Output Transformer wiring - Secondary

[Bob M.]

I've got an tube amp with a new output transformer. The secondary has both 4 ohm and 8 ohm outputs and the amp has three jacks on the rear panel. I'd like to keep the Fender idea of having a switched jack ground out the hot out if nothing is plugged into the amp/jack - probably on the 8 ohm tap. I guess I'd like one jack for 4 ohm, one for 8 ohm and one for ext. spkr. @ 4 ohm.

Does someone have an amp drawing or schematic with dual secondaries that I could copy?

Thanks,

Bob M.

[J Martin]

You can use a power resistor of about 250 ohms accross the output. While not a direct short, it will prevent the flyback effect of an open output that can cause voltage peaks that can burn holes in the winding insulation.

The way I wire output jacks is to use a standard jack for eight ohms and a "transfer" jack for the four ohm jack. When you plug something in to the four ohm jack, both jacks are then connected to the four ohm tap. This way it allows use of a single eight ohm cab in the eight ohm jack, or two eight ohm cabs into each jack, or a single four ohm cab in the four ohm jack.

Sorry I don't have a diagram. Go to one of the big electronic suppliers and look at the Switchcraft page. They will have a diagram for the transfer jack.

[Bob M.]

J Martin,

Is what you mean by a 'transfer jack' to be one that contains switches (one or more) so you can manipulate the secondary windings different ohmage taps?

Thanks,

Bob M.

[J Martin]

Think of it as a single pole double throw switch. When there is no plug in the (4 ohm) jack the ungrounded part of the jack is connected to the eight ohm transformer tap and also feeds the the other (8 ohm) jack. When a plug is inserted into the four ohm jack, the ungrounded part of the jack is pushed up to the four ohm tap which still feeds the eight ohm jack making them both four ohms. A simple thing, but hard to explain.

[Steve Conner]

That is neat! I kind of guessed something like a "transfer jack" must exist, but I never could find it because I didn't know the name of it. I guess it's Switchcraft part # 13A?

http://www.switchcraft.com/products/jack-85.html

[Tom Phillips]

I guess it's Switchcraft part # 13A?

Yes but they are really had to find in stock anywhere.
Tom

[J Martin]

13A is the one. I have purchased from Mouser and Digikey. There are plastic ones around too, but I would never use those on an amp intended for live performance.

jm

[bob p]

You can use a power resistor of about 250 ohms accross the output. While not a direct short, it will prevent the flyback effect of an open output that can cause voltage peaks that can burn holes in the winding insulation.

I'm not sure what you meant -- could you clarify this for me? Are you recommending keeping a 250 ohm resistor across the output at all times, or are you recommending switching the 250 ohm resistor out of the circuit when a plug is placed into the jack?

for safety reasons, I like to keep a low wattage 8-10 ohm power resistor wired-up to the OT as a dummy load unless a speaker cabinet is plugged into the amp. that way the amp won't die if its inadvertently left at idle with no speaker cab plugged in, or if somebody trips on a speaker wire and pulls it out of the jack. for an amp that only has one output jack, a normally-closed SPST type of switching jack works well. (IIRC that's a Switchcraft number 12A.)

for amps that have multiple output jacks I suppose you could use a 13A in each position and wire the normally-closed shunt leads of all of the jacks in series between the tap on the OT and the power resistor. that way a power resistor would provide a dummy load if nothing were plugged in, but it would get switched out of the circuit as soon as a cab is plugged into any of the jacks. then you'd need a fixed Z or a rotary Z selector.

BTW, Tom - Mouser has them in stock and lists them for $5 compared to $12 at Digikey. (!)

[J Martin]

I have seen the resistor method used in other amps, Traynor comes to mind. The jack shorting method won't help you if you have a blown speaker. And yes its left connected.

[Enzo]

When i wanted Switchcraft connectors, I called my rep at Rapco. Rapco is now part of RApco/Horizon. I suspect any of the cable/connector companies would have stocks.

[Bob M.]

Yeah, what I think we are talking about here is a jack with an independent switch that is neither hardwired to hot nor ground. The switch is a SPDT and has a normally closed contact and a normally open contact. When a plug is inserted into the jack, the normally open and closed switches reverse, allowing (in this case) multiple ohmage choices with limited speaker out jacks.

I ended up copying a Sunn amp from my schematics that used multiple secondary ohmage taps for my project. I was lucky enough to have three output jacks to start with.

My local electronic supplier carries a jack with two independent SPDT switches per jack as described aboved, plus hot (tip) and ground. I always stock these as they are good for doing switching magic in amps. They are good for stuff like installing a preamp out that can standby the power amp so you can use an amp head in the studio like a preamp or rackmount without worrying about damaging the power amp with no speaker wired up.

Bob M.

[bob p]

The jack shorting method won't help you if you have a blown speaker. And yes its left connected.

thanks for clarifying that for me. yes, a blown speaker would be really bad. that's a good example of not being able to protect yourself from every possible problem. in my case, i just want to protect myself from killing my amp in a situation that's far more likely to happen to me than a blown speaker -- like me doing something really stupid, like switching amps and cabs, and powering on one of my heads while I've forgotten to hook up a speaker cabinet.