Classictone PT for Bassman Clone - Wiring Questions

[The New Steve H]

I'm doing the wiring for my Bassman (5f6a) clone. I have a Classictone power transformer, model 40-18029. I have schematics and layouts from Fender and Weber.

Here's the deal. The Fender schematic calls for 325 volts into the rectifier tube. The specs for the transformer appear to say I have to choose between half of 701 volts (355) or half of 610 (305). Maybe I'm wrong; I'll post the transformer specs.

1. I don't know which wires to use. Seems like 355 volts is an unexpectedly large deviation from Fender's 325. On the other hand, 305 is not that close, either. Suggestions appreciated.

2. I'm confused about connecting the power cord. I assumed there would be two input wires, one at each end of the primary, plus a ground. I see that orange is the ground, but there are five wires going into this thing: red/black, black, brown/white, black/white, and brown.

At the upper and lower ends of the primary, I have red/black and brown/white. That part, I think I understand. I think these go to the power cord wires. But I can't see how to connect the upper and lower halves of the primary. It looks like the upper part terminates at the black/white wire, and the lower part terminates at the brown wire, making it necessary to connect these wires in order to connect the halves of the primary.

On the other hand, maybe I just connect the red/black and brown/white to the power cord and cap the others?

Help!

[The New Steve H]

I checked around and found out I can use the 710v connections for the rectifier, but I am still digging on the primary connections.

[The New Steve H]

Now I'm thinking the halves of the primary have to be paralleled, but it still looks weird.

[Phil_S]

If you are using a GZ34, rectified B+ will probably be less than 15V higher. I can't see that as a problem. Use the 710v secondary.

The orange wire isn't part of any winding. It is a shield that should be grounded. I'd give that wire it's own ground bolt.

If you are in the US, to connect the primary, do the following:
1) Connect the Black to the Brown PT wires and then take that junction to the switch and then to the fuse. Connect the hot side (black wire) of the wall supply to the other side of the fuse. I like to interrupt the connection from the PT to the fuse with the switch so the fuse is on the line side of the switch. This puts the fuse where it should be, between the line supply and the amp as soon as you plug it in. It will blow when you plug it in if there is any sort of short. Review of this one from the wall: Wall plug > black wire > fuse > switch > PT primary Black + Brown.
2) Connect the Black/White to the Brown/White. Connect that to the neutral (white wire) from the wall supply.
3) Connect the Green wire from the wall supply to a dedicated chassis ground bolt, using a ring lug (not a spade lug) to secure the connection. Make this wire so it has a bit of extra length (not too much) so when the wall supply gets pulled out in an accident, this ground wire is the last to go. Dedicated means you only put that one wire on the bolt and nothing else.

What you have here is a PT with two primary windings. They are connected in parallel for USA 120V and in series for Euro 240V.

If you are able to do it, I highly recommend using an IEC inlet. Old computer cords are free and take care of the run from the wall to the IEC. This greatly simplifies the problem of using proper strain relief on the cord -- you don't need any. When (not if) someone steps on the power supply cord, it will probably get yanked out of the IEC connector, saving your amp from a real accident. Those computer cords are short. You probably want to keep a good 14 AWG extension cord around. Extension cords with molded plugs and heavy wire gauge can be bought at Target for very little money.

Do a search on how to cut the rectangle in the chassis for the IEC inlet. Lot's of opinions are out there including mine.

Good luck.

[The New Steve H]

Okay. I'm joining the red/black wire to the brown wire, and I'm joining the black/white and brown/white wires. I am capping the black wire. I think this will give me two 120v primary halves in parallel, and using the red/black instead of the black will enable the thermal cutout. I am capping the black in order to avoid shorting the thermal cutout out of the circuit.

If I explode, I will come back and post photos.

[The New Steve H]

Thanks for the help. I wrote my last post while you were posting. Looks like I have it right.

Am I correct to cap the black wire and use the red/black instead? It looks like this will enable the thermal cutout for the rectifier but not the heaters and so on. Looks like it's impossible to use the cutout to protect them.

I'm using an IEC outlet, but I don't like them because you have to cut a huge non-round hole in the chassis.

[Phil_S]

I think, when you cap the black, you are bypassing the thermal cut out (a fuse like thing). Attach the black and the brown. Then use the red/black to attach to the external fuse. That's what I'd do.

[ToneMerc]

I'm using an IEC outlet, but I don't like them because you have to cut a huge non-round hole in the chassis.

Now that's funny considering the way that you love to throw large amounts of aluminum chips!


FWIW, I think you are in the homestretch now.

TM

[The New Steve H]

You're right about me enjoying the machining, but machining holes that are neither round nor square is a pain. The inlet or receptacle or whatever I use has a pentagonal shape. Maybe I need to look for a better one. The neat thing about using strain relief is that you can go to Home Depot and buy wire to make a 20 foot cord if it makes you happy.

Thanks for the vote of confidence. The soldering is turning out to be much more aggravation than I thought. I didn't realize that if I used short wires, I would have to solder things to the sockets while the sockets were screwed in. I can't take them out and then put them back after the wires are on. That makes for some serious microsurgery and gymnastics.

[rdjones]

Just follow the wiring diagram in the PDF.

For US 120VAC connect the BLK/WHT and BRN/WHT together.
These are the "low side" and connect directly to the neutral side of the AC, white color coded lead on a cord, the "N" terminal on the IEC.

The BLK and BRN connect to each other and nothing else, cap this connection off.

The RED/BLK wire is the "hot" and connects to the switch, the switch connects to the OUTSIDE (ring) terminal of the fuse holder, the TIP of the fuse holder goes to the "high side" hot of the AC, which is the black color coded lead of a (properly color coded) AC cord or the "L" terminal of the IEC.

Please follow proper safety guidelines and don't bypass the transformer's cutout, or wire the hot side in any other manner than described above.
Safety codes, guidlines and regulations are there for our protection and that of everyone around us.

SAFETY FIRST !

rd

[rdjones]

I'll also add that following an old (older than about 1980) layout will have you breaking a number of safety codes.
When in doubt follow the modern AC wiring conventions.

rd

[The New Steve H]

Thanks for the very specific instructions.

It just looked crazy, paralleling the primaries or halves or whatever they're called. My brain could not accept the idea. And I still don't understand why it looks like there's a red/black wire with thermal protection and a black wire without it.

I was really hoping to hear this thing today. Now that I'm soldering with tweezers and a microscope, I'm starting to wonder when it's going to happen.

[ToneMerc]

The soldering is turning out to be much more aggravation than I thought. I didn't realize that if I used short wires, I would have to solder things to the sockets while the sockets were screwed in. I can't take them out and then put them back after the wires are on. That makes for some serious microsurgery and gymnastics.

I hate to say that I told you so.............remember this.

However; if you are using the Belden 9 pin, the distance from the mounting flange to the very tip of the socket pin is only around .395. Thus, it's going to be real interesting when you go wire heaters, attach grid rsistors or snubbers.

You are a glutton for punishment

The first amp I built I called myself saving money so I bent, folded the chassis and drilled all the holes. I came away from that experience saying never again. The extra upfront cost of having piece mind to be able to comfortably work with a prepped chassis is usually less than any money and time you will waste in frustration going the cheapest route.

TM

[The New Steve H]

Oh, okay. I'm stupid. The cutoff precedes the junction of the black and brown wires and protects both coils.

[The New Steve H]

Are you saying the sockets themselves are the problem? I thought you were saying the chassis would interfere with them. That's not happening.

I picked the sockets because they were cute. I thought this was a sound basis for the decision. Also, the Fender jobs looked like something even the Chinese would be embarrassed to sell.