Anderton '64 reissue JTM145 - any info?

[Matec]

Hello Littlewyan

Very good your circuit. His solution was well thought out, it seems more JTM45, than JTM145 own. We hope soon to hear the results.

One problem that I have identified.

You will have to reverse the secondary for the amplifier does not oscillate. With this configuration of power, the transformer must have inverted terminal, as it is in my schema. At low gain, it does not oscillate continuously, but appear strange noises already reported by Smokebreak.

Good luck!

Matec.

[Littlewyan]

Ah yer I've had that before, however it was on a Trainwreck Express that I built and it oscillated when the volume was on 0.1!!! Also I can't build a JTM145, my chassis isn't big enough, so just a JTM1 for now.

[Smokebreak]

Here is the schematic I have come up with for the JTM145, sorry its not very clear I made it in LTSpice. With the two volume controls I can only assume that is how they have done it. I've simulated this in LTSpice and the controls work as they should so it is possible that they've done it this way.

Smokebreak - Your right the components I've removed are very easy to put back into the circuit so I should be able to easily install them if I run into any issues. It'll be good to see if I don't need them though. Don't forget the board which Marshall use for this amp is very small, especially for the amount of parts they use. I was actually quite shocked when I saw the size of it! I suspect if you removed C15, C17 and C19 from your amp you wouldn't notice any difference. C301 could be a keeper though as this will/should dump all RF Interference from the input to ground, which in a way enables them to keep the grid stopper R28 small. As for the Volume Pot I'm going to stick in a 500K and keep one of the 1Meg resistors. Really can't see the point of C18 though as it makes near enough no difference taking it out. Attached the schematic of the amp I'm going to build.

Liked the audio clip of your's by the way, was that an Airbourne song you were playing?

Edit: Actually I see what you mean about the bright cap. I might just put back the 1Meg Resistor that I took out as I can easily mount it on the volume pot like you said. Now with your amp would you miss the Bass and Middle Controls if you removed them or do you use them? Also did you use a 470pF Cap for your Tone Stack or 250pF?

I paralleled to get to 480p in the stack. If you put back the 1M(s), you may consider changing the pot back to 1M, or change the value of the coupler, if you want to maintain the response of the original design.

Well, as far as missing the B and T controls, I usually have them straight up, but it's a pretty thick sounding amp, so I do find I like to roll down the bass a hair a times, but if I didn't have the panel space, I think I could live with fixed values.

Thanks for the compliment on the clip. I can't believe I've never heard of Airbourne, but I have now! Thanks for turning me on to them. They rock! I wonder if they grew up in Australia listening to ACDC I was just making up riffs and stuff.

Your schemes look good, but I'm wondering about the paralleled couplers coming off V1a. Say you had one put turned all the way down, wouldn't you lose all signal to ground, as that would be the path of least resistance?
I think the decription of the amp states that either volume can be used(I'm assuming to full degree, even when the other pot not in use is turned down), or you can blend them. If that's true, my theory goes out the window.
Are you ditching the 2K7/.68 for your 145 ideas?

Anybody out there own one of these?

[Littlewyan]

Well when the pot is turned all the way down there is 1Meg Resistance for the coupling cap and then 470K Resistance for the grid of the next stage so it works fine. I took away the 2K7/.68 as the JTM45 never had this originally and I think it was only used on the JTM1 as that only had 1 volume control, but who knows! They all came out last year so why aren't there any official schematics yet!!!!!!

I'll change the Pot back to 1M as well by the way, which is good as I already have 1M Pots whereas I would of had to buy the 500K Pot.

Edit: By the way I tried adding a 47uf cap to the 470ohm cathode resistor for the 12AU7 in my simulation and it didn't make any difference to the waveform at all. Odd as it should have given the amp a bit more gain surely?

[Smokebreak]

Finally got this thing in its home.
I realized when I was poking around before I mounted it that never did put in the 470p bright cap! I put it in and good lord it's very very bright at lower gain settings, at that was with humbuckers. It's too bright, I would say, but then again I never put them in Marshall builds. It added some harshness as well. If I build another i'd put a 150p and 300p on an on/off/on switch. I kept the 1M on the cathodyne grid as well.

I've been diggin the wooden faceplates lately. I found a lazer etching place right around the corner who did this one for $25. I just used the Minwax Polyshades to finish, which people seem to hate, but it works well for this application.

Are we gonna do the JCM1 next?

Lonestar for size reference only.

[Littlewyan]

That looks pretty dam smart. Wish I could get hold of or build a Cab like that.

http://www.ampmaker.com/infocentre/thread-32.html

This is the cabinet that I'm getting, except the guy is going to make the chassis bottom mounted for me. I'm also going to use a wooden faceplate, although mine will probably be a bit lighter than your's. I was going to get letters wood burned onto it, but I'm going to look for a laser engraving place now that I've seen your's .

470pF is what I prefered in my Marshall 1987xl, HOWEVER, I always ran that thing with the volume on 8 so the bright cap didn't sound as harsh. On my Trainwreck Express that I built I never use the 470pF Cap I always switch on the 100pF Cap.

Read through a Q and A regarding the JTM145 the other day and they do specifically say there are two gain stages with the two volume controls. I wonder if they reused one of the gain stages and perhaps bumped up the gain another way? Must have changed the circuit quite a bit if they did.

Edit: By the way, your amp definitely isn't missing the Bright Cap. The start of the clip where you've got the gain on 3 it rocks. Might try leaving it off mine at first.

[Matec]

Smokebreak
My God!
Did you?
It's perfect!
I am ashamed of my work here. The cabinet that you did for the amplifier is very professional.
Congratulations.

The bright capacitors, they are not part of what I may call the components that I had absolutely sure of its values. Simple, in the photos, I see them, but I can not read their values. This value I got for testing, but I could be very wrong.

If you find a better value I put this option in my schemes.
I have no problems with that. Rather, it is a good way to improve the design.

Thank you.

Matec

[Smokebreak]

Edit: By the way, your amp definitely isn't missing the Bright Cap. The start of the clip where you've got the gain on 3 it rocks. Might try leaving it off mine at first.

Exactly, that's why I wanted to give you the heads up. My amp is bright enough as is!

I got in touch with some pyrographic artists at first and prices were about as much as it cost to build the rest of the amp. A few amps ago I used a trophy shop in town to engrave, but they were a bit pricey, too.

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

[Smokebreak]

Smokebreak
My God!
Did you?
It's perfect!
I am ashamed of my work here. The cabinet that you did for the amplifier is very professional.
Congratulations.

The bright capacitors, they are not part of what I may call the components that I had absolutely sure of its values. Simple, in the photos, I see them, but I can not read their values. This value I got for testing, but I could be very wrong.

If you find a better value I put this option in my schemes.
I have no problems with that. Rather, it is a good way to improve the design.

Thank you.

Matec

Oh no, I had a buddy build the cab. My woodworking skills are not that great. I'm lucky I didn't ruin the faceplate !

For the bright cap, I didn't try any other values but I would think, depending on the player, and the amp, that 100p-300p would be plenty.
The bright cap did add some clarity to the neck humbuckers that was nice.. Maybe I will drill another hole

[Littlewyan]

Hmm I'll have a look around. There us a pyrography artist near me, haven't spoke to them yet but thought as I only want 5/6 letters on my faceplate they wont charge much. But i'll see if I can get it laser engraved as that looks smart.

As for the bright cap the standard JTM45 value is 100pF so this might be better suited to this amp. Later 60s amps had 470pF and 70s amps had 4500pf!!

Also what wattage resistors did you use for the low power mode? Looking at pics of the PCB they'd be no bigger than 1watt.

[Smokebreak]

I used 2W resistors, but honestly, I'm not exactly sure how to calculate the necessary power handling for each resistor, in this part of the circuit. I figured all the resistors(and the speaker) were dissipating around 1W of output power, so I went with 2W. I never did measure the voltage drops accross them.
Hopefully someone can chime in with some expected values based on the scheme.

[Littlewyan]

yer that's the rule of thumb that I've heard of, although it isn't scientific it seems to work . However I THINK I've found out how to work out the wattage rating required. I haven't built the amp yet so cannot take any voltage or current measurements from the output, so I just kept doing the wattage calculation (V X I = P) until I got 2W.

So 4V / 8R = 0.5A, 10 x 1.25A = 2W

Our Voltage divider on the 16ohm tap is 82R and 18R. So 4 / 82 = 0.048A, 4 x 0.048A = 0.19W. So both resistors need to dissipate 0.095W. Can someone please confirm that I'm doing this correctly? It'd make sense if the wattage rating was that low as it looks like Marshall only used 1/2W / 1W Resistors.

[arjepsen]

Hey.
I've been building this little thing too, according to matecs schematic.
Fired it up first time last night.
It was way too bright for my taste, so I've been trying to fiddle with that.
First I removed the 470p bright cap.
It helped a lot. Now it's nice enough at low volume, but it actually still gets a bit too bright and harsh when I turn up the gain.
Next, I tried removing the .1 µF cap over the second 33µF cap.
It took some more of the brightness away, but now the bass is getting a bit too loose, so I want to put it back in.
I changed the 470p in the tonestack to 220p.
Any other suggestions on how to tame the brightness?

Regards
Anders

[Smokebreak]

Hmmm I would think the small caps over the larger ones would actually help with HF.
You could try 100p-500p across the first plate load resistor, or change the values in the conjunctive filter. You've got 1M strapped across the gain pot, as well as another from wiper to ground??

[arjepsen]

As far as I've understood, the .1's help tighten up the bass, but also adds to the treble.
I once built an october studio from over at ax84.
For the first cap I used a big film cap, and that amp was quite bright.
On the forum it was discussed, that using films in the psu would actually brighten the amp.

I'll try the 100-500 you mentioned - is it instead of the 47p going from plate to cathode?
You mention trying different values of the conjunctive filter. My knowledge about it is limited - do you have any suggested values I could try?

Regards.