Adding aux input to an old tube radio
17/07/2012 4 Comments
I’m a sucker for all things tube and I love the warmth of glowing filaments with blu-ish hues from electrons being boiled alive and going bonkers in a glass cage. Apart from guitar amplifiers, I’d always wanted to have a nice old tube radio to listen to but these seem to have sky-rocketed in price, especially FM receivers which can still be used in Singapore (no one broadcasts in AM here anymore, unless you live close to M’sia or Indonesia. More on AM and FM radio). As you might have imagined, this doesn’t really appeal to my fiscal sensibilities. Besides, I hardly ever listen to radio anyway so all I really needed was the amplifier section in an authentic looking box, to which I could hook up an auxiliary input for a Bluetooth receiver or a player of some sort.
Attempts at finding a non-functional radio for cheap to fix up were futile, but after many hours on Ebay I managed to get my hands on an old AM receiver from a local collector. This is a Philips 196A, which was designed to run on A/C or on 90V batteries (!) and be portable. The case is a cardboard box with resplendent faux leather covering and kinda looks like an old handbag that your grandma might carry to church:
This handbag doesn’t contain the usual lady’s accoutrements:
You can see that this was manufactured for use in Australia from the Aussie state initials on the station dial:
Not your usual suspects in the tube line-up (1R5, 1T4, 1S5, 3V4):
It even conveniently came with instructions on how to remove the chassis:
Circuit-wise, all that needs to be done is to feed an audio input into the grid of the first gain stage of the amplifier section, where the output of the tuner section normally goes. If that last sentence didn’t make sense, now would be a good time for a disclaimer: THESE CIRCUITS CAN CONTAIN LETHAL VOLTAGES, EVEN WHEN TURNED OFF AND NOT PLUGGED IN! IF YOU’RE NOT SURE WHAT’S GOING ON, JUST DON’T!
With that out of the way, here’s a typical circuit for a radio of this sort:
The first audio gain stage is the 1S5 and you can tell by the absence of Intermediate Frequency Transformers (IFT 1 and 2) at the plate (always check the tube datasheet). In this instance, the connection I’m looking for is conveniently located at the potentiometer (R3) that serves as a volume control.
Alright, time to get down and dirty. Some jiggling and choice swear words later, the chassis is out of the handbag and onto the bench. Cue gratuitous gut shots:
Some of these resistors and capacitors actually look good enough to eat:
The potentiometer I’m looking for is thankfully located outside of the rat’s nest that is the main circuitry. A cannibalised 3.5mm jack with the stereo signals combined via 10Kohm resistors at the potentiometer and grounded via the shielding braid will be the auxiliary input.
And there we have it, a funky-looking player with a glorious 0.25W of output. Of course, it’s not going to blow away any audiophile set-up worth the GDP of a small country (if any of you hi-fi nuts are reading this, don’t you have 24K gold-plated connectors to polish?) but it oozes plenty of old-school mojo.
I’m sure some of you out there are wondering…yes, you can connect a guitar input here as well (with the appropriate grid resistor) but you might want to hook it up a bigger speaker as these little ones on the radio weren’t meant to be abused.
I’ve christened it with some Bill Monroe and the Bluegrass Boys, which sounds right at home on this radio. In the meanwhile, it’s time to start planning my next misadventure into thermionic emission.