B&H 179 schematic question

[angelodp]

Hi, I am trying to understand what the additional tap on the AC input is doing? The AC plug is only 2 blades and the third center hole has no connection, yet the AC socket in the amp has that center pin in the circuit as shown on the schematic. What is the reason for that configuration.

I want to implement a grounded plug for this amp. I do not want to disturb the existing socket. My though is to cut off the rubber socket with a foot of cable, then splice to a three prong plug and connect the ground with a ring terminal to screw in the back.

I have a quick mocked up idea, it would be properly soldered and shrink wrapped etc.

[Phil_S]

Look closely at the schematic. The two outer terminals are for 117V. The middle with the outer terminal designated as common (the one with the fuse) is for 91.4V. It seems weird, 91.4V, but that schematic is dated 1945. Maybe it was an export contingency? In any case, the connector you show only has the two outer terminals, so that's the 117V and there is no connection in the middle. You can safely ignore the middle terminal. If I were working on this one, I'd disconnect the middle lug inside the chassis and cap it with heat shrink. Remember to move the fuse to the hot side of the mains connection, or just add a second fuse. There is no harm fusing the common, but there absolutely needs to be a fuse on the hot side. While you are at it, you might think about cutting out the connector on the chassis and replace it with a modern IEC inlet. If there's enough room, that's what I'd do. The white Sprague cap above the existing connector is the death cap and you'll be clipping that anyway. Once the death cap is gone, you should have enough room for the IEC.

I wouldn't splice onto the existing. It's old and should be considered unreliable. Besides, it won't work, see above, the middle has no terminal.

[angelodp]

Thanks for the reply. This unit was in storage and practically unused. The cord is in pretty good shape. If I did splice to the old cable and switch neutral and hot from the new cable, and run an external ground wire to the chassis it should work. And cap that middle terminal.

No?

Ange

[Phil_S]

How are you going to make the mains ground and do it in a "workmanlike" and safe manner? IMO, there's no fooling around with this. Make the mains ground properly. Safe trumps preservation every time.

[sluckey]

Perhaps this amp was made for USA and Japan. The amp is wired for either. But you need the proper power cord. Just a thought...

[angelodp]

Excellent points. I plan on attaching the ground wire with a star washer and nylon nut. This amp will sit on a producers bench, once plugged in it will never move and get unplugged.

BTW this amp sounds amazing

[angelodp]

I do not understand this part of the schematic, it seems to indicate that the common and hot are connected, as well as that center pin?

If I understand correctly the death cap comes out and that center connection gets shrink wrapped.

[sluckey]

I think that's just a poor way to indicate the three wires terminate inside a connector and was not meant to indicate the wires are connected together. Does this look better?

[angelodp]

Yes, by miles.

Thanks

[angelodp]

Another schematic error... Yes? Now amended. 6V6 cathode bias R 250 ohms should show that dot.

[angelodp]

Please explain this. The left hand speaker connection is 16ohm, the right hand says 8ohm dual.... meaning?

Can I use the right hand connection for 8ohm speaker?

Ange

[angelodp]

Phil, can you see any harm in simply leaving that export center connection in place and the death cap and ignoring it. I am proceeding with creating my pigtail and external ground connection, with a solid ground screw and nylock nut and star washer.

A

[angelodp]

Another question, the schematic shows 16ohm tap for a single speaker, and it says Dual 8ohm. What does that mean? Is it that if I were to plug in two 16ohm speakers I would be reflecting 8ohms? Or is it that I also have an 8ohm tap.

A

[martin manning]

I believe that indicates that there are 8 and 16 ohm taps. The “dual” is likely two 16’s in parallel.

[Phil_S]

Ange,
Of course, you can do what you want. I hear you saying you don't want to do anything to disturb the vintageness (not a real word) of the amp. I disagree on this point. I'm doubtful anyone cares, even if they know about it. There is every good reason to modernize when it is a good improvement that doesn't booger up the amp.

BTW, I wouldn't make a ground with a nylock. Nylon is an insulator. Use two regular nuts, one to bolt down the ground wire and and the other to lock the first nut. Remember to use a metal star washer and a ring lug on the wire.

In the case of the mains plug, the connector on the chassis has 3 pins, but the wire looks to me like there contacts only for the outer two. If there really is nothing in the middle position on the wire, then it doesn't matter if you disconnect what's on the inside. As for the death cap, I suppose you can clip it and leave it there, but what for? No one is looking. I'd remove it. It's just an accident looking for a place to happen.

I don't think you showed the plug connector before. That looks to me like changing it for a modern IEC will be a very easy mod. Some careful work with a file should do the job nicely. That solves **all** the problems with the mains cord, as long as you ground the middle lug on the IEC properly. When you remove the death cap, there should be more than enough room for the IEC.

I think the speaker diagram says this. (Single) If you use one 16Ω, plug it into the connector designated for 16Ω. (Dual) If you use two 8Ω plug one into each connector. I see Martin posted his thought on this and it's different. The reason I came to this conclusion is that each half of the winding is marked .57, suggesting to me it splits the secondary into an equal number of turns. An 8Ω tap will have .707 x the number of turns on the 16Ω tap. A 4Ω tap will have half the number of turns on the 16Ω tap. So, I make it for one 16Ω, or two 8Ω. The two 8Ω will be parallel, showing the OT a 4Ω load. I'm not sure I got this one right.

I don't know if there is room inside for regular 1/4" jacks. I'm pretty sure you won't want to do it. If there is room, if this were mine, I'd look to replace the connectors with jacks. It looks like it could be a very good fit. Not something someone will notice.

Good luck,
Phil