Anderton '64 reissue JTM145 - any info?

[Smokebreak]

Sounds great buddy! Good work!

[Littlewyan]

Thanks. I'm thinking about my next project already! At the moment I'm thinking of either a 1 watt Fender Bassman or 1 watt Hiwatt CP103 or DR103.

Edit: Just put a 25W Bulb into my light bulb limiter, used it on the amp and amazingly it lowered the B+ to 250V. Bargain! So I put the Cathode Resistor back to 470R in the Output Stage and I have to say it sounds almost the same as when the B+ is 330/340V but with less clarity. Took the limited away, started trying larger cathode resistors such as 2.2K, 2.7K and 3.3K. 2.2K was almost a winner as it had the sort of 400V Plexi sound (brown soundish), however I miss the clarity at times. 2.7K/3.3K was the same but quieter (think the 12au7 was barely turned on with these values!!). One thing I might try is the 2.2K again but then with the 2nd gain stage bypassed with a .68uF Cap. I tried the .68uF with the 1.5K resistor and it gave the amp a nice bit of bite.

[Matec]

Congratulations Littlewyan.
I am following your mods, and I was interested in the results. Keep posting your experiments.

Just recently did a JCM800 / 1.5 W. Does not sound the same, especially in the Hi channel, the amp has a lower overall gain. The sound is pretty nice, I like how it turned out.

Cheers

Matec

[Littlewyan]

Matec

Start a new topic and then post your schematic. I might be able to help as I did do a simulation for the JCM1 and it did have quite high gain so looked promising. Haven't built a prototype though so can't say how good it sounds in real life! Or did you not want anymore gain?

One thing I may also try is adding a bypass capacitor to the 12AU7 Cathode Resistor. This should make the distortion a bit more crisp.

[Matec]

Yes, I will.

I just need to put in order the documentation.

Within a few days I will open a topic in the area of the Technical Discussions. Not only for the JCM in particular, but for "Lo Power" in general.

Matec

[Littlewyan]

Low Power amps are becoming increasingly popular it has to be said!

I tried the .68uF Cap and decided in the end that I didn't really like it. Was too much, made it sound a bit mushy. I did however try bypassing the 12AU7 Cathode Resistor and didn't like it as it made the sound quite harsh. Another thing I experimented with was lowering the Output Impedance by mismatching the speaker. Plugged a 4ohm cab into the 8ohm socket so the Primary Impedance seen by the 12AU7 would have been 11K and it sounded alright to my ears. I can see how 18K would sound better than 22K (My OT).

I have the say though the distortion on this amp doesn't have very much clarity, not sure if thats due to me leaving out the 100nF Capacitors on the HT or if thats just how it is. I also don't have very much treble but I think that could be because of the low power mode. Our ears generally don't hear higher frequencies as well at lower volumes. Found this with a few amps before.

[Matec]

There is much to be said, and do, with regard to "Lo Power".

For example, in JCM1, a bypass capacitor is 47uF. (Yes, I did the schema of the power JCM1 ), this can be attempted on your amp.

In the tests that I and a friend made ​​the biggest power gain we got was with 18k ~ 20k primary impedance of the OT. But timbre tests are more complex. Need several transformers and good ears. I had neither.

In the scheme of Jtm1 there is a change, C15 becomes part of a DC filter in the audio signal path, along with R36, but it is a precaution manufacturers. For us, it can be forgotten ... (or not).
C17 retains its position.

Matec

[Littlewyan]

Well I'm hoping that I'll be able to test this amp on high power mode soon so will find out then what it really sounds like. I did once mod a Marshall 1987 to sound good with an attenuator in the bedroom, however when I took it to band practice it sounded like crap. So not making that mistake again. I have however just looked in my parts bin and found a 100nF Orange Drop Cap. Its huge but it will do for testing which I'll hopefully get done one night next week to see if I can hear a difference.

At the moment I'd say when the amp is at full volume it sounds quite compressed, whereas with the volume on 7 it sounds nice and raw and has a nice bite. But again this could be due to the low power mode.

Edit: Been looking at the ECC802 Valve today, its difficult to get here in the UK but I've found a few suppliers. Might be worth a try in my amp as it can handle slightly more power than the ECC82 and might add to the sound a bit more.

[Littlewyan]

Worked out why having a cathode bypass cap on the 12AU7 sounded weird. Seems to have a hard time charging up the cap. If I play a chord and then mute it I can hear the amp go silent and then the hiss come back after a few seconds. Very weird. Any ideas?

I will say the amp sounds quite compressed but I think thats just the nature of the 12AU7 with the conjunctive filter.

[jazzkramer]

I was thinking...
...maybe is there an Anderton's CS JTM145 buyer who could open the head, unscrew pots and boards and take very detailed photos of both sides of each board?? Am I dreaming?

Bye, jazzkramer.

[Littlewyan]

We haven't had anyone take any detailed photos of the rest of the 1 watters yet . Not that I've seen anyway. We need someone who owns the amps that can make schematics for us, as Matec was having to go by photos (of which he did a great job of). Someone that owns an amp could at least confirm that Matec's schematics are correct. I'm surprised no one has done this yet. Tried emailing Marshall asking for a schematic (well you don't ask don't get) for the JCM1 but they said I need to be a registered engineer. Plus any registered engineer that releases a schematic will be going against their policy. So its unlikely that we'll see an official one for a while ;(

[jazzkramer]

Hey Littlewyan... you gave me an idea ......maybe I could know a "registered engineer" I have to ask....

[Littlewyan]

Sneaky. Dodgy though, if anyone ever found out it was them that released the schematic they'd be in big trouble with Marshall. You could maybe get them to verify Matec's schematic.

[jazbo8]

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

[jazzkramer]

You're right Littlewyan, but asking is free. I will try and see what I get . In the future that schematic will sure be public, even if not the "original paper". A not so far from here shop is selling a JTM1 used. I don't have all the money right now, but if I found it I will buy it and will open it and will look at the real schematic. I would like to build a sort of "preamp pedal" by that schematic becouse the "head" assemble is not pratical to me. So, it's only a matter of time: we will discover that schematic (I hope!).