OT impedance switch question

[deci belle]

I want to use the vibrato foot-switch hole in the back of my bf Showman for a speaker impedance switch.

It is a 3/8" hole, so I hope I can use a high-enough rated toggle switch; on-on-on, or on-off-on?.

I want to be able to switch between 8 and 16 ohms, but 4-8-16 in a 3/8" bushing toggle sw would be cool!

I'm not sure at all what kind of switch or what current rating it needs to be except DBL or TPL throw, huh?

Would anybody please have any ideas or recommendations for this mod I want to do?

[Jazz P Bass]

Is there a multi tap output transformer in a Showman?

[deci belle]

Dangs~ I better answer this one right!!

haha!! Yes, Jazz!! There is in this one, cuz I put a Low-pwr tweed PT and OT w/ multiple taps, heehe!! It sounds sso cool. With the dual 5U4GBs, the voltages across the board are actually identical, in some places, to the specs I measured with the original '66 Showman's PT on the chassis— only a couple volts difference max, at other measurement points.

So now I have two rectifier tubes and two 6L6s. the OT primary is 4kΩ, and it has 4-8-16 taps. Just a two-position switch would be good; a three position switch, optimal?

It has to be a switch with a 3/8" bushing cuz I can't drill the chassis, right? Any ideas?

[Tom Phillips]

One solution for you would be the 3 way rotary impedance switch shown at the bottom of the page at http://www.hoffmanamps.com/MyStore/p...ER_ID=!ORDERID!
Tom

[deci belle]

Hi Tom, thank you for that! I was pretty certain that the rotary switches would be too big in size and also the bushing too, cuz a conventional CTS potentiometer barely fits back there (really tight fit at the top of the lip on the bf-style chassis.

I wasn't even going to consider the style you mentioned because of their size, but I will check it out anyway, thanks for the suggestion (and the link)!

Otherwise, I guess a SPDT or SPTT (if there is such an animal) is what I'd need. But I still don't know enough to make intelligent choices concerning current values for such a switch. I'll figure it out, though!

[J M Fahey]

As a seat of the pants "rule" a switch meant to commute "signal" such as most pushbuttons used in amps to select mono/stereo or similar duty, can not be trusted to carry speaker level currents.
On the contrary, most switches meant to switch *power* , usually rated 125 or 250V but more important, at least 2A, with 3A , 4A and even 6A better, if available, have contacts strong enough.
For rotary switches I use those made for motors (such as fan speed controls) with good results, but YMMV , of course.
And the beefier ones may be too large.

[pdf64]

Instead of using the hole for a switch, why not use it for a jack socket wired to the 16ohm tap?
Leave the regular speaker and ext speaker sockets in parallel on the 8 ohm tap, but remove the 'shorting to ground' link on the switched contact.
The ony downside that that arrangement is that it won't accommodate 2 x 32 ohm cabs in parallel (unless an additional 'Y' adaptor being used) but that's a fairly unlikely scenario.
Wiring directly to sockets eliminates a significant failure mode.
Pete

[deci belle]

Brilliant!!! Better 'n a switch, Pete!!

[deci belle]

Using your data Juan, I did find a very cool SP3T 1/4-40 (7.4mm) bushing switch at Mouser M2024LL1W01-RO NKK Switches | Mouser that has a locking lever and uses a double-pole base. It's rated at 125/250v @6A.

Thanks Jazz, Pete, Juan and Tom for the suggestions!

[pdf64]

Additionally, good mitigation for an open circuit on the speaker circuit is to fit a ~470 ohm 20 watt resistor permanently across the 16 ohm output.
Pete

[deci belle]

ok, I'll order that with the switch— I wouldn't have thought to get one that big! Thanks, Pete!

[Gregg]

Most rotary switches have two ratings:
1/ Contact (withstand, not switching) current rating
2/ Switching current rating (during switching).

Those two may differ substantially. In datasheets you'll usually see the second.
For a Lorlin 4POLE, 3 POS rotary switch (part #CK1032 ) for example 1/ is 5A and 2/ is 150mA.
Considering that you're not switching the impedance during playing you can consider 1/ for our purposes.
In this example you can take a 3x4 section switch and wire all 4 sections in parallel to maximize the current and be on the safe side should any of the sections fail.

[Bruce / Mission Amps]

470 @ 5 Watts is big enough.

[tubeswell]

Or you could use a conventional DPDT switch and wire the respective poles in parallel (in order to provide more insurance against switch contact failure)

[pdf64]

'470 @ 5 Watts is big enough'
Sorry, I saw 'Showman' and thought '4 tube amp'.
Pete