OT Wiring.....

[steven21042]

I noticed on this JTM48 that the layout :

http://ampgarage.com/forum/files/mojoto ... ng_186.pdf

calls for the Brown wire from the OT be connected to the # 3 pin of the V4 Power tube.

AS I'm following the build here:


http://acapella.harmony-central.com/sho ... ons./page3



They connect the brown lead to Pin #3 Of the V5 Power tube.

Is there a difference?

[plexitone]

Yes, it makes a difference. If you're using the Mojo supplied OT (part #7021300) there's a good chance that the colors of the primary (blue and brown) are backwards. Heyboer makes those and has been known to mess up the colors.

I helped someone with a Mojo JTM 45 kit recently and his needed to be opposite of the Mojo layout (blue V4, brown V5) so I'd do that.

If you have them backwards at the end of the build you will hear an obnoxious, loud hum or squeal when you turn the amp fully on (not in standby). If this happens, unsolder one end of the negative feedback resistor (27k that connects to the impedance switch) and try the amp again. If the squeal is gone you need to reverse those OT wires.

[Structo]

Yes so leave those wires long until you are sure because if you trim them to fit now and they are backwards one wire will be too short to reach the socket.

[pdf64]

If that did happen, the wires to the control grids could be swapped over (between V4 and V5).

[tubeswell]

Brown and blue are pretty much the 'universal' colours for each side of the primary winding in PP OTs. For 6V6/6L6/KT66 etc, Pin 3 is the plate pin. So (either) the brown or the blue wire goes to Pin 3 of one socket, and vice versa for the other socket. Some factories have the blue and brown leads attached to the 'other' sides of the OT primary. Its all much of a muchness until you get the feedback loop squealing, and that's the only way to know for sure which colour wire needs to go to Pin 3 on which socket. (For amps without a NFB loop, it doesn't make any difference whether you attach brown or blue to whichever side)

[martin manning]

Nothing wrong with confirming which half of the primary is which and wiring it correctly the first time...

[The New Steve H]

I looked at Steven's layout, and it seemed like the output tubes were wired exactly the same. I knew about the squealing thing, but I never knew why it happened.

[tubeswell]

Nothing wrong with confirming which half of the primary is which and wiring it correctly the first time...

And 50% of the time, the way that one ends up doing that is to hook it up and see if it squeals or not.

[Structo]

If that did happen, the wires to the control grids could be swapped over (between V4 and V5).

Or you can swap the wires to the phase inverter as well to reverse the phase.