Output Transformer Question - unused tap? (and now PT question)

[BobL]

Ok, so I've been building a 5E8A from a mojo kit, and it's going pretty well.

I'm a little stumped on what to do with the output transformer, as it has both a 4 and an 8 ohm tap, but the amp will just be wired for one setting, not be adjustable.

Schematics:
OT:
http://www.mojotone.com/transformers/Fender/Output/MT-FENDER-MOJO766.pdf

Circuit:
http://mojotone.com/Amp%20Kit%20Schematics%20and%20Wiring%20Diagrams/Tweed%20Kits/MK-TTLP-SCH.pdf

Looking at the schematic of the circuit, and the schematic of the transformer, I believe that:

Blue will go to V5, and Brown will go to V6, and Red will go to the board. Is that right?

Black will go to the sleeve of the speaker output, and I'll wire green to the tip for an 8ohm speaker load... so my question is... what do I do with the Yellow 4 ohm tap?  Heat shrink it and let it sit under the board or something?

[TIMBO]

Hey BobL, To have some options, I would trim the 4ohm lead long enough to put in another speaker jack and heat shrink it and zip tie it to a place where it can't cause any problems.

[mat janssen]

I think there is a mistake in the circuit diagram of the amp.
You forgot coupling capacitors at the output of the input gain stages.
Regards,
M

[sluckey]

Blue will go to V5, and Brown will go to V6, and Red will go to the board. Is that right?

There's a 50/50 chance that the blue and brown wires may need to be swapped so don't cut them to length just yet. Instead, temporarily solder them full length to the sockets. Be prepared to swap them if you have any NFB issues.

[BobL]

Good deal - thanks for the replies.

[BobL]

I have a question about the power transformer... the info from mojo is hard for me to understand, and the layout, etc. doesn't account for all the wires coming out of the PT.

Here is the PT schematic:
http://www.mojotone.com/transformers/Fender/Power/MT-FENDER-MOJO752.pdf

So the green/yellow/black wires are all easy enough to figure out.

Am I right in thinking that the red/blue wire is the bias tap, and that the red/yellow and green/yellow wires just run to ground?

Can someone help me understand what each of these wires is actually doing?

Thanks!

[sluckey]

Am I right in thinking that the red/blue wire is the bias tap, and that the red/yellow and green/yellow wires just run to ground?

That's correct.

Can someone help me understand what each of these wires is actually doing?

The two black wires are the primary wires and connect to the 120V wall juice. Think of them as the input to the transformer.

The yellow wires step down the wall voltage to 5VAC and are used the heat the rectifier tube filament.

The green wires step down the wall voltage to 6.3VAC and are used to heat all the other tube filaments. The green/yellow wire is a center tap for the green filament wires. It is connected to ground to help reduce filament induced hum.

The red wires step up the wall voltage to 355VAC and are used to develop the B+ high voltage for the tubes. The red/yellow wire is a center tap that connects to ground. It is necessary for producing the B+ with a conventional full wave rectifier tube (or SS diodes). The red/blue wire is another tap on the red wire winding. This tap provides 50VAC (as measured with respect to the red/yellow center tap wire) that is used by the bias circuit.

[BobL]

Thanks, sluckey!  Extremely informative reply - things are starting to click.

So this build had me run two 100ohm resistors from the pilot light to ground, which I have read is to compensate for a transformer without a center tap and protect the transformer - is that right?

From what you've said, it sounds like my center tap runs to ground, so these  resistors are unnecessary - is that right?  Will leaving them there help/hurt anything?

[sluckey]

If you want to use the green/yellow centertap from the PT then don't use the resistors. If you want to use the resistors don't connect the green/yellow centertap wire.

The 100Ω resistors (we call them an artificial center tap) do offer a degree of protection to the PT in the event of a tube plate to filament short (usually pin 2 and pin 3). If you use 1/2 watt resistors they will burn out in the plate/filament short scenario and maybe save the PT, OT, or other expensive power supply components.. The heavy duty wire used in the green/yellow centertap doesn't offer any protection. I think it's probably more likely the unsteady hand of a technician checking plate voltage that causes the short between adjacent tube pins.

What would I do? I only use the 100Ω resistors in the event my PT filament winding does not have a real centertap.

[BobL]

In the 'plan ahead, idiot' category...

Got everything set up, and then pulled my (what I thought were) 16 ohm speakers out of the boxes... and they were 8...

At which point I had the 4 ohm tap completely buried under the board and zip tied to a standoff... I was able to use a mirror and a hook probe to eventually slip the zip tie off the top of the standoff without completely removing the board and desoldering everything, but man... that was a pain in the butt. 

Things to remember to confirm in the future... heh.

[PRR]

> Things to remember to confirm

IMHO.... you can't confirm everything. When possible, keep your options open.

I just wired a house power monitor. I completely littered it with terminal blocks. Every wire can be swapped-around. And I *almost* know what I am doing.

There's one point I can't safely pre-confirm. (Actually I can, if I shut-down the entire house; I prefer not to re-set EVERY clock and VCR in the house AGAIN.)

If I had wires for 4 and 8, I'd leave the "un-needed" wire where I could get to it. Neat is nice, but standing there with the "wrong" wire and no music is distressing.

[BobL]

So very true...