3 Speaker Jacks

[C Moore]

I rebuilt a 1968 Bassman for a friend. It needed a new OT. I bought a 50 watt, multi-tap from Magnetic Components........4, 8, and 16 Ohms.
I am wondering the best way to use the three separate taps. The chassis will not accept a rotary Z Switch. So I thought to use the existing main and extension speaker jacks, and add a third jack.....so as to use all three OT windings.
For the life of me, I cannot get straight in my head....in theory....do I still just use ONE shorting jack, on the 4 Ohm tap, for the 3 speaker jacks.?
Thank You

[martin manning]

Whichever one you put the shorting jack on (shorting to ground) would have to be in use, so to allow use of any of the three individually, you can't put a shorting jack on any of them. I'd use non-shorting jacks for all three, and put a resistor of a couple of hundred ohms from one of the taps to ground.

[telentubes]

Thanks Martin. I've wondered about that configuration as well.

[C Moore]

Whichever one you put the shorting jack on (shorting to ground) would have to be in use, so to allow use of any of the three individually, you can't put a shorting jack on any of them. I'd use non-shorting jacks for all three, and put a resistor of a couple of hundred ohms from one of the taps to ground.

OK...
So I run the common to all three, no shorting jacks, and run a 200 Ohm resistor from one of those commons (all 3 are connected) to ground.?
Thank You

[martin manning]

No, the common is grounded and connected to all three output jack sleeve terminals. The OT taps go to their respective tip terminals, and from one of those, say the 8-ohm tap, connect a resistor, say 150 ohm to ground. That's 20x the speaker load, so it will not affect the total load to any significant degree, but it will protect the OT from an accidental no-load condition. I don't know what the optimum value would be, but that should be safe.

Edit: Scratch that; make the resistor 10x the speaker load (80-100 ohms on the 8-ohm tap), and at least 5W (1/10 of the amp wattage). I tried a sim and 20x still gets primary voltage way over 1kV

[C Moore]

No, the common is grounded and connected to all three output jack sleeve terminals. The OT taps go to their respective tip terminals, and from one of those, say the 8-ohm tap, connect a resistor, say 150 ohm to ground. That's 20x the speaker load, so it will not affect the total load to any significant degree, but it will protect the OT from an accidental no-load condition. I don't know what the optimum value would be, but that should be safe.

Edit: Scratch that; make the resistor 10x the speaker load (80-100 ohms on the 8-ohm tap), and at least 5W (1/10 of the amp wattage). I tried a sim and 20x still gets primary voltage way over 1kV

10-4
Thanks.....not the first/last time you have helped me.
Obviously I did not understand the purpose of the resistor.
I do now.
Thanks Again

[martin manning]

No sweat. You might convince yourself that you can't hear the effect of the resistor by A-B'ing it with and without the resistor connected and report back. Using 20x the speaker load might be okay if the OT can take a spike of 1.5-2 kV. BTW, a further advantage of this set-up over the usual shorting jack type is that you are also protected against the other end of the speaker cable being unplugged, or a failed cable.

[surfsup]

Look at any of the ax84 schemos, the HO for example. They use a 220R...

[martin manning]

They use a 220R...

...on the 16-ohm tap.

[Gibsonman63]

I hate to see you drill a hole in that amp. How about trying to squeeze an impedence selector where the ground switch is? You may have to re-route some of the AC power around it.

[surfsup]

martin, fyi, my post wasn't meant to be contradicting you. I was just pointing out if he had a 200R resistor already (it read like he did), to check that out.

Sometimes (most of the time) using my phone, with the little keyboard, I truncate my messages due to convenience and laziness. I post using my phone more than my PC.

BTW, I use that 220R resistor on all my builds (faithfully copied from ax84), and it possibly saved me several times (though I'll never know for sure, its cheap insurance for $1).

[martin manning]

I'm with you Gibsonman, maybe just wire in the 4 and 8-ohm taps using the two existing holes? That should cover most situations.

Surfsup, that's cool, I just wanted to make clear that the choice of value depends upon which tap it's wired to.

[Richie]

In an old bassman, you could remove the ground switch,and put in a speaker impeadence switch.
The switch holes are usually bigger than the jack holes. If you put in a 3 prong cord,and change the wiring after removing the ground switch,the ground switch is not needed. Now this way, if ever needed,the amp can be put back to orignal.

[M Fowler]

https://tubeamparchive.com/viewtopic.ph ... hlight=k60

In this thread the photos show I added an impedence switch into a Mojo Bassman chassis, maybe that would work for you as well.

Mark

[Roe]

I've added 4/8/16 ohms switches to my twins