Using a Lamp to get max brightness out of a tx
Old timers often talk of tuning up into an incandescent bulb for max smoke, max brightness.
But doesnt this mean theyve just tuned for that specific impedance which wont be matched by the antenna system they subsequently switch to? What am i missing? David |
Re: Using a Lamp to get max brightness out of a tx
Don't you mean tuning up with a lamp in the secondary of a current transformer in lieu of an ammeter so that you can tell when you're approaching multiples of quarter-wave (max current at the tx end)?
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Re: Using a Lamp to get max brightness out of a tx
As I consider myself an "old timer"...and a "young pup" to others..It was general practice to test a transmitter, into a "lamp load" if nothing else was available. In those days a dummy load was way too expensive.
However when you were reasonably happy that you could light the lamp, and test the modulation if fitted. Then it would be time to connect to a aerial and tune up properly. |
Re: Using a Lamp to get max brightness out of a tx
Maximum smoke was what you got if you did have a (probably underrated) dummy load. Or possibly from the rubber insulated wire to the bank of 12v headlight bulbs.
With a suitable match anything from wooden legs to damp string ATU it could also be gotten from a beefburger matched to load a Racal TA 1860 during morning tea break in the transmitter lab...... |
Re: Using a Lamp to get max brightness out of a tx
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B |
Re: Using a Lamp to get max brightness out of a tx
I see. I think I'd misunderstood the procedure.
Thanks |
Re: Using a Lamp to get max brightness out of a tx
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A couple of pages from the book 'Amateur Radio, a Beginners Guide' by J Douglas Fortune published in 1940, this is for a single 6L6 final TX.
Scans not perfect but I didn't want to split the books spine. John |
Re: Using a Lamp to get max brightness out of a tx
Fab. Thanks John
D |
Re: Using a Lamp to get max brightness out of a tx
Some years ago, I owned a Z-Match antenna tuner made by an outfit called Southern Radiocraft, previously Olympic Radio and before that Tiger Radio.
The tuner had a tune or dummy load selector, which switched in a bulb, I'd guess about 100W rating. Oddly, the bulb was painted blue (not tinted glass, painted). 73 Roger/G3VKM |
Re: Using a Lamp to get max brightness out of a tx
I still remember the thrill of tuning my first transmitter and lighting a small lamp in series with the aerial, and hearing it on the R1155. It was probably VFO and 6V6 made from scrapped domestic wirelesses. So encouraged I soon passed my RAE and Morse Test and made more advanced transmitters. A neon bulb on the aerial lead-out was a popular indicator too.
73, Andrew, |
Re: Using a Lamp to get max brightness out of a tx
For a 10W input transmitter (as they were specified back then) I used a handy little lamp load made up of four 12V 2.2W bulbs soldered together in series/parallel - not far off 70 ohms.
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Re: Using a Lamp to get max brightness out of a tx
A bulb wired in series with the antenna [usually end end-fed wire] is a good impromptu indicator of antenna current so you can tune-for-max then short-circuit the bulb.
Another version I remember was a bulb wired in the cathode return of a HF PAto act as a current-indicator; you used it a part of the tune/load process when matching, and then when the output was properly loaded the bulb would glow dimly but brighten slightly on modulation peaks [useful before the days of audio-ALC circuits, and often easier to watch out of the corner of your eye than a flickering meter-needle would be]. It also acted as a fuse to protect the valve in the case of gross antenna-mismatch! |
Re: Using a Lamp to get max brightness out of a tx
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D |
Re: Using a Lamp to get max brightness out of a tx
One WWII-era military radio I remember had a sprung "tune" button: when pressed it switched the bulb in series with the antenna, put the TX into transmit, and also introduced a resistor into the screen-grid feed to the PA to reduce maximum power/protect the PA valve from excessive dissipation if the antenna was grossly mismatched.
Release the button after tuning: the bulb was taken out of circuit, and full screen-grid voltage was restored to the PA stage. |
Re: Using a Lamp to get max brightness out of a tx
Wireless Set 46, if I'm not mistaken? Also the post war A40 manpack had a detachable lamp for tuning up.
73 Roger |
Re: Using a Lamp to get max brightness out of a tx
Until around 40 years ago it was very common practice to use light bulbs as "indicating dummy loads" for testing transmitters. Valve finals are very tolerant of the wide variation of lamp impedance with temperature (and therefore the applied power).
----- There was also the technique used in the Whaddon Mk 7 "paraset", which has two pilot lamps as tuning indicators. One is inductively coupled to the anode tank and the other to the aerial tuned circuit. The anode lamp indicates correct PA tuning but maximum brightness is not necessarily the best condition for oscillator stability (you can get chirp or poor starting as it is keyed). The aerial lamp should always be tuned for maximum brightness, as it is effectively measuring aerial current via a current transformer. |
Re: Using a Lamp to get max brightness out of a tx
Fond memories of clipping a bulb across the thick copper Final inductor of Marconi SWB 10kW transmitters and tuning the drivers for maximum brilliance. You had to reduce spacing of the bulb clips as you tuned further. A stern ear bashing from the Chief if you blew a bulb! The RN VHF gear 691 used a small pea bulb coupled to the lecher lines for tuning purposes, after which the coupling could be rotated to extinguish the bulb.
Forgot to mention - the SWB transmitters were in a large hall, you took the greatest care to NOT blow a bulb then just as you were about to finish tuning, someone would fire up another SWB close to frequency. Yep you guessed it - a blown bulb, not your fault but another ear bashing and sniggers all round from fellow engineers. |
Re: Using a Lamp to get max brightness out of a tx
The bush radios as used in Australia (flying doctor, etc) used a small lamp to indicate ae. current to tune up for max. current into whatever was available for an aerial.
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Re: Using a Lamp to get max brightness out of a tx
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Re: Using a Lamp to get max brightness out of a tx
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Still in use in my shack this vintage dummy load when tuning up my valved transmitters.
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