New Build: JTM45 VSoZo ABCC LarMar IEC LarrysGnd Turret Fuse

[ldanmartin]

Did you notice that George reverses the 5th .022 SoZo cap and usually the 1st .1 SoZo cap (the SoZo "line" is towards the front/pot side). Those two caps are next to each other.

I have read these "lines" need to go toward the tube sockets (though the manufacturer says it doesn't matter).

I see that in the Archives pictures:
http://plexireplicas.com/amplifier-archives/" onclick="window.open(this.href);return false;

My next step is to start populating the PTP board. I am just reviewing the instructions and various amp pictures... and noticed this about George's Plexi Replicas builds.

I wonder why? He's pretty consistant in doing that.

[demonufo]

It doesn't really matter which way round they go, BUT they can be slightly quieter (reject more noise) when the ground end (the end with the line, which is connected to the outer foil of the cap) is connected at the lower impedance connection of the two (which is usually the closest of the two points to ground).
This ensures that the outer foil of the cap is the closer to ground, and therefore better at shielding the cap from parasitics (amongst other reasons).

However, from this reasoning, you've got the cap on the presence the wrong way round, but in all honesty, I doubt very much that you'd be able to notice the difference. After all, mustards weren't marked up so as you could tell which end was the outer foil, and something tells me that the builders aren't very likely to test all the individual caps with an oscilloscope prior to fitting, just to find the right polarity.

Just follow what George does, and you'll be okay. It doesn't really matter, but there is a POTENTIAL difference there. But like I said, whether or not you'll notice the difference...

[ldanmartin]

Thanks for the update demonufo.

The Presence cap is already on the way George does his. I had wondered about the direction of the Presence cap before putting it on, so I put it on in the direction George did it per the Plexi Replica Archives.

Here's some shots of George's builds with the two SoZo caps reversed and the direction of the Presence cap...

http://plexireplicas.com/amplifier-arch ... py.jpg.php" onclick="window.open(this.href);return false;
http://plexireplicas.com/amplifier-arch ... 00.jpg.php" onclick="window.open(this.href);return false;
http://plexireplicas.com/amplifier-arch ... 50.jpg.php" onclick="window.open(this.href);return false;
http://plexireplicas.com/amplifier-arch ... 51.jpg.php" onclick="window.open(this.href);return false;
http://plexireplicas.com/amplifier-arch ... 18.jpg.php" onclick="window.open(this.href);return false;
http://plexireplicas.com/amplifier-arch ... 63.jpg.php" onclick="window.open(this.href);return false;
http://plexireplicas.com/amplifier-arch ... de.jpg.php" onclick="window.open(this.href);return false;
http://plexireplicas.com/amplifier-arch ... 19.jpg.php" onclick="window.open(this.href);return false;

I'll leave the Presence cap the way I have it, and place the other two caps reversed like George does his, as you suggested.

[ldanmartin]

http://img820.imageshack.us/f/dsc0078jk.jpg/" onclick="window.open(this.href);return false;

DSC_0078.jpg (77.74 KiB) Viewed 1857 times

[Flames1950]

Damn nice job, did you fire it up yet?

[ldanmartin]

Thanks Flames1950!

Testing voltages now. Problem at Testing Part 1: Powering Up - Step D: -06.9V DC on V4 and V5 pin 5 w/bios full clockwise and GZ34 installed. Lar/Mar PPIMV on 10.

V1-V3 pins 4,5,9 at 3.0V AC.
V4-V5 pins 2, 7 at 3.0V AC.
4.4V AC pins 2 & 8 on rect tube socket.
293V DC Standby.

Checking things now to see why there is -06.9V DC on V4 and V5 pin 5... but can't find anything.

Rewired/Resoldered Lar/Mar PPIMV and resoldered the turrets where it connects - no change.

Any ideas?

[Flames1950]

I do see the two blue caps in your bias section are backwards, didn't see that on the small version of the picture.......

[ldanmartin]

Thanks again Flames1950.... that was the ticket! Measuring -37.9V DC on both V4 & V5 pin 5.

On to the next step!

Good catch! I checked all the resisters against the chart, but missed the direction of the two caps...

http://img535.imageshack.us/i/0924102117.jpg/" onclick="window.open(this.href);return false;

0924102117.jpg (91.05 KiB) Viewed 1842 times

[ldanmartin]

Step E: V3 pin 1 = 225 (OK), V3 pin 3 = 44 (Low), V3 pin 6 = 217 (OK), V3 pin 8 = 44 (Low)

V3 pin 3 = 44 (Low) and V3 pin 8 = 44 (Low) are supposed to be 10% higher than a value of 48 (they are connected and should have the same value, it's just 10% lower, instead of 10% higher).

Shall I proceed, or does this too low of a value need to be addressed?

http://img697.imageshack.us/i/dsc0081j.jpg/" onclick="window.open(this.href);return false;

DSC_0081.JPG (97.93 KiB) Viewed 1840 times

[ldanmartin]

I just swapped the preamp tubes V2 & V3. Same low reading on V3 (it did not follow the tube).
V2 checks out ok with the original V3 tube I was using.

I am tempted to continue. Reading the forums and can't find anything specific to indicate if this is ok or bad...

[Flames1950]

Don't get excited about the V3 readings. Hell, I never even take readings there.

[ldanmartin]

It sounds good!

It does have a high pitch squeal when Presence is above 6...

Standby voltage was 384... so bias'd at 44.0 and 44.6...

[Flames1950]

The squeal is common if the chassis is out of the head case -- the shielding on the bottom of the case helps prevent it.

[ldanmartin]

Yes, the amp head was out of the case and sitting directly on the speaker cabinet.

When I moved the head off the cabinet the squeal went away (still out of the case).

Now to find a perfect sounding Marshall 4X12 cabinet for it. (My AVT412 doesn't sound the best).

I have been reading the forum and other sites for weeks and can't come to a conclusion.

I want the cabinet to work on a 12xxx series head too... so I know to get the higher watt speakers...

That seems to only leave the 1960V cabinet option, as most say the 1960A/B doesn't sound good with the JTM45.

Otherwise it's a custom cab like Scumback or Avatar 4X12, or upgrade my existing AVT412 cab speakers.

I like the sound of the Scumnico's and H75 & H55... anyone know if either of those sound good with a 12xxx amp?

[ledzep443]

I've been looking at the Marshall Handwired 4x12 (I'm using an AVT412 for all my amps atm) since it has greenbacks in it. The 1960AV has Vintage 30's in it, which sound different from greenbacks and may or may not be to your liking. If you can afford it, George makes 4x12's now and you can get them with either celestion greenbacks or scumbacks. (They're on the plexireplicas site in case you haven't seen them yet)