Little Monk JTM584

[Big Monk]

So I had previously posted my amp I was calling the 5F184, which was basically my modified Valve Junior circuit from over the years. Since then, i've done some further experimenting and come up with what I am calling the JTM584.

It's essentially a 5W, SE EL84 amp but with some circuit tweaks to put it into cranked Plexi territory. As opposed to some of the early "Marshallized" VJrs, I've tried to strategically control the signal gain to accentuate and ape certain aspects of what I hear when I hear a cranked Plexi.



My hope is to try and finalized my "desktop" 1590DD design with my existing transformers and use a JJ 40/20/20/20uF cap can to save some space.

V1a cathode has an 820/680nF combo while V1b has an 820/2.2uF combo. Over the years I've tried almost every combo in these spots and these two pairs seem to be the most balanced to my ears between lower volume Tweed/Plexi cleans and grind when cranked.

The signal path of the 33k input resistor, 470k/330pF, 500kA Volume pot, 470k grid leak and 5.6k grid stopper was arrived at after years of trying stuff out. The 470k and 330pF treble peaker seem to provide the right frequency response. I've tried 100 pF, 220 pF and 470pF there over the years and 330 pF just sounds right. The signal limiting across the audio path and the 470 grid leak resistor seem to provide the right balance of signal slamming the grid and power tube distortion.

I've added a 1k screen grid resistor which was notably absent on all versions of the VJr.

The 40uF filter cap and 220uF EL84 cathode bypass cap strike a balance between softer/saggier Tweed response and the very firm sound of a Plexi power amp.

I've added a switchable Bright cap for low volume playing. I may also do a High/Low input as well for when I want it to be more pedal friendly.

Lastly, I need to determine how much (or rather how little) negative feedback to add. I'm using the AA784 Champ Amp style feedback insertion, keeping the V1b cathode bypass cap. The goal here will be to fine tune the cranked amp tone such that the notes don't get totally shredded when turned up. I'll likely tack in a pot and listen for the onset of slight damping and then sub that value.

 

[Gordo]

I’m interested to hear this Monk.

 

[Guardians of the analog]

Little Monk

 

[Big Monk]

I’m interested to hear this Monk.

It really does give a very good Sonic approximation of an old cranked Plexi.

I think the biggest component to the success of this goal is NOT over driving the second triode.

It has a rich and fairly clean tone until you get up over halfway on the volume, where it then translations to big, bold and crunchy power tube distortion.

My goal is to also replace my Celestion G10N-40 with a WGS ET10.

If the overall experiment works out like I think it will, I’m also contemplating buying a 4x12 cab and a larger OT. Probably the Hammond 125GSE, which is a big old 25W OT.

 

[Gordo]

I can't say enough nice things about WGS. I've put them in one of my amps (Traynor) and in a bunch of customer amps and I've always been impressed.

 

[Giorfida]

So I had previously posted my amp I was calling the 5F184, which was basically my modified Valve Junior circuit from over the years. Since then, i've done some further experimenting and come up with what I am calling the JTM584.

It's essentially a 5W, SE EL84 amp but with some circuit tweaks to put it into cranked Plexi territory. As opposed to some of the early "Marshallized" VJrs, I've tried to strategically control the signal gain to accentuate and ape certain aspects of what I hear when I hear a cranked Plexi.

View attachment 77442

My hope is to try and finalized my "desktop" 1590DD design with my existing transformers and use a JJ 40/20/20/20uF cap can to save some space.

V1a cathode has an 820/680nF combo while V1b has an 820/2.2uF combo. Over the years I've tried almost every combo in these spots and these two pairs seem to be the most balanced to my ears between lower volume Tweed/Plexi cleans and grind when cranked.

The signal path of the 33k input resistor, 470k/330pF, 500kA Volume pot, 470k grid leak and 5.6k grid stopper was arrived at after years of trying stuff out. The 470k and 330pF treble peaker seem to provide the right frequency response. I've tried 100 pF, 220 pF and 470pF there over the years and 330 pF just sounds right. The signal limiting across the audio path and the 470 grid leak resistor seem to provide the right balance of signal slamming the grid and power tube distortion.

I've added a 1k screen grid resistor which was notably absent on all versions of the VJr.

The 40uF filter cap and 220uF EL84 cathode bypass cap strike a balance between softer/saggier Tweed response and the very firm sound of a Plexi power amp.

I've added a switchable Bright cap for low volume playing. I may also do a High/Low input as well for when I want it to be more pedal friendly.

Lastly, I need to determine how much (or rather how little) negative feedback to add. I'm using the AA784 Champ Amp style feedback insertion, keeping the V1b cathode bypass cap. The goal here will be to fine tune the cranked amp tone such that the notes don't get totally shredded when turned up. I'll likely tack in a pot and listen for the onset of slight damping and then sub that value.

Keep posting updates! Sounds like a cool idea!

 

[Big Monk]

Down and dirty clip, warts and all:

Only thing missing from the proposed final version is NFB. my thought is that a very subtle amount between 220k-1M will suffice.

You can tell from the phone clip but there is a hint of raggedness/shredded nature on the notes when cranked that a subtle amount of NFB should solve.

 

[Aquaticwombat]

Fitting that in a 1590DD is impressive. I just did an AC4 and needed the 10" chassis to make it work.

 

[Aquaticwombat]

Also, the ET10 is an awesome speaker. I have one in my 6v6 jcm800 combo.

 

[Big Monk]

Fitting that in a 1590DD is impressive. I just did an AC4 and needed the 10" chassis to make it work.View attachment 77601

Mine is not currently in a 1590DD. I believe mine is in a 12x7x2 Aluminum Hammond.

I think the 1590DD idea is a bust. Just not enough space.

 

[Aquaticwombat]

Mine is not currently in a 1590DD. I believe mine is in a 12x7x2 Aluminum Hammond.

I think the 1590DD idea is a bust. Just not enough space.

I was wondering how youd get transformers on there. Point to point you could def do the guts, but the tubes and transformers would be.... Rough.

 

[Giorfida]

Down and dirty clip, warts and all:

Only thing missing from the proposed final version is NFB. my thought is that a very subtle amount between 220k-1M will suffice.

You can tell from the phone clip but there is a hint of raggedness/shredded nature on the notes when cranked that a subtle amount of NFB should solve.

Dude! Great job!!!
I definitely hear the super bass/jTM vibe going on.
You are right a bit of NFB will help take out some of the harshness.
Others you won’t get out- there is just a bit of magic that the cathode follower, long tail PI and push pull output do that we can’t “naturally” get from a single ended amp. But. There are tricks to get around that. NFB helps a lot? As well as I’ve heard conjunctive filters used effectively- the trick with them is keep it “less is more”.
Still surprised at how much Marshall vibe is in what’s essentially a hot rodded champ circuit! Good stuff!

 

[Gordo]

Oh yeah, great demo to get the idea of what you're doing. It has that ragged top end and the Marshall grunt. You leaned into a bit of the cleans and it seemed to clean up pretty nicely. This is very cool and really makes me miss building amps. Thanks Monk.

 

[Aquaticwombat]

Also, if you want to save a few bucks, that 8w single ended transformer on antique electronic supply is 20 bucks and actually sounds great in my AC4.

 

[Big Monk]

Dude! Great job!!!

Thank you!

I definitely hear the super bass/jTM vibe going on.

It’s definitely got the same kind of grit and grind.

You are right a bit of NFB will help take out some of the harshness.

And it won’t take much. I do t actually want all that much anyway and the amount I do use won’t really be audible. It’ll be mostly affecting the feel of the amp and just tightening up a bit of the speaker distortion.

Still surprised at how much Marshall vibe is in what’s essentially a hot rodded champ circuit! Good stuff!

I think the biggest thing to remember is that the 5F1 Champ architecture doesn’t have any loss, so you can’t get carried away with too much signal gain.

In this amp in particular, or any two tube 5 watt amp in general, there IS such a thing as too much of the WRONG signal gain.

If you want the biggest and most stout amp overdrive sound, it has to come from driving the power tube and NOT the preamp tube.

 

[Giorfida]

Thank you!



It’s definitely got the same kind of grit and grind.



And it won’t take much. I do t actually want all that much anyway and the amount I do use won’t really be audible. It’ll be mostly affecting the feel of the amp and just tightening up a bit of the speaker distortion.



I think the biggest thing to remember is that the 5F1 Champ architecture doesn’t have any loss, so you can’t get carried away with too much signal gain.

In this amp in particular, or any two tube 5 watt amp in general, there IS such a thing as too much of the WRONG signal gain.

If you want the biggest and most stout amp overdrive sound, it has to come from driving the power tube and NOT the preamp tube.

Agreed on all points.
And you bring up a good point- in the champ topology, it definitely is power tube being driven, where in most push pull amps, the phase inverter acts as a limiter and breaks up while keeping the power tubes from clipping.
But simple amps like this have so much room to tweak- and with the simplicity, the smallest changes can be heard.
I’m just glad to see others sharing the joy (and frustration at times) of amp builds! Hey with enough of us here, perhaps it’s time to petition the powers that be to add an amp building section! One can only dream

 

[Big Monk]

Agreed on all points.
And you bring up a good point- in the champ topology, it definitely is power tube being driven, where in most push pull amps, the phase inverter acts as a limiter and breaks up while keeping the power tubes from clipping.
But simple amps like this have so much room to tweak- and with the simplicity, the smallest changes can be heard.
I’m just glad to see others sharing the joy (and frustration at times) of amp builds! Hey with enough of us here, perhaps it’s time to petition the powers that be to add an amp building section! One can only dream

I’d be inclined to agree with you but DIY amp building was already in decline when I got into it back in 2008 or so.

It’s certainly not dead but every year it gets more and more expensive to build amps.

 

[cooder]

Great stuff and soundz, very cool!
I do have a few low power single ended OT lying around, will have to have a look into this. Sweet!

 

[Big Monk]

Great stuff and soundz, very cool!
I do have a few low power single ended OT lying around, will have to have a look into this. Sweet!

I’ll let you know when the boards come in and I can send you a BOM and one of the boards.

 

[cooder]

Oh man that would be super sweet! I'd be keen.
What PT have you been using, what voltages are on HT? I do have a bunch of old radio PTs that might just fit as well.
This is how I built my wee champ with radio PT / OT. Coold wee thing in a 1590XX.
It's big enough to use at band practise and I still hear the occasional 'can you turn down a bit?!' haha