DIY Guitar companion amp - JCM800 2W combo

[Nick Kitchener]

So the plan was always to follow up building the DIY guitar with a DIY companion amp.

The requirements:
* Marshall JCM800 -- this has always been my iconic amp
* Playable in a small mancave without annoying the street

My research brought me to Rob Robinette's great site (fenders and Marshalls etc) which had a design for low wattage fender and Marshall amps - including the JCM800 at 2 watts (yes 2 watts) with all the Marshall pentode push-pull sound.

So after along time (much longer than the guitar) of designing and experimenting with LTSpice I got to a point where I was happy and pushed the order button on a number of places around the planet for components.

Building the chassis and positioned the hardware:

Cutting forward...

[Nick Kitchener]

And the rectifier..

So a normal JCM and most tube amps have the power going to the power stage, then to the pre-stages through successive filtering. The issue is the little EF80 pentodes can't cope with the high voltages so we have to have two separate power supplies - one for preamp (390V) and one for the power supplies (300V). This is why there's two chokes and two sets of power filtering.
A second difference from rob's design is that I'm using a EZ80/EZ81 rectifier tube to provide some rounding and sag - it also means I don't need a standby switch as the tubes B+ comes in slow as the heaters heat up.

Well first startup with the G12M-65 that needs break in showed it has alot of hum (as you will hear).. but it's worth a listen for that marshal sound:
https://youtu.be/QX_Y7_L2mLY

That's a 2W amp into a 97dB/W speaker - plenty loud enough!

I have since found a few issues and solved them - the main one being two large ground loops on the jacks. I've also replaced some of the internal single core unshielded signal wires with shielded teflon 1000V wire.

This how shows (at full pre-amp volume, full master volume, and full presence) some oscillation or interference at 20kHz which I believe is coming from an interaction between the heater supply that's elevated.

There's still some work to be done - I need to order some additional parts (some were scavenged and reused). The Marshall officinardos will recognise the lack of treble peaking caps etc. I also have some mods that I want to add - specially a 4th pre-amp tube to provide additional distortion (akin to the Mr scary v2 mod), cold clipper switchable cathode resistor values and a post tone stack distortion plus a bypassable effects loop. Addition to this there's going to be some attentuation - initially resistive but later a reactive dummy load to allow speaker less DI work.

[Nick Kitchener]

So some background noise, power on, then pre and master volume noise floor now I've had a go at reducing the hum. It's probably still uploading but has about 5 mins left..

https://youtu.be/Aw74li3kovc

In short
* open chassis with out full shielding or shielding on the power cabling or EF80s.
* still has the mains 20khz noise - that I'll have to think of someway to low pass that or notch it.
* the mains him is now very very low, below the hiss of the amp reallly - so full volume pre + master + presence and tone stack is now humless but instead has hiss.
* the hiss is the next target.. although we're limited in what we can do (no running dual triodes for each stage to reduce the noise floor.

I also did a 20V shottky mod to test that.. hehe that gave it 1980s metal vibes all over. so that's going on a switch.

[Nick Kitchener]

And update following changing the NFB to sit on the 16R speaker tap, this improves the sound greatly:
https://youtu.be/GvhyhcgSZhg

So I'm happy with 'version 1' of the amp.

I have some odd now in mind that I'll order parts for and add in. The additional parts will also increase the B+ for an additional sharper sound plus I'll add some additional mains filtering.

The experiments also have me thinking about adding a some switchable mods for adjusting for a more bluesy sound.

[Jarno Verhoeven]

Keen on seeing the schematic for your amp, surprised that the EF80 only has 2.5W max plate dissipation, they are bigger than the smaller double triodes, and ECC82/12AU7 even has a higher max plate dissipation.

What did you use for the output transformer?

[Nick Kitchener]

I have some TAD 12bh7-str and JJ ECC99s that also fit the same long B9A socket format.

The transformer is a Hammond 125B. A 5W push-pull that has a few options for use:

The differences between Rob's EF80 JCM and mine, are that this one has:
* Higher voltage power transformer (and larger VA for a reason)
* tube rectification (actually it's got UF4007 as backup on the plate loads) using an EZ80 at the moment.
* JCM800 pre stage voltages
* two power supply rails - one for the higher voltage V3/2/1 of the JCM to match the real JCM voltages (~390 B+) and the second rail provides the lower voltages for the EF80 (which should sit at about 270V at the plate). This is why it has two chokes.

I will change to the EZ81 which can cope with more voltage and current. This will allow me to increase the B+ but also add a fourth pre-amp tube (for a unity gain shaping stages). The amp itself doesn't need anymore gain.. but the additional tube enables some active mods rather than simply using diodes etc.

[Jarno Verhoeven]

WOW, that is a schematic! thanks for sharing.
Might be a fun project to try a power stage like that with some really old (50's vintage) speakers I still have. In a dovetailed speaker enclosure
Built this a while ago:

Padouk and basswood, no name speaker, and a class d amp at the back. Test purposes.

[Nick Kitchener]

Nice! (the schematic shows the two additional stages for the V2 mod that I will adjust)

Next will be the enclosure, for the design there's two options:

1. Separate boxes - a head box for the amplifier and a 1x12 cabinet for the speaker, however a 1x12 looks odd and the amp is easy to knock off the cabinet without bolting it down. A 2x12 looks better:

This allows both open back (better for low volume/smaller room) or closed back (better for higher volume/projection in larger spaces).

Or

2. Combo - a combined amp and 1x12 speaker in the same cabinet.

Typically they're open back for ventilation.

After much deliberating - I'm going to go for a combo. The amp controls have deliberately been offset to allow for this. I don't need massive volume, I do need it to be easy to move and have a high spouse factor. Most combos are a little too small for the speaker, so I will make one that's 12" in depth and sized to add an inch or two to stop the speaker sounding boxy.

It will be made out of Baltic Birch ply (the favoured guitar cab material rather than MDF) and aged pine (15+ years) from an old coffee table for the internal structs/framework.

The amp+speaker has enough frequency range but due to to the frequency response for the OT it does lack some bass depth for drop tuning that gets starts becoming a bass guitar I noted yesterday that drop Octave E etc started showing the limits (in part this is probably due to needing 68-72 gauge strings. Also the speaker isn't designed or built or bass guitar frequencies (where long excursion 8" or 10" speakers are used - which aren't good for guitar higher notes). I could make a 2x12 but they're bulkier but can offer a better lower register due to the two speakers pushing air.

Standard low E on a guitar is E2 (82Hz), the 7 string with normal tuning provides a low B1 (61Hz) and the guitar with 62 gauge strings can do F#1 (46Hz) and with thicker strings could push that to D1 (36Hz). However I'm happy with the current ranges/smaller drops - the extra string just opens some string transitions rather than shift hands. The output transformer is small, and hence can't support that low frequency extension anyway. It simply doesn't have the iron and flux to support it. It will register the higher frequency components but the -dB at that point is pretty much not happening

[Nick Kitchener]

An update on the amp - I went for a combo amp.

https://youtu.be/TsT1MaZPGHs

It's now assembled, although the front baffle needs fixing once I have the speaker cloth and tolex done. I have some shielding I need to add too.

Specs:
* 1x12 using a Celestion G12M-65 (12" 65 watt speaker)
* 585Hx560Wx316D (approximately 99 litres internal capactity)
* 1/4 finger joints on the sides
* Left over 12mm baltic birch plywood for the baffle
* 12mm beech plywood sheets for the other sides
* 5x12" rear opening
* pine from a reclaimed table as internal structural for the rear sheet support