First Build Transformer Problems

[brokennarsil]

Oh yeah that OT looks like a heck of a good deal! Way cheaper than the ones I've found similar thusfar. Thanks a lot!

[JHow]

It looks like our posts crossed in cyber space. Regarding the mojo tone xformer, it looks like it would work, but I would ask them about the power rating, since you want to run two 6L6s.

[brokennarsil]

So I'm finally on my way! I've ordered 6 new 8-pin sockets, as the old ones had been pop-riveted into the chassis and had a nasty lug that I couldn't drill out where the bolts need to go. Not to mention the gob of solder on all the pins...

My hangup now is that one 6sl7 I have in place of the third 6sc7. The difference is that the 6sl7 has one cathode per grid, whereas the 6sc7 has one for both. What I did in my planning of the layout was to bridge those two together to create one for both. Would that work?

Also, with the 6sc7 pin 1 is the shell, which is ground I think. The 6sl7 has no such ground, but both show pin 8 as a ground as well. I thought 8 was a heater filament. What's the deal? It shows this pin 8 thing on those schematics I posted earlier.

[JHow]

What I did in my planning of the layout was to bridge those two together to create one for both. Would that work?

If by this you mean you connected the cathodes externally, I think that is fine.

Also, with the 6sc7 pin 1 is the shell, which is ground I think. The 6sl7 has no such ground, but both show pin 8 as a ground as well. I thought 8 was a heater filament. What's the deal? It shows this pin 8 thing on those schematics I posted earlier.

I alluded to this in the comments about the ground scheme. Pins 7 and 8 are heaters on 6SC7. The orginal layout shows one wire connecting one side of the heaters (pin 7 in this case) and then taking the other half of the heaters to the chassis. I was suggesting you use a pair of wires to connect the heaters (which are in parallel) - completing the circuit not through the chassis, but back to the transformer. Twist the wires together in the runs between the tubes, keep the runs away from other wires, particularly grids, closer to chassis is good. Use 18 ga. solid wire. You want to plan your runs efficiently, not taking up a lot of space or getting too near other parts of the circuit, going from tube to tube and including your pilot light in that as well.

I was looking for a good picture. Have a look at the "heater wires diagram" here:

Common hookup info

This is illustrating 12ax7s, but the idea is the same. This diagram is a hook-up diagram, however - you will be twisting the wires between the tubes. The way to do that, by the way, is to twist first from the middle, then solder the ends. If you solder, then twist, you can tweak those new tube sockets lugs.

[brokennarsil]

Yeah as far as the cathodes, that's what I meant. I'll run a small strip of wire between the two pins for that.

As for the heater wire, I was going to twist them anyway, but didn't know that supplied the ground as well. Since that's the case, that will simplify things a lot for me.

[brokennarsil]

So my build is progressing. My new sockets just came in today, my OT yesterday. I've been wiring the heaters and the PT this evening, and have hit yet another snag.

My first is with the OT. I have the pinout of the old one, but with the new one, that's no good. I've got the OT40PP from Muscial Power Supplies, and on it is printed a list of each wire and where it goes.

Red=B+, Brn&Blue=Plate 4k2, Gry&Wht=Plate 3k2, Blk=0Ohm, Grn=4Ohm, Yel=8Ohm, Org=16Ohm.

I've figured that the Ohm wires go to my respective speaker resistance, but the others are confusing me. On the schematic of the amp I want to buid, it only shows four wires from what I assume to be the OT. One goes to the speaker as I mentioned already, but the other three go to the 6l6s and 5u4 respectively. I assume the listed 'plate' wires go to the 6l6s but which to where? And the b+ goes to the 5u4 maybe? And finally, would the black 0Ohm wire be ground?

And the other problem is really embarassing, since it's a very simple matter. Concerning the power switch and fuse, are there any polarity issues with these? I know the wall current is ac, so does it matter which way I wire these from the mains of my pt? Following my schematic, it only shows how the wires go, not to which of the two pins. I would like the 'north' direction of my toggle switch to be 'on' but I'm not sure how to tell if I'm wiring it properly.

Thanks agian, and sorry for all the hassle.

[JHow]

Let's do the simple one first. On your American three-wire power cord black is "hot" and white is "neutral", green is your chassis ground. The switch and fuse do not have polarity, but you should make your run from black to fuse to one side of DPDT switch to power transformer primary, back out to the other side of the switch and back to the white on the power cord. A picture is better:

Common hookup info

With respect to the output transformer, it has more wires than the orginal because it has two options for input impedance (4K2 or 3K3) on the primary side and then options for 4-8-16 ohms on the secondary side.

The hookup diagram is here:

P-P OUTPUT XFMRS - A Thermionic Webplace

I think you want to use the 3k3 (grey and white) for two 6L6 in PP configuration. Maybe the gurus can weigh in on that. It also depends on the speaker load. Anyway, whichever option you use, just shrinkwrap the others and tuck out of the way.

[brokennarsil]

Okay I think I've got the switch/fuse question cleared.

As far as the transformer goes, let me see if I have this correct. For my application, I will connect the red B+ to the rectifier pin 8, the grey and white 3k3 wires will go to each 6l6 plate(does it matter which wire/tube?), and the yellow 8 Ohm and black 0 Ohm will go to my speaker. The others I disregard?

[JHow]

Okay I think I've got the switch/fuse question cleared.

As far as the transformer goes, let me see if I have this correct. For my application, I will connect the red B+ to the rectifier pin 8, the grey and white 3k3 wires will go to each 6l6 plate(does it matter which wire/tube?), and the yellow 8 Ohm and black 0 Ohm will go to my speaker. The others I disregard?

Yep.

Red to B+ (rectifier pin 8 in orginal drawing)
Grey or white 3K3 to plates (pin 3), I think either one can be grey or white
Black to speaker jack ground lug
Yellow to speaker jack "tip" lug

Others just shrinkwrap the end and move out of way. Don't cut them short until you know the output impedance works like you expect. Also, you may want to run the other two speaker outs to two additional jacks. That way, if you get a 16 ohm cab or something, you could use that.

[JHow]

Oneother thing, when you get close to being done and you are ready to fire up, here is a handy SOP for lighting up your amp for the first time:

Paul Ruby Amplifiers

Also, if you haven't already, search "light bulb current limiter" on this forum and make one. Its just extra insurance in case something gets sideways.

[brokennarsil]

Excellent! Thanks a lot. I've been trying to be as careful as possible with the progression of the build, since I really don't want to go back and troubleshoot later. I've got plenty of time though, since the resistors/caps I ordered won't be arriving for a while

I'll try to post a pic or two when I get a little more done so maybe you could tell me at a glance if anything looks terribly wrong.