Anderton '64 reissue JTM145 - any info?

[Matec]

Hey jazzkramer.

Now I'm feeling that someone wants to take action!

If 2 or 3 more people do so, we will all schemes 1W we want.

Cheers

Matec

[jazzkramer]

Matec, I hope I can do it, and hope others can do it too . Before or later we will have the schematic, even if the best and most difficoult to get is the CS JTM145 by Anderton...
Bye

[jazzkramer]

TADAAAAAAAAAAAAA: Ok guys, I found a guy who own the JTM1. NOW I only have to convince him to open it and.... Stay tuned!!!

[Matec]

Great!

When you have to open the amp:
In the case of the back part, take photos directly on the plate.
The important thing now is to know the details, especially small values ​​of capacitors. As for the scheme itself, I have no doubt about the amount of hits.

When disassembling the panel has a set of allen keys. Maybe what you need them to remove the knobs.

Take photos of both sides.

Take lots of pictures!

If you have questions, write.

Good luck!

Matec

[jazzkramer]

Thanks Matec, as soon as I can I will take tons of detailed pictures both sides all plates!! If possible I will copy all the condenser values (they're not so much)

Probably he will lease me the amp for a couple of days... so, I will post fotos and you (or others) will ask me if something else specific that maybe I forget to take fotos is needed, or something that is not enough clear

At the moment don't know when I could do it but sure I will do!

[Littlewyan]

I have to say that this amp has always sounded a bit like there is a blanket over the speakers and I think I know why. No presence control! Using LTSpice I simulated a normal Marshall 1987 and then my JTM1 (with 1987 pre amp spec) and found that the normal 1987 had much higher treble. Take away the presence control and its the same as the JTM1. I've attached screenshots to show what I mean.

I have looked at ways at adding a fixed value presence but can't see how it can be done with this circuit. Any ideas? I reckon the JTM145 sounds so much better because they gave it a fixed presence control.

[Matec]

Jazzkramer

No doubt I'll be waiting anxiously for these photos.

Littlewyan

In the graphs you did not specify what is the full line, nor the broken line.

You could make the effect "presence" of two forms.

1 Insert an RC filter in the NFB circuit. It is done so in 1987 and others. But the low performance of the NFB in this circuit, do not allow much "presence".

2 In the photo I posted, the CSJTM145 appears C1, the bypass capacitor of the output tubes. Being an electrolytic capacitor. should not have a low value. You could try putting a value to give some presence, (maybe 2,2uF, or whatever) without much difficulty.


(In their simulations of tube circuits, Norman Koren models are used?)

Cheers

Matec.

[Smokebreak]

There is an ebay seller, deerhunter56, I believe, that will cut any size wood plate, at any thickness for a few bucks. She has a lot of different kinds of wood, too. I get mine 1/16", which is thin, but just fine once you finish it. No more trying to find pots you like that will be long enough, or trying to get more than 1 thread on a standard Alpha. Not sure about shipping to UK

Would you still have the link for the wood panel? Search on eBay turned up nothing... TIA

http://www.ebay.com/usr/deer56hunter

[jazbo8]

http://www.ebay.com/usr/deer56hunter

D'oh, I swear I typed in the name and nothing came out, oh well, thanks for the link, gonna order me some boards.

[Littlewyan]

The full line is the frequency response of the circuit I guess you could say and I believe the dotted line is the phase. I ignore the dotted line if I'm honest as I don't fully understand it.

When Cathodyne PIs are used the NFB is generally connected to the Cathode of the Driver Stage. I wonder why Marshall didn't do this? As you could easily insert a presence control there and in theory it would have the same effect as it has in it's current place. I don't think we can add a filter to the current NFB loop as not only will it filter out the NFB but will also filter out the signal coming from the anode of the driver stage.

Also I was going to ask about the 560k resistor off of the Cathodyne grid. In a lot of schematics I've seen this is connected before the 470K resistor and not after. As at the moment its acting as a voltage divider in a way.

Edit: Attached a schematic to show you what I mean.

[Smokebreak]

http://www.ebay.com/usr/deer56hunter

D'oh, I swear I typed in the name and nothing came out, oh well, thanks for the link, gonna order me some boards.

Haha no worries. I had the "56" in her name in the wrong spot. I actually did up a board yesterday for another amp. I've been using the all-in-one polystains on these and it works well for me.

[Matec]

Littlewyan

Marshall made ​​this circuit to be practical and serve for two configurations with the same components. LTP PI would use a valve to more.
Let us not forget that the same board, and most of the components are used in a slightly different form, in JCM1.

In JCM1, this bias resistor of the PI, is not 560k. but of 470k.

I do not remember having seen this circuit somewhere. When configuring the Marshalls, which is also underused, might not have the expected result. Anyway, that's not how I saw it on the motherboard.

[Littlewyan]

Matec, you're probably right. I might experiment with moving the feedback loop to the driver stage's cathode. However at the moment I think the feedback loop is simply there to stop the 12AU7 being pushed into distortion too quick. So if I move it to the driver stage it will stop doing it's job as it'll just give the driver stage more headroom.

I have been experimenting with changing the tonestack to JMP values, bypassing the second gain stage cathode with a .68uF Cap and increasing the first coupling cap to .022uF. The .68uF gives the amp more presence but doesnt sound nice with the 56K/220p configuration. There have been amps in the past with no presence so I'm sure there is a way round this.

[Littlewyan]

After a lot of experimentation I have come to this conclusion. The original values work fine . Basically I found that if I changed the tonestack to JMP values and added a .68uF Cap to V1B's Cathode then I got a nice hot plexi sound, but also found that it was only good for overdriven tones. So I put everything back to stock except for V1B's Cathode Resistor (820R). This was better as I could get a bit of clean (I have hot pickups so its difficult), changed the Cathode Resistor back to 1.5K and not only did the cleans improve but I seemed to gain more upper mids. So its now staying stock.

For those of you interested I did try changing the NFB and adding a presence cap. I moved the NFB to V2A's Cathode, bypassed C11 and changed R15 to 10K. The result was a very rough sounding amp, had more high end but wasn't nice. Added a .1uF Cap to the Cathode and I got even more high end and upper mids but again was very harsh.

Another experiment I did was using a .022uF Coupling Cap for V1A, however I then suffered from a bit of blocking distortion and it sounded like the 12AU7 couldn't handle the extra bass. So if you're looking for the boomy bottom end that a plexi delivers then this isn't the amp for you. But if you're looking for a great bedroom amp then this is perfect.

[Matec]

I can only say that I was happy with it all.

But I'm also impressed with the research and experiences of Littlewyan.

Congratulations.

Matec