real nightmare.  But I got there with some particularly nasty chemicals, and after some wood repairs where screws and rivets had ripped the pine up a bit the cabinet was ready for some new tolex and a grille front for cooling.  The back plate was fine so I left that.  Added another vent to the bottom, and filled any "love dents" and sanded it smooth.

[Amped]

Next was the turret board - needed that for the hole spacing in the chassis (and final fitment, better to find out now if something doesn't fit than later...).  Cut, drilled and turrets/standoffs installed.  Fitted in nice, everything seemed good.  Changed my approach this time a bit, thought I'd mount the board first, then install the components (usually do it the other way around), there was enough room and it seemed simpler this way for this build. 

On to the aluminium chassis.  It needed a bit of work, the layout was good but it looked pretty ratty so I drilled out the Noval socket holes for the 6V6's, drilled out any other holes I needed (turret board, switches etc.), there's a few small holes left from the last build but I managed to use most up or cover them.  I painted the chassis black, and this time I built the faceplates in advance.  They are a pain to make, really fiddly but if I don't then I solder everything in and it makes it too hard to put them in later.  I'm committed now, can't go changing anything...

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Now to build it.
Heater wiring along sockets first, don't solder the first socket so you can run from the lamp to the first socket later (I'll fly over the top for this - shorter run).  Then inputs, my new switch wired up and shielded wire for the sensitive runs done.  Then basically start hooking up the turret board and everything else (transformers/caps/jacks/impedance selector/bias/...).  I also found out that NKK have stopped making my favourite impedance selector switches now, damn.

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I left the Tolex to last - usual story for me - not as much fun as the circuitry.  But got it on, it's stuck much better this time.  I used some left-over tolex so it was a little short and I had no backup, but I made it work by panelling the bottom only. 
Put the chassis back in the cabinet.  Time to fire it up.

[Amped]

Works!  I had three issues;
1) The OT secondary leads were around the wrong way (found that pretty quick),
2) Although it worked it sounded pretty rubbish - almost like V1A was smashing V2 - but at lower levels too.  V1B input sounded fine-ish?  Thought "Uh-oh, my fancy switch...).  I messed around with it for a bit and after changing a few values I put it all back how it was, voltages were right, the circuit was pretty stock, any changes I made just changed the tone for the worse.  It had to be something else.  I went over the stock 2104 schematic Vs. the Thor - then I saw it.  I'd changed V1A and V1B around - the original (and Thor) has V1B as the first "High" input and I'd put the snubber on V1B like the drawings but because I'd changed them it was supposed to be on the V1A side.  Swapped it over and it opened up.  Can't believe I did that firstly, and then I missed it over and over again when I was checking.
3) The Presence control is wired backwards in the Thor schematic - which I copied.  I have some build notes for the Thor and they did it on purpose (I have to wonder why?), so I swapped it around and it works great.

[Amped]

Anyway it sounds good now, not amazing but definitely good enough for me.  The only mistake still present is on the faceplate, I got the front panel switch "Low" and "High" backwards, my OCD kicks in every time I use it :-(.  One day when I'm bored I might make a new faceplate and play with some of the Thor tone tweaks and see if I can get some extra magic back into it.  Or maybe I'll just leave it, like I said it still sounds pretty good.

[glass54]

Very Neat job Amped.
...and nice soldering 
Kind regards
Mirek

[tubenit]

I respect your tenacity and diligence in re-doing your amp!  Well done and bravo! 


With respect, Tubenit

[Amped]

Very Neat job Amped.
...and nice soldering 
Kind regards
Mirek

Thanks Mirek

[Amped]

I respect your tenacity and diligence in re-doing your amp!  Well done and bravo! 


With respect, Tubenit

Cheers Tubenit 

[imgumby001]

Great job brother!!! I have a similar build that I never got working right. Maybe I should pull her back out and give it another try!!!

[Amped]

Great job brother!!! I have a similar build that I never got working right. Maybe I should pull her back out and give it another try!!!

Heck yea go for it!  No point that investment sitting there looking all sad at you, might as well have another go.  This is the 4th re-imagining of that particular amp (first 3 we’re the same build, just never quite got there…).

A few different builds later, back to it, change tack-and success!

All the best for your journey too 😁