Build Your Own Clone Message Board

Hey guys,
I have an idea and want to run it by you all before I cook an OT. So on the champ there is one output jack. Yet I have taps for 4ohm, 8ohm and 16 ohm outputs. Normally you would choose the one you want and cap off the other 2. However, if I wanted to, could I have 3 jacks on the back ( a 4ohm, 8ohm and 16ohm)? I know that you could only plug into one jack at any time. If I were to do this would I simply just hook each tap up to its own jack, and the wire from the 22K resistor on the board to each jack as well? This seems to me like it would cause the different taps to connect to each other, which would be bad. Is there a way to do this so that I can have a range of choices for speakers?
-Patrick

If I'm recalling correctly, mine has only 4ohm and 8ohm taps. My plan is to attach a small project box to the chassis where the output jack goes with two screws and a grommet, and place the output jack at the bottom of that. Then make the output impedence switchable. If it's an either-or situation between output taps, it seems like making it switchable is a simple solution with regards to output impedence....and the small project box provides the space that ain't on the chassis. Or am I missing something??

Yeah, I have a bigger OT with 3 taps and a larger chassis so space is not an issue. The big thing that I see as a problem is the 22K feedback resistor needs to be switched at the same time as the OT Secondary. If you were to connect the 22K to all of the jacks, you would create a short between all of the tips and it would connect the OT secondary's together (a big no-no). If I were to do it on a switch, I would need to go rotary I think so that I can have 3 different choices and the 22K Feedback connected to only one OT secondary at a time.
Just trying to wrap my head around it and see what other solutions may exist.
-P

Patrick switch wise could you not use an on/on/on toggle?

I'm trying to think of a way to isolate each tap with that resistor but can't. Even if you use 3 resistors, one for each tap, still you are linked. Interesting for sure.

I'm thinking that I will end up with two jacks that the 22K is always attached to. Then use a 1P3T Rotary switch to switch the 3 taps. Needs to be a break then make type switch. The only problem with an on/on/on toggle would be that the middle lug would always be on, connecting two of the OT secondarys (bad).
-P

The NFB resistor always comes off of one tap, there's no need to switch it. If you did switch it when you switched taps at the output jack, it would completely change the NFB circuit. The taps to the output jack can be switched as you'd like to. If there are any details that need to be addressed, the schematic for your OT will have them.

Can you elaborate a bit? If I am hearing you correctly, then if I put 3 jacks on the back (4ohm, 8ohm, 16ohm) I can add the NFB to the 4 ohm only and it will function just fine? How would that work? It seems to me that the NFB resistor needs a path to the speaker, unless the whole point it to give it a path to the OT, in which case it makes no difference if a speaker is attached to that particular tap. Like this:

That is correct. A speaker is not needed to complete the NFB because is it in parallel with the NFB loop. If you chose to use a switch for the duty of selecting the OT taps, put the NFB resistor between the OT and the switch so it is always connected to the OT.

If I understand that correct your saying put the NFB wire on the common of the switch?


If so that is incorrect.
The amount of NFB (NFB ratio) will change depending what tap is used.

Chris, will the little diagram I have up a few post work? My preference is to have the 3 jacks rather than a switch.
-P

That's not what I'm saying. The NFB resistor should always be connected to the tap it was designed to work with. To accomplish that and allow switching of OT taps with a switch, the resistor should be placed between the OT and the switch where the connections are static, not between the switch and the output jack (which would result in changing the amount of NFB voltage).

Since the ultimate goal is to keep things simple (a good thing) and not use a switch (sorry for introducing that, I thought that was the ultimate end goal), the diagram you have above will work fine.

Sweet, thanks!
-P

Right ok so what I said you want to keep the NFB ratio constant.
Sorry I can't understand what your saying with where the NFB is going, but I know what your getting at.

Not my most elegant work, but I think it illustrates the circuit well enough. The NFB is always connected to the 4R tap thus is always giving the desired NFB amount. Since ground is always connected, the NFB circuit from the 4R tap is always present regardless of the output selection switch's position. This is a trick Hammond uses on the 1650 series of OTs to allow NFB to be used.

Note, this schematic implies a grounding scheme where the output jack is isolated from the chassis and is ground referenced where the NFB is inserted.

So if I am using non-isolating Jacks does it still make sense to run a ground to the ground point, or am I going to cause issues with a ground loop ( I figure the chassis is not a good way to ground anything, so I was going to take all 3 jacks to a single ground point where the PT Center taps meet.
-P

Grounding is a very individualistic topic. You can ground the output jack to the chassis directly and omit the wire, or you can insulate the jack and add the wire. Up to you. Fiber shoulder washers are handy to have around for jack isolation. Mouser number: 534-3108.

You don't actually even need to bring the OT common to a chassis ground.

If you use a insulated jack you can just attach the common to the sleeve. No mechanical connection will be made back to the chassis (ground).

This is another option...

Floating the output will still allow the speaker to work, yes, but with NFB there is no circuit. I think as best you'll get an NFB loop that sorta works but likely injects noise into the output stage or the NFB just won't work because there isn't a circuit.