FINISHED (Just about) - compact JTM45 build.

4114Effects

NOW FINISHED! Scroll down to see.

Just planning my Christmas project. Have all the parts to build a JTM45 with added master volume. Already have a small head and chassis from a different project, so to save me making another I wondered if I could make an ultra compact JTM45 ...

So here are all the parts laid out - what do you think - too close together? Any issues you can forsee noise wise. I know it will be a tight build, I'll be paying extra attention to lead dress inside, but would this work as a chassis layout?

I'm thinking I'll get away with it, but I don't want to spend hours building to find that everything hums from proximity to the transformers!

@ICBM @MJW maybe one for you guys if you have the time to have a little look.

Thanks!

Winny_Pooh

FWIW there is a technique some builders use to run low voltage through the circuit and listening on headphones move the orientation of the OT & choke for lowest interference before drilling their mounting holes.

here is one post:

http://www.tdpri.com/forum/shock-brothers-diy-amps/493592-dragon-slayer-custom-jcm800-build-3.html#post5889776

4114Effects

Aye. I'm pretty confident that the PT and OT are far enough apart, plus they're at 90 degrees to each other. I'm more worried about proximity of transformers to the preamp and output valves.

Modulus_Amps

Can be done, but for a first time build I would suggest going with a known layout.

you are missing the rectifier valve

You can buy a JTM45 size blank chassis in the UK for not too much money.

if you want to check for hum use the headphone trick:

From the AX84.com FAQ: THE HEADPHONE TRICK

"First set up the power transformer. If it's not installed yet, all the better, but if it is, this will still help. If the PT is not installed, wire up the primaries to a power cord, but do not wire up any secondaries-- tape the ends of the secondaries. If the PT is installed, fine, just make sure there is no circuit on any of the secondary leads (pull all tubes, and disconnect leads and tape them as necessary).

Now set up the signal transformer (output, reverb, whatever). Hook one set of OT secondaries (your choice, but I usually go from ground to the highest impedance tap, don't worry about impedance matching here!) to a set of headphones. Tape the primary leads.

Put the headphones on, plug in the power cord, and start moving the signal transformer around. Try it all over the chassis, and also rotate it. While you might expect the least hum with the transformers as far from each other as possible and at right angles (and that may be the case for you), it might come at an odd angle and/or position. Sometimes the least hum occurs when the transformers are side by side. When you find the quietest spot, use a permanent marker to mark where the mounting holes should be."

4114Effects

Thanks for the advice @Modulus_Amps

Funnily enough, I bought most of the parts for this build from you!

I should clarify though, this isn't a first time build. I've built 7 or 8 valve amps so far (from scratch, not kits), so I'm confident on wiring etc. I was just checking if putting things this close together was likely to cause any issues.

My cab is only a 1x12 - so a full sized Marshall head would look a bit silly overhanging both sides of the cab

thermionic

Some serious iron on that chassis... Have a Wow.

martinw

4114Effects said:

 I was just checking if putting things this close together was likely to cause any issues.

My cab is only a 1x12 - so a full sized Marshall head would look a bit silly overhanging both sides of the cab

Unfortunately, no-one can tell you if that layout will work unless they've already tried it with those exact components. There are so many variables, that's it's (in practical terms) impossible to predict what will interact with what. Consequently, we could only offer gut feelings - mine is that it's too cramped and I wouldn't do it. Heat dissipation would be one of my prime concerns, even if noise isn't.

In your favour, the JTM45 is a low-gain amp, and is quite tolerant of less-than-ideal layouts. A lot of Hifi amps and other non-audio vintage equipments use what we would think of as very cramped layouts without problems, so maybe it would be fine.

I would definitely recommend a headphone test, as you might find simply turning the OT on it's end (for instance) makes a huge difference to background noise.

BTW I put JTM45 style amps on 17" chassis in 53cm cabs that rarely overhang a 1x12.

Good luck and stay safe.

ICBM

4114Effects said:

I'm pretty confident that the PT and OT are far enough apart, plus they're at 90 degrees to each other.

No they aren't - all three are parallel in that pic. It's the direction of the winding axis that matters, not the orientation of the core plates - all three are currently vertical to the chassis.

Luckily you can easily fix it - leave the OT like it is (axis vertical), rotate the choke so its sticker is facing the OT (axis left to right) and rotate the PT onto the other set of mounting holes (axis front to back).

But even then, if you can I would be inclined to build it temporarily without bolting anything down and make sure - sometimes they don't follow the rules as martinw said.

4114Effects

Thanks guys. Bigger chassis it is :-)

ICBM

I like the idea of building it that small - as long as it works.

The unnecessary size of things like a Marshall 100W chassis has always annoyed me - let alone the size of the boxes they put them in, especially the later 50W amps.

I don't think heat is a serious issue in an amp that relatively low-powered, as long as it's that way up and has good ventilation. Lack of a valve rectifier will help, and isn't necessary anyway - if you want to introduce some sag, put a couple of 50-ohm resistors in the HT feeds. It's not exactly the same but close enough.

4114Effects

ICBM said:

I like the idea of building it that small - as long as it works.

The unnecessary size of things like a Marshall 100W chassis has always annoyed me - let alone the size of the boxes they put them in, especially the later 50W amps.

I don't think heat is a serious issue in an amp that relatively low-powered, as long as it's that way up and has good ventilation. Lack of a valve rectifier will help, and isn't necessary anyway - if you want to introduce some sag, put a couple of 50-ohm resistors in the HT feeds. It's not exactly the same but close enough.

Totally agree, the Marshall heads and chassis have always seemed like a massive waste of space. 

Yep, I was going to put resonably large grilles in the top - and the back would be one large grille, so heat shouldn't be too much of an issue. The PT is over-rated anyway so it shouldn't be putting off much heat at all.

I've never been too bothered about valve rectifiers, apart from if it's a 5Y3 in a 5E3 circuit where I have been able to hear a small difference at higher volumes. There's so little sag with a GZ34 that it just seems like a waste of a socket.

I'll probably mount the can capacitor internally too, or just use two decent 32uf axial caps instead. Having the cap above the chassis seems like another waste of space!

Hell, I might just try it like this. The worst that will happen is I'll have to take it apart an install in a bigger cab if it doesn't work out.

4114Effects

Updated layout ... might just throw caution to the wind and go for it

ICBM

4114Effects said:

I've never been too bothered about valve rectifiers, apart from if it's a 5Y3 in a 5E3 circuit where I have been able to hear a small difference at higher volumes. There's so little sag with a GZ34 that it just seems like a waste of a socket.

I have been able to hear it - even the difference between two different GZ34s - in an amp I'm very familiar with. I got someone to help me with a blind test once. (OK, one was a proper GZ34 and the other was a Sovtek which are known for not being accurate to the original spec.) There is *something* in it, but it's pretty subtle. I'm pretty sure I wouldn't be able to tell which it was in a new build I didn't already know the sound of, at least if you roughly matched the amount of sag.

What I find slightly hilarious is that the most valuable rectifier valve of all is the Mullard GZ34, which is the *least* valve-like - it has the lowest forward resistance and hence is the closest to a solid-state one...

4114Effects

@ICBM Just done the headhpone test, zero hum with it laid out as above. Putting the PT where the choke is now gave noticeable hum, so the headphone method works well

So with that in mind, I should be good to go? The only question I have is whether transformer hum can be picked up via close proximity to the preamp/output valves? 

ICBM

4114Effects said:

@ICBM Just done the headhpone test, zero hum with it laid out as above. Putting the PT where the choke is now gave noticeable hum, so the headphone method works well

Interesting - in that case leave it! Although I would still be inclined to build the amp without bolting them on (or not using bolt hole positions that don't apply to any alternative orientation, anyway), if you can.

4114Effects said:

The only question I have is whether transformer hum can be picked up via close proximity to the preamp/output valves? 

Not from the OT - although it can from the PT - but it could potentially cause instability.

Modulus_Amps

Try and keep the OT primaries away from any pre-amp wires, is is going to be tight around those three 9 pin sockets so sketch out your layout before you start, let us know how you get on. Trainwreck amps have the OT and PT right next to each other, and though I don't believe it most guys say there is no hum

4114Effects

So following on from this, I think it's turning out rather well.

Had to do some thinking outside the box, front layout was rather cramped, so the switches have moved to the back ... I think this works pretty well, plus its a couple less runs of AC wire from the front to the back, so probably makes sense for the layout.

Covered it in red tolex and managed to get the vent directly above the valves, which should help take care of any heat issues from such a tightly spaced build. Even managed to include the GZ34 rectifier socket. 

Got some polished silver corners on the way which should complete the look.

Pretty chuffed with how this is turning out so far

ICBM

A post-PI MV works well on this circuit, and I see you've left a space for it .

Use the 'Ken Fischer' version with a dual-gang 250K pot replacing the bias feed resistors and 1M safety resistors from bias supply to wiper on each half.

4114Effects

@ICBM that's exactly what's on order ... although the schematic I have uses 2m safety resistors :-)

simonbeckwith

This looks mega........ As always

Andyjr1515

No idea whatsoever what 7/8 of the threads are talking about, but it looks great and I suspect it will perform just as well