First off, diagnosing vacuum tube [sorry "valve"!] falures is not too hard. It is the filament that gives out. If it doesn't glow and get hot, it's dead. Unfortunately, some designed wire all the filaments in series, so, when one dies, they all appear dead. So you ned to pull each one and check its filament. A bit like Christmas tree lights ..
Is that an ASR33 Teletype outside your office? If so, why??
I love my old Magnavox. Tubes have such a nice sound when playing an old Chuck Mangioni album. However, I must admit that I rigged it so that I can feed in Air Play from my Mac via my wireless network. The kids really get a kick out of playing Jazz from their iPhone on an old Tube amp. My daugher in music school is hooked on the sound.
@Max: You would be wise to replace all the paper and electrolytic capacitors BEFORE applying power. You don't want to fry a hard-to-obtain vacuum tube. And there is a lot more that goes wrong with tubes than just their heaters/filaments.
That Grundig radio probably has a power transformer so the heaters will be in parallel, unlike the series connected heaters in the "All American 5".
There is a Yahoo group called "Fun with tubes" that have a lot of good information on restoring antique radios and designing guitar amplifiers.
When I was a little kid my parents had a Grundig radio that looked very similar.
I seem to recall the similar Grunding of my parents back in the mid 50's had a row of 5-6 knobs at bottom center which controlled various audio frequency bands. The visual indications were small beads tied together with red elastic strips behind the glass panel. As the knobs were adjusted the beads moved up or down about 1-2 cm, the elastics stretched and followed to form a mechanical visual display of the resulting frequency resonse.
If you feel brave, you can put it on a variable autotransformer and slowly crank it up. Once the rectifier tube lights up enough to work (maybe 25%) you might be able to reform the caps by letting it cook for a few hours, gradually upping the voltage. No guarantee.
If you don't have an autotransformer, try putting an incandescent light bulb in series with it. Try a 25 watt to start.
Your caps might work fine, might blow up, or (most probably) just won't filter well and it will hum badly. Time to get out the scope (check the voltage rating on the probe!) and see what caps have a lot of ripple on them. Also check casthode bypass caps. They can short, or have high ripple.
If you give up, don't throw out the case. Put an mp3 player or internet radio in it.
you might be able to reform the caps by letting it cook for a few hours
This might work for electrolytic caps, but an interstage paper coupling cap that has absorbed moisture will leak and drive a following stage control grid positive, resulting in excessive current and possible damage.
Max said: "...a vacuum tube radio from the early 1940s. Amazingly enough, I have the perfect spot for it. The photo below shows the bay outside my office. The comfy chair and ottoman in the foreground allow one to take a few minutes' break now and then."
No! Not the Comfy Chair!
...anyway, as the proud owner of a whole pile of vintage hp and tektronix equipment, I can only echo the cautionary sentiments of the previous posters.
You don't want to take the chance of losing this opportunity. Think of all that you'll be missing, should it become permanently inoperable!... as the store-owner from this classic Charles Rodrigues cartoon ( from the "Over & Out" series in the long-defunct "Electronics Illustrated" magazine, this one from January 1970! ) may have been trying to suggest:
I still have two tube radios. I've got a Hallicrafters S-38, 1940's (I think) vintage short wave radio. I haven't turned it on in a few years, but it worked just fine the last time I did.
I've also got a 1929 Atwater Kent TRF (tuned radio frequency) radio. Unfortunately, I don't have the cabinet. Just the radio. I haven't turned it on since about 1984, but it did work then. The sound wasn't so great after my rather lousy job of re-coning the speaker. I'm a little afraid to turn it on now, because of the old capacitors.
Tube circuits are pretty easy to fix, even for a digital guy. Replace all the caps. Replace all the tubes.
Most of the coupling and filter caps can be replaced with 630 volt film caps. There's probably a chassis mounted single or multi-section electrolytic. Leave it in place, so you don't have an ugly hole. Just disconnect the positive end and use a modern cap underneath the chassis; it will be a lot smaller. Change the mica caps too, but leave the ceramics alone. If the radio pops through the speaker, it's moisture being driven from the ceramics and will go away.
NOS (new old stock) radio tubes are totally available, either from online tube stores or on e-bay, and cheap. As TV repair men pass away, their grand kids sell the garage full of tubes to a broker who is looking for the few specific make/model valuable audio tubes. The rest are TV and radio tubes or audio tubes with an unimpressive pedigree that are pretty much worthless. A lot of DIY guys are designing amplifiers with TV sweep tubes because they sound great and are cheap. Audio tubes are still in current production and are pretty cheap.
Generally you can do the fix without a schematic, but it's likely to be glued to the inside of the back. If there's no power transformer you have a 'Hot' chassis. You get a 50% chance of the chassis being at AC line. When a little kid pulls the knobs off, the metal shafts will be at AC line too, so replace the power cord with something polarized like an IEC-320 connector. Leave the ground disconnected, but you can at least get AC neutral on the chassis, assuming the building is wired properly.
Yeh when I was ~8 yrs old, my schoolmates dad owneda radio/TV repair business , we would go dumpster diving and wire up "radios" using old valves , (well we could get the filaments glowing anyway witha 6V battery !).
Built a Tesla coil (first of several) at age 15 used a 806? 810? triode (octal base with top cap anode) it used about 3 scavenged valve radio transformers to get the 1000v B+. Made a friedd with a tech at the local radio station, he gave me some bigger valves (Used in sets of 4 as the exciters I think? they discarded the whole set when weakest valve couldn't keep up)
Not really done much with valves since.
The 110ohm observation was with a Chinese made hot air station, it had a handful of carbon composition resistors in the triac speed control. One resistor looked kind of heat stressed and was brown brown brown (and measured an erratic 63k) , while another resistor of the same size nearby showed brown black orange and measured 10k. I swapped the suspect one with a 2W metal film 10k resistor and the speed control worked properly again.
That sparked a recall from decades back with colleagues who had also noted propensity for drifts in carbon composition resistors that ran hot. By contrast modern metal film resistors can be cooked till the paint flakes off and show no drift.
On a related note , be wary of metal film resistors > 100k operated at 250v or more (The Tectronix SC501 tiny scope was always having trouble with a set of 4 voltage dividers on the HT) You need to replace with "high voltage" resistors, (and preferably physically bigger resistors as this lowers the electric field)
@salbayeng: On a related note , be wary of metal film resistors > 100k operated at 250v or more (The Tectronix SC501 tiny scope was always having trouble with a set of 4 voltage dividers on the HT) You need to replace with "high voltage" resistors, (and preferably physically bigger resistors as this lowers the electric field)
@KField.... "Elevenses".... reminds me of an Andy Capp cartoon (Andy Capp was a working-class British layabout forever shirking work, drinking beer and chatting up barmaids, and a cigarette hanging out of his mouth). He had a long suffering wife called Flo who had cleaning jobs to bring in a bit of money. I remember one cartoon with Flo carrying a tray of 11 beers to Andy, with the comment "Elevenses!"
I'm sure even Max would struggle with 11 G&Ts for breakfast.....
@kfield: You mean you were pouring it directly down your gullet, bypassing your lips entirely?
Hey -- I'm not gauch -- my mom brough tme up propper -- I know which color drinking straws and which paper umbrellas go with each type of cocktail -- and when it comes to a formal dinner in which white, red, and dessert wines are served with each course, I know that you use the drinking straws starting from the outside -- LOL
@Max hmmmm....go right ahead but I don't think I'd quite keep up with you. I had 9 G&T's once and then DROVE round (this was way in the past...) to my then girlfriend's place and later went to sleep. I can't have been that bad 'cos she is now my wife. But she has not let me forget that.
Andy Capp is alive and ... well, he's well between the football matches.
He appears in various US newspapers (there still some in print) but you should be able to find him, Flo, and the others at Comics.com. Alas, that's one site embargoed by our net nanny software, the same software that wouldn't let me access a site for the famous WWI infantry unit known as "The Fighting 69th."
@David Ashton: "(Andy Capp was a working-class British layabout forever shirking work, drinking beer and chatting up barmaids, and a cigarette hanging out of his mouth). He had a long suffering wife called Flo who had cleaning jobs to bring in a bit of money."
Andy resides at gocomics.com. Here's today's story:
Edit: The picture doesn't show on my PC, but I don't know if it's our net-nanny software or not.
Some AC-DC tube radios were wired so that the chassis was connected to one side of the power plug when turned on, and to the other side of the power plug when turned off. One side of the plug connected to the power switch. The other side of the power switch was connected to the chassis. The heater series string was connected from the chassis to the other side of the plug.
I recapped and rewired one of these to run safely and cooly off a 12 VAC 1A wall-wart supply.
I rewired the small-signal tubes (12BE6, 12BA6, 12AT6/12AV6 or 12SA7, 12SK7, 12SQ7) with their 12 V heaters in parallel across the 12 VAC. I used three silicon rectifiers to triple the 12 VAC to 36 VDC for B+ for these tubes. The gain is lower, but they work fine. I removed the rectifier tube (35W4 or 35Z5) entirely.
I relpaced the audio output tube (50C5 or 50B5 or 50L6) and output transformer with a LM384 running from 12 VDC (halfwave-rectified 12 VAC). I used a resistive divider at the LM384 input to reduce the gain to the unity voltage gain exhibited by the audio output tube and transformer. I reused the output transformer primary as a filter choke for the 36 VDC supply filter.
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