JM Signature started

[telentubes]

Slow going on my end as well, but I'm getting closer.
Bummer when you forget to order a part or two. I do that more often than I would like.

[telentubes]

I got the JM fired up late last night. Had to reverse the OT primaries and raise the tail resistor on the bias from 27K to 33K for a more usable range, but other than that, things look good. A busy day today so won't get to spend any time playing but initial voltages are very close with the tired start up tubes. I've got a 4AB3C1B plugged in to the reverb and 2 others to try. I'll add finished pics when I can.

[Matt J]

I got the JM fired up late last night. Had to reverse the OT primaries and raise the tail resistor on the bias from 27K to 33K for a more usable range, but other than that, things look good. A busy day today so won't get to spend any time playing but initial voltages are very close with the tired start up tubes. I've got a 4AB3C1B plugged in to the reverb and 2 others to try. I'll add finished pics when I can.

How were you able to tell the OT primaries were backwards? I have been worried about that with the build I've been working on.

[norburybrook]

I got the JM fired up late last night. Had to reverse the OT primaries and raise the tail resistor on the bias from 27K to 33K for a more usable range, but other than that, things look good. A busy day today so won't get to spend any time playing but initial voltages are very close with the tired start up tubes. I've got a 4AB3C1B plugged in to the reverb and 2 others to try. I'll add finished pics when I can.

How were you able to tell the OT primaries were backwards? I have been worried about that with the build I've been working on.

Ha...you'll know if they're wired wrong.....there'll be no doubt .......

I always put headphones on when switching on to protect my ears from the howl....


MC

[telentubes]

My understanding is that it is the luck of the draw when you first hitch up the primaries. 50/50 chance of getting it right.
On this first power up, and flipping "standby" to "on", the amp instantly began to howl loudly. I hoped the problem was with the OT primaries and not something else, as switching the primaries is an easy fix.
I powered down, drained the caps, switched the wires, powered up again, and it was dead silent and ready to try with a guitar. I usually lightly solder the primaries during the build in case they are backwards, then do a permanent job once I know they are correct.

One of the pros on this forum (I'm definitely not) may know of a way to establish which way to wire the primaries before hand, but for me, it's always been a crap shoot.

I did get to play through this amp quietly this morning, and first impressions are very good. Luscious tone and great feel under the fingers. I plugged it in to a Celestion G12-65. I think this will be a good one.

[norburybrook]

great

I had a 100% record of having them the wrong way round until my last Dumble build where the odds went in my favour...hence my headphones when powering up


I'm going to start on mine soon so am looking forward to hearing your thoughts on yours.

Marcus

[amplifiednation]

I got the JM fired up late last night. Had to reverse the OT primaries and raise the tail resistor on the bias from 27K to 33K for a more usable range, but other than that, things look good. A busy day today so won't get to spend any time playing but initial voltages are very close with the tired start up tubes. I've got a 4AB3C1B plugged in to the reverb and 2 others to try. I'll add finished pics when I can.

How were you able to tell the OT primaries were backwards? I have been worried about that with the build I've been working on.

Don't worry, you will be able to tell.

[Matt J]

Thankfully, I didn't have any of those issues you've described when I first fired up my Small Special, but I at least know what to look for now!!

[ToneMerc]

My understanding is that it is the luck of the draw when you first hitch up the primaries. 50/50 chance of getting it right.

TT, it doesn't have to be a 50/50 shot at it. Here's a tip, have you ever looked at the winding diagram of an OT and seen a dot at the beginning of one of the windings? That dot indicates which primary lead is the start winding, the one which should be wired to the inverted side of the LTP/GNFB config input. Many modern Fender style transformers will have the start primary winding wired to the blue lead, some of the early Fender transformers did have the brown lead as the start winding.

When I looked at Taylor's JM diagram I noticed that brown lead is wired to the inverted side, so if one uses a blue start winding OT, the leads would have to be switched.

TM

[norburybrook]

Killjoy .......................


actually that's something new I've learned. Unfortunately InMadOut transformers whom I've used on all my last builds don't put a dot on their diagrams, they are very comprehensive as well.......strange.....I suppose I could ask them which is which when I next order.


I see Hammond do however.


M

[martin manning]

The key is the phasing of the primary and secondary, so you need to see where the dot is on the secondary winding too. Assuming the PI is a LTP with GNFB applied in the usual way, the primary lead that goes to the inverting side of the PI is the one that is in-phase with the hot side of the secondary, regardless of where the dot is. In other words, if the the grounded side of the secondary has a dot, then the primary lead that doesn't have a dot is the one that goes to the inverting side of the PI. If the diagram doesn't have phasing dots, then it's easy to find out what the phasing is: Connect the primary's CT the the hot side of the secondary, put some AC voltage on the secondary, and then measure voltage from the secondary ground to each plate lead. The lead with the higher voltage is the in-phase side. It's worth checking even if you have a diagram with phase dots.

[galtjunk]

I must have all of the luck.
Of the dozen or so PP amps that I have completed, all of them have been correct on the OT wiring.

Of course that will change now that I have made this statement.

[telentubes]

There you go. I never knew about the dot.

[telentubes]

Here is the finished amp, pre combo cab. Got to spend more time with it and don't feel like I need to change a thing. Really great well balanced circuit. The OT I got from Aaron plays a big roll as well. Wish I'd bought more than one.

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[Bombacaototal]

Looking great! This 50 Watt Schumacher OT must sound great