wards airline radio hack.

[DummyLoad]

friend gave me a wards airline radio rescued from the scrap heap. said give it a makeover al-la champ.

problem is PT i want to use has only 5V filament; it's the hammond 263AX - 200VCT @100mA w/ 5V@2A

so, there i was...

if i use 5CZ5 @250VDC that yields about 5W into 5K load. so, for preamp tubes we use 4AV6 and add a load resistor to burn off a little less than a 1V. filament load comes in at 150mA over budget, however, we're not concerned. snagged the tubes off ebay for about $2.75 ea. - they should be here by friday.  

what is presented here should fit in the original chassis without being too close-in, doing a preliminary mock-up the PT and OT also fit with room to spare.

last problem is that big honkin' tuning dial - do i somehow mechanically link to the master vol. or just fill it in with bling...at this time, i'm leaning towards bling...decisions, decisions.

using a FWB sim shows about 270V and the first filter, 260V at node A, 255 at node B, and about 240V at node C. we'll see.

schematics attached. will post pics of chassis soon.

respectfully,

--pete

[sluckey]

How much voltage does the 5V winding actually put out if you use the 115V primary tap?

[DummyLoad]

How much voltage does the 5V winding actually put out if you use the 115V primary tap?

don't know stave, the PT i have is older and has just the two primary leads.

--pete

[jjasilli]

I think as long as you end up with 5 filament volts loaded, that's OK.  If less then maybe full wave rectify the filaments.  E.g. 4.9VAC x 1.414 = 6.9VDC, still OK.  EDIT:  the point is that unusual tubes need not be used, if the amp is for the typical end user.

Tuner dial use. Probably a PITA to implement, but the coolness factor is hard to ignore.

[DummyLoad]

but the coolness factor is hard to ignore.

true.

going to look into coupling a pot to a gutted OEM tuning bank...

--pete

[PRR]

Pots turn 270. Caps turn 180.

[jjasilli]

Or maybe use pulleys with string like an old Ernie Ball volume pedal; or some old radio tuners (Fisher comes tom mind).

[DummyLoad]

Pots turn 270. Caps turn 180.

clearly we need some reduction then...    

i thot on most pots rotation was about 300 degrees without a switch - more with a switch.

--pete

[DummyLoad]

Or maybe use pulleys with string like an old Ernie Ball volume pedal; or some old radio tuners (Fisher comes tom mind).

was going to review re-using the shaft, pulley, string & spring that came with the radio. the front end of this radio is built as a single assembly.
the dial string is wound three turns over the tuning cap shaft and has a single loop over the pulley attached to the end dial.

--pete

[jjasilli]

OTOH ideas: maybe use that dial to trigger a SW or a several position rotary SW. Maybe use a long shaft & coupling*, or some basic metal working skills a rod could be fitted to the rear of the dial, then coupled to a pot or SW.

* available ready made somewhere, memory failing: used for some vintage preamps & current production hi-fi diy or kits -- to keep pots & rotary SW's near the rear panel of preamps to minimize lead length to minimize noise.  The additional long shaft & a coupling enables the knob to be at the front panel, with the pot or SW several inches behind near the rear panel.  Maybe this concept could be plagiarized for your build.

[DummyLoad]

OTOH ideas: maybe use that dial to trigger a SW or a several position rotary SW. Maybe use a long shaft & coupling*, or some basic metal working skills a rod could be fitted to the rear of the dial, then coupled to a pot or SW.

* available ready made somewhere, memory failing: used for some vintage preamps & current production hi-fi diy or kits -- to keep pots & rotary SW's near the rear panel of preamps to minimize lead length to minimize noise.  The additional long shaft & a coupling enables the knob to be at the front panel, with the pot or SW several inches behind near the rear panel.  Maybe this concept could be plagiarized for your build.

thank you. suggestions are much appreciated. i

was considering a coupling shaft. was thinking about using surgical tubing as a starting point.

--pete

[PRR]

300, whatever. More than 180.

I suppose you could build the amp with 50% more gain than it needs, then just use the first 2/3 of the volume control, end up at the right maximum gain. But you'd *know* there was more gain inside after the pointer hit 1610, and that's intolerable.

It's geared-down 10:1. What knob on an amp do you want to make six knob-turns to get from end to end of a pot? Yes, I've known knobs that could have been 2-turn even 3-turn because they were too touchy in one spot, but that's what "taper" should do (doesn't always do it right).

The tuning capacitor may have some value on eBay.

[DummyLoad]

300, whatever. More than 180.

agree. have decided that it is not worth the effort. too much machining & intricate custom fabrication. additional reasoning follows below...

The tuning capacitor may have some value on eBay.

a closer look at the front end - this unit has no tuning capacitor - tuning knob pulls strings over idlers to move two slugs in and out of two inductors. seems too intricate for a $13 model.

after drilling most of all the holes and mocking up all the input jacks, tubes, output jack, power switch, OT, PT etc., this is going to be too cramped, seriously thinking about scraping project. cramped is ok for a radio; not a higher gain amp.

--pete

i guess "portable" is relative to the period...  

[jjasilli]

But how much does the radio weigh?