JTM45 Build

[pdf64]

I put together a JTM45 simulation following the Metro schematic and found the PI wants to oscillate at ~90kHz with high Treble and low Bass and/or Middle settings. If I drive it hard enough to square-off the output the oscillation will stop, and under some lower drive conditions the oscillation will damp out. I also found I could kill it by increasing the fizz cap (47p across the PI plates) to 220p.

When square wave testing mine, I noticed a slight 'ringing' on the leading edges.
It was a while ago and unfortunately I didn't make notes, but concerned about parasitics / margin of stability, I remember that a 100pF fizz cap made it worse; without a fizz cap it looked fine, so I left it like that.
Tone control settings would have been adjusted to get the best square into a resistive load at a level below clipping into a resistive load, eg Normal channel, Treb 0, Mid 10, Bass ~2, Pres 0.

[M Fowler]

My JTM45 just sits in it's head box and collectd dust because it's not much of a crunch amp pretty mellow tame amp which I suspected it to be when I built it.

The JTM50 EL34's gives up a little more.

[ArrogantOwl]

Reel, did you get yourself another amp garage account?

Nope. I am not Reel. I do lurk a lot though.

Can find me on TGP under the same tag.

[M Fowler]

TGP banned me and others so no going there. Haven't figured out why we got banned unless it was that last TGP thread posted a while back

Mark

[pdf64]

Can find me on TGP under the same tag.

Curiosity got the better of me; a search for that member name (and a few variations thereof) came up blank?

[Littlewyan]

Why would it take you 2 hours to read through this thread? Unless you're an extremely slow reader it wouldn't take you longer than 15minutes. Also why would you read this thread, find out it wasn't an issue and then read through the same thread on the Tube Guitar Amp Builder page?

Looks like I'm going to have to fit PPIMV controls to my amps. My Express had to be turned down as far as it would go on the attenuator to make the volume low enough so it could be miked up. My attenuator gets pretty hot when you turn it right down, altho luckily last night we played outside and it was pretty dam cold so it stayed cool.

[pdf64]

What speaker/s are you using?

[Littlewyan]

Eminence GB12s in a 2x12. I think I worked out they're a bit more efficient than Greenbacks (Eminence and Celestion efficiency ratings differ).

[pdf64]

A 1x12 would be ~3dB quieter, and I can recommend the much derided (and so cheap secondhand) G12T75 as having medium 97dB/W sensitivity and a great tone that's close to a greenback and can take a lot of power, without nasties such as cone cry.
The top end is a little more muted than a greenback, as it lacks the sizzle that a small dust cover seems to provide, so it's probably similar to your GB12 in that respect.
So a single G12T75 may bring your level down 4 -5dB.
You could mod the cab so that there was one socket for 8ohm 1x12, the other for 16ohm 2x12.

[Littlewyan]

Haha............There used to be G12T-75s in that cab until I swapped them for the GB12s . I wish I took some measurements with a dB meter before and after to see if there was a difference. I don't think there would be that much difference as Eminence get their speaker efficiency ratings in a different way to Celestion. I did work out the difference ages ago but can't remember what it was now.

I'll just stick Master Volume controls in my amps as it'll make bedroom playing a lot easier and if I don't want to use them live I can just turn the master up to 10 and use the attenuator. Plus parts for a Master Volume control are cheaper than new speakers .

[guitarmike2107]

What attenuator are you using, are you gigging at below bedroom levels?

Go to BnQ and get a sheet of 6mm plexi glass to put between the speaker cab and sound desk, problem solved, if you need to you can use a heat gun to put 2 bends in it to make it free standing and provide better isolation, then if you want to get even more fancy you could attach your mic clip to it ....

[norburybrook]

my JTM45 definitely takes pedals very well. I love mine through a marshall 4x10 cab, with any kind of clean boost and a Zendrive it's a great crunchy amp.


The clean tones on the treble channel with a strat are wonderfully sparkly , the normal channel is very thick and not much top end, but they blend well together with a jumper.

I had Nick Kershaw's guitarist here recently ( he was a big 80's guitar pop star in the 80's in the UK) and he thought my JTM45 was the best amp he'd ever played. Needless to say he wanted to buy it but I said it wasn't for sale


also without wanting to name drop, I work with Mark Knopfler a lot and his main 2 amps are 2x Reinhardt JTM45's through 2 4x12's all covered in lovely red tolex.

funny how people's experiences vary so much


Marcus



P.s it was cold last night I was gigging at Gawsworth Hall and I could see my breath when singing!!

[ArrogantOwl]

Can find me on TGP under the same tag.

Curiosity got the better of me; a search for that member name (and a few variations thereof) came up blank?

I screwed that up

It's Adviseable Owl on TGP.

It's the one place I didn't use that naming convention haha.

You can try that on Reddit. Or ConspicuousHippo on MLP.

[Littlewyan]

The attenuator is a custom made one that can take the amp down to bedroom levels but it gets quite hot when I do so.

For the last two gigs I played the sound man wanted to mic up the amps, especially for the last one as he was recording us.

How would the plexi glass work with the microphone? As I think the issue was the amp was too loud for the mic. I could just install a PPIMV which I could use with the attenuator.

[norburybrook]

Your amp won't be too loud for any dynamic microphone. A dynamic mic which is what is usually put on guitar amps, SM57 etc is almost impossible to overload. You'd be looking at a SPL of 180db which is very close to the theoretical maximum SPL on earth of around 190db due to our atmosphere.


The plexi screen will mean you can crank your amp top a level it's working at for you but the sound won't be killing the front row of your audience or pissing off the sound man or other band members. Other option is to have your cab facing off stage if it need top be loud and have the mic'd signal fed back through your monitor. That would be an extreme case though or a production decision if amps are not wanted to be seen by the production/set designed on a big tour.

Get down to B and Q and try it. Also close backed speakers are a bit more friendly to others too. A favourite guitar player of mine lays his 2 VOX AC 30s on their backs facing directly upwards, makes them almost closed back and keeps the sound controllable for everyone.



Marcus