Anderton '64 reissue JTM145 - any info?

[Littlewyan]

What software are you using for this?

[jazbo8]

What software are you using for this?

pctube by Ayumi Lab, but LTSPice could do the same thing, I think...

[Littlewyan]

I had a look but its all in a different language. Anyway just tested the amp, I put a signal through it whilst measuring voltages and the HT drops down to 326V and the dissipation of the 12AU7 goes up to 3.5W whilst the volume is on full. I'm guessing this isn't good?

[jazbo8]

I had a look but its all in a different language. Anyway just tested the amp, I put a signal through it whilst measuring voltages and the HT drops down to 326V and the dissipation of the 12AU7 goes up to 3.5W whilst the volume is on full. I'm guessing this isn't good?

Yup, it's in Japanese, took me awhile to Google translate the articles and learn to use the program... Anyway, it probably won't kill the tube just shorten the life a bit, how does it sound though with the high plate voltage?

[Littlewyan]

It does sound a bit harsh I will admit but still good. I'll increase the bias resistor size from 1K to 1.5K and see how it sounds. Might end up having to replace the PT which will be a big pain!

Edit: Well when I say harsh I think thats partly due to the low power switch. Being at home with parents means I have to use it all the time . Will change the resistor in a sec.

[Littlewyan]

Ok if I change the bias resistor to 1.5K then the idle bias sits at 1.7W, on full whack it goes up to 2.97W. Next size up is 2.2K in my stock and on full whack this dissipates 2.2W. Bit low compared to the stock value and 1.5K seems to sound quite nice. Will solder that in and give it a proper test.

I think it sounds even better now! Although the HT has now risen to 340V/345V it does drop down to 330V when running full blast. Has a real nice crunch on full volume, however anything below doesn't sound as good. Going to try upping the bright cap as its currently 100pF and I used to like 470pF on my old Marshall.

[Littlewyan]

Here is the schematic. As you can see its more JMP than JTM but then the original circuit was more JMP than JTM anyway. I changed the bright cap to 470pF as I preferred this, changed to the normal voltage divider configuration in the pre amp as it sounds near enough the same as the original circuit just less components, left out the extra stabilizing stuff as the amp is stable/quiet without it and changed the tonestack cap to 470pF as the extra mids just sound so good!

I did try and change values to be proper JTM so I shorted out the 820ohm resistor on first stage so the cathode cap was essentially 220uF and changed the coupling cap to .022uF and it was ok but I found I just kept turning the treble and mid up so changed it back. The way the original is setup is definitely JMP style so its odd they market it as a JTM. This also has a lot of gain on tap, mine might have more than the original, either that or its the medium/hot pickups in my guitar.

[Littlewyan]

How could I make the low power switch attenuate even more volume? I saw that on AX84 James (Marshall engineer) said to put a 10R or 8R2 resistor in parallel with the speaker, but id rather adjust the resistors already in the low power circuit. Increase the size of the 82R and 47R resistors?

[jazzkramer]

Hi Littlewyan, I really would like to listen to some samples of your amp, may you maybe post an audio? Of course we are not here to judge the guitarist, but only to apreciate the amp result. Thank you!

Jazzkramer.

[Littlewyan]

Will do soon, just need to tweak it bit more first. Thinking about changing the slope resistor in the Tone Stack to 33K and might change the cathode resistor on the 2nd gain stage to 1K to give it a tiny bit more gain. I also need to adjust the low power mode, going to try increasing the 82R Resistor on the 16Ohm Tap to 100R. If that gives the results that I want then I will increased the 47R Resistor to 68R. Will report back once I've done this.

[roberto]

How could I make the low power switch attenuate even more volume?

Change resistors values from 39R to 18R (whatever you find between 18 and 22R), from 47R to 150R and from 82R to 330R (R means Ohms).

You will switch between 1W and 20mW.
More attenuation? Double/triple the 150 and 330R values.

[Littlewyan]

Thanks Roberto. I'm already using 18R, i've definitely got 100R on hand to try for the 16ohm tap, i'll see what else I can dig out. How can you work out the attenuation? As it isnt in a normal voltage divider configuration.

[roberto]

You have a certain amount of power (current and voltage) on the secondary of the OT. Just consider the current that flows through the resistor+speaker and the current that flows through the 39R to ground (the current splits as usual), then square the rms current flowingh through the speaker and multiply by the impedance.

edit:
Take a look at the current divider on the next post.

[roberto]

Take a look at this, it will simplify your understanding:
[img:399:181]http://www.learningaboutelectronics.com ... vision.png[/img]

[Littlewyan]

So say I have 5 volts on my 16 ohm secondary.

5 / 18 = .27A

5 / 98 (82 + 16) = .051A

How do I work out the RMS Current from this?