Choosing an Impedance Selector Switch

[keithb7]

Hi folks, I am assembling a 6G2 Princeton circuit. I chose to use a Hammond 1760E output transformer. It has taps for 4/8/16/ ohm. I have a rotary impedance selector switch, and it seems easy enough to wire in, however it is a little bulky for the Princeton chassis. I am not trained in electronics but do alright, and get by with help from members here. I am hoping someone can help me with a some type of smaller impedance switch. Would a 3 position toggle switch work? If so, I am unsure which switch to buy. 3 position single throw? Would I need 4 connections on the switch? One for each impedance, plus the common wire? I would like to have one output jack to the speaker, and be able to switch impedance. My theory on how to wire this up properly is limited. Any help appreciated.

Transformer info is here: https://www.hammfg.com/files/parts/pdf/1760E.pdf

Just for fun here is my progress so far. Thx.

[Leo_Gnardo]

Would I need 4 connections on the switch? One for each impedance, plus the common wire? I would like to have one output jack to the speaker, and be able to switch impedance.

That's the answer, you got it. 3 output impedance taps plus the output wire. Perhaps the bulky switch you have is intended for high power amps. The brown Princeton delivers about 10 watts at clip - you could use a light duty switch here and it will work just fine. Excellent looking build in your photo, good work! I've owned 2 original brown Princetons, very enjoyable amps. Take a look at P-H395 from Antique (tubesandmore.com). It's 3 position but has 3 poles - you could parallel the sections for more current handling capability. A little cumbersome I know but I don't see any switches that aren't light duty (0.3 A contacts) or heavy duty (6A contacts.) Perhaps another MEFster will have a good suggestion.

Another challenge is finding a good spot for your switch. You'll want to keep it away from the input circuit. Bringing output leads in proximity to input will invite oscillation.

[Enzo]

Will it fit in the extension speaker jack hole?

Or pick two, like 8 and 16, and wire one to the speaker jack and the othe to the extension jack. No switching required.

The switch you describe is a one pole three-throw.

[keithb7]

Thanks folks. I could put a switch in the extension speaker jack, yes. I like your idea Enzo about just picking 2 settings, probably 4 and 8 ohm, and wiring each to separate jacks.
I think that's what I'll do. I own both 4 and 8 ohm speaker cab options. I have nothing in 16 ohm, so that'll do it.

[Chuck H]

FWIW I always just do 4 and 8 ohms for my own stuff. It satisfies anything I'll do neatly. But I've had customers complain about it. They want to be able to plug into a 4x12 Marshall cabinet. Granted, new ones will switch between 4 and 16 ohms. Older ones don't. So they would need to rewire a vintage cabinet. That would "obviously" ruin their mojo.

I only mention it because it's something I've had to deal with. A non issue for a personal decision, but some players want (need?) all the common impedance options.

[Enzo]

Easily switchable though. How about an internal "switch". Wire 4 ohms to the main jack, then wire 8 and 16 to terminal strip lugs through the hole in the strip, leaving the top loop clear. Now run a wire from the second jack to a quick connect female. The kind of connector used on speakers. Now you can push the female connector over the lug for either 8 or 16 ohms. If needs change to the other option, we open up the chassis and move the wire. A two post terminal strip is eaasy to mount with a single screw. Now we have not eliminated any choice, but didn't have to mount a big switch.

For that matter we could run all three windings to a terminal strip and have each jack with a quick connect, so the two jacks can be any combination of two impedances you prefer, and changed easily.

[SoulFetish]

I have just the thing for you. In fact I'm using these going forward unless they either become astronomically expensive or unavailable.
I found these after scouring the earth for a rotary impedance selector after scouring the earth for a flush-mount selector switch which met my criteria for voltage/current ratings, compact size, and COST.

Schurter - part# 0033.4032
http://www.mouser.com/ProductDetail/...89ILqcJQ%3D%3D
http://www.schurter.ch/bundles/snces...__Frontpl_.pdf
mouser has 12 in stock for $12.31
Online Components sells them for $11(and change)

I like the NKK rotary switch which has become a popular option over the pass couple of years and is commonly found in your Mojotone type amp-parts-retailers. But they come at the cost of chassis real estate both inside AND outsided on the back panel (or where ever). For the amp I'm building now, I really didn't want to compromise the goals I had for each aspect of the design. To keep this short (I just deleted a paragraph after I realized I went on a tangent, turning this into an essay on "what I really want to achieve in my amplifier design". See even the title of that essay is insufferably long. Getting back to it....)
Bottom line, I wanted a flush-mount rotary switch. With the coin style actuator similar in style to the ones Marshall uses. I don't know if you've ever checked out the average prices for those switches, but I have. After that experience the only thing I can reason is that they must think I'm stupid. They must think that we are all f*cking maroons to pay an average of $38 for that just because its "Marshall style". The reality is that Schurter (who makes the switch I posted above) is also the manufacturer of this line of switches as well.
http://www.schurter.ch/bundles/snces...__Frontpl_.pdf
They are usually in the range of $20-40 depending on application etc.
I will say this though, in the course of writing this I have some breaking news on the "marshall" impedance switches. Someone at CE/antiques must know that we're on to 'em and has these priced at $16.88. I don't know if it's a mistake, but if you needed a replacement (or were committed to those for some reason?), but that's a steal compared to what I commonly see.
https://www.tubesandmore.com/product...ement-marshall

For what it's worth, I still think the one I use has important advantages over the traditional Marshall style.

edit: in case you want to see what they look like mounted. This was a quick mock up to compare results to what what I'd drawn-
http://music-electronics-forum.com/t...99/#post440499