Anderton '64 reissue JTM145 - any info?

[jazbo8]

Thanks guys... looks like Tube Town got me covered.

FWIW, I tried a PT with a 250V secondary which is way above the spec'd 180V, and the sound was too harsh imo.

[Littlewyan]

When you tried the 250V Secondary did you adjust the bias of the 12AU7? In my simulation software I needed a 2.2K Resistor in place of the 470R to get the bias right.

[jazbo8]

When you tried the 250V Secondary did you adjust the bias of the 12AU7? In my simulation software I needed a 2.2K Resistor in place of the 470R to get the bias right.

Yes, but I left the preamp stages as is, perhaps that contributed to the harshness, in any case, the PT I have is too large and is a bit of a waste to use on this project. But do let us know how your PT works out with the proper adjustments.

[Littlewyan]

The issue I had was the PTs that were perfect for the job were too big! Altho I didn't see this one on Tube Town, dammit! O well very worst case I'll replace it. I'm also using a 22K OT opposed to an 18K OT. Its going to be interesting. I'm hoping the rest of my parts will finally arrive today so I can get started.

[jazbo8]

I found another good small OPT for the build from Radio Spares. A lot cheaper in the UK for some reason.

[Littlewyan]

Sweet! I didn't think Radiospares did Output Transformers anymore. Would have to do some speaker mismatching to use that one though but you could make it work. I'm in the middle of drilling holes in my chassis at the moment, only got the turret board mountings and earth/ground lug holes left to drill and then I can finally start assembling it!

[Littlewyan]

The amp is finished and sounds great! The circuit is slightly different to the original, i'll post a schematic later. The HT is running a bit high, 340V but I've lowered the bias so the 12AU7 is only running at 2.2W dissipation (2.75W Max). Surprisingly it worked first time, nice low level of hum, only change I had to make was swapping the anode leads around (nfb was pfb).

[Smokebreak]

Yea! Congrats buddy.

[Matec]

Yeah!

Congratulations to Littlewyan for his work.

He got a great look!

Damn! All of these amps, I saw, the one that got ugly is mine!

Very nice!

Samples?

-------------------------------------------------- -----------------------------

Yesterday I was doing tests, along with a friend, in my amp.
I own a Squier, and he has an Epiphone.

I always thought the amp with too much treble harmonics, using the singles, and I was sometimes in doubt about the choice of the components of the filters.
However, when doing tests with humbukers, the amp seemed another. It sounded like the videos of Marshall.

It appears that one has been done for the other.

I will not change anything.

Cheers

Matec

[Littlewyan]

Thanks guys. Still jealous of Smokebreak cabinet tho . The amp does have a lot of high end, in its current form the filters are from a 70s JMP. JTM45 would have a .022uf coupler on first stage, 250uF Cathode Cap and .1uF Couplers on the output. Odd that Marshall didnt do this.

[jazbo8]

The amp is finished and sounds great! The circuit is slightly different to the original, i'll post a schematic later. The HT is running a bit high, 340V but I've lowered the bias so the 12AU7 is only running at 2.2W dissipation (2.75W Max). Surprisingly it worked first time, nice low level of hum, only change I had to make was swapping the anode leads around (nfb was pfb).

Congrats! The amp looks great... I tried to draw a loadline using your figures, and it shows the plate dissipation is well over the 2.75W Pda for most of the operating cycle, would that be a concern?

[Littlewyan]

Well I checked the idle bias of the valve and it is running at 2.2W Idle which is lower than what Marshall biased it at. How are you working out the bias? The valve isn't red plating at all but does get bloody hot compared to the others! Which I heard was normal with these amps anyway.

[jazbo8]

Well I checked the idle bias of the valve and it is running at 2.2W Idle which is lower than what Marshall biased it at. How are you working out the bias? The valve isn't red plating at all but does get bloody hot compared to the others! Which I heard was normal with these amps anyway.

Here is what I came up with, hopefully it is not too far from the actual measured values...
[img:549:550]https://dl.dropboxusercontent.com/u/132 ... 5V_22k.png[/img]

[Littlewyan]

No that looks almost spot on, I get 14V on the Cathode of the 12AU7. Now I did all this but just couldn't work out how Marshall had done it. If you calculate the bias based on the parts they've used when their amp is operating in Class B its also in the danger zone. My Amp may also suffer from a bit of PT Sag whilst playing as the HT was SUPPOSED to be around 320V but its at 340V. Might check later to see if it drops at all.

[jazbo8]

If you calculate the bias based on the parts they've used when their amp is operating in Class B its also in the danger zone.

It's still Class AB, just a tad over max Pda at full power, here is the loadline using the numbers from Matec's schematic (should be close what Marshall used:

[img:553:550]https://dl.dropboxusercontent.com/u/132 ... 0V_20k.png[/img]