Impedance Selector switch

[Alonzo]

Is there enough room on the Mainline top plate to replace the impedance "Hi/Lo" switch with a rotary multi position switch? I realize I may have to use a slimmer knob to not cover the volume plate, I just want to make sure there's enough room underneath to not conflict with the other switches.

[Paul Birkeland]

It would seem pretty likely that you'll hit the wood base if you install a rotary switch there.

The bigger issue, in my mind at least, will be what the knob on the top will look like.  It will overlap all of the text on the control overlay, and you may end up having the choice of having a large enough knob to cover up the extra hole for the anti-rotation pin in the rotary switch, or having a knob that is small enough that it doesn't overlap the wood base either.

For a two position switch, using a rotary seems like an odd choice.  Do you have other plans in mind?

[Alonzo]

Why yes I do.  I'd like to put my TL-404's in the Mainline, be able to try the different outputs for the headphones I have (HD-650, HD-800, Grado and AKG-702's).  I had planned on bending your ear for advice on the install after I get it up and running stock.  Since it's in transit I'm trying to get the parts in place.

[Paul Birkeland]

How are the TL-404 secondaries configured (I can't quite remember at the moment).

You'll end up losing the balanced headphone output and balanced headphone jack, so you maybe able to have two toggle switches available for impedance switching purposes.

-PB

[Paul Joppa]

There are miniature rotary switches - I have two in front of me right now, made by Grayhill. Switch diameter 1/2", mounting hole 1/4", shaft 1/8". I think I was going to make a miniature Foreplay at one time, using some of those tiny tubes with wire leads - never got around to it though.

[Doc B.]

We have some miniature Paramount plates if you are interested.

[Alonzo]

PB, the 404's are 4, 10, 50, 200 and 500.
Paul, thanks.  I would have to put in washers to bridge the mounting hole differences?
Doc, thanks, yes I would like to order it if I can. I'd like to make this as pleasing and professional as possible, this won't be one of my jury rigged amps but the headphone amp for the immediate future for my den listening.

[Doc B.]

I was just kidding PJ. Somehow we ended up with a set of Paramount panels that were cut at about 50% of normal size. Everything is small, including every hole. Worthless, but sort of an interesting curiosity to show around.

[Alonzo]

Oh, OK.  I going to have to find a way to make this work, and look good... would the impedance switch boards work here?

[Paul Joppa]

The Postcard Paramount - gotta love it!  :^)  save the panels, I'll see if I can come up with something. Triode-wired 5902s?

Now I remember - the late, great Terry Cain built some half-size Abby speakers - I think they used FE-108Sigma drivers? I thought it would be cool to make a mini-Bottlehead system to match. Other priorities intervened, alas.

[Paul Birkeland]

PB, the 404's are 4, 10, 50, 200 and 500.

There are DP3T toggle switches, I'd get yourself one and use it with the 4, 10, and 50 Ohm taps.  Buy another 1/4" TRS jack, and install that where the 4 pin XLR jack is on the stock amp.  Next, you can use the stock switch in the BAL/UNBAL hole and feed the 200 and 500 Ohm taps to that switch, then to that jack.

This will leave you with one jack that has the three low impedance taps, and one jack with the two high impedance taps.  It also uses all of the holes, and doesn't involve the arduous task of wiring up a rotary switch.  (Just don't use both jacks at once)

-PB

[Alonzo]

Thanks,
I'll run by RadioS and pick up a new switch to try out. 
Just finished the stock build.  Almost perfect.  Got a problem with the voltage check.
All resistances were within spec.  Heaters light up.
All voltages were good except Kreg on B side is 2.7.  A side is 10.8.  Voltage at 20 trimmed to 145.  Voltage at 30 is at .08 and the lower right led on the bside of the c4s on the C side doesn't light.
That's confusing...1 out of 4 leds won't light, I'll take a picture to show.

[Paul Birkeland]

You'll want to double check all the jumpers, especially on the center PCB's.  One very common mistake made is to forget to bend over the two little leads on the trim pot.

-PB

[Alonzo]

checked the jumpers, re-wet every solder bump on the center and C side board, swapped tubes and put in a new 12au7.  Kreg on the B side still measures low, now at 3.1 vdc.
Any ideas?  Replace LED that's not lite?

[Paul Birkeland]

Are you getting the correct voltage at OA?  What voltage range can you get on OB?