Upgrade from heat belching 5U4

[reeferman]

Just curious how many of you have ever replaced 5U4(s) with solid-state rectifiers. As I recall, Philips ECG line was the first to sell a plug in replacement. I started doing it long before that with individual rectifiers. I would take a bad 5U4, bust it up, then solder the rectifiers in. That way a 5U4 could be put back in if desired. I despised the heat those things put out, especially if you had 2 of them sitting on top of the Pwr Xfmr. As I recall, I had to readjust the screens & drives, but usually posed no problem as the set was in for service.

[RVonse]

Although your modification is far superior from a technical standpoint it certainly raises flags as to how far one should go with the repair of an antique set. Because if you start to modify or "molest" an original chassis it may become a slippery road. Kind of like a judgement call attempting to figure out, just where do you stop? Personally, I would replace bad parts with authentic parts (and from the appropriate era) if at all possible and I would stay away from redesign of the chassis unless you find no other way. In addition to this, I would also keep all the tubes with the set making certain someone later has them to plug in if needed. IMO a lot of the nostalia of owning an antique is simply to witness how it was originally built. I have already seen some older tubes on ebay that are going for big money even though they don't work. The 5U4 you crushed may become like that someday, who really knows?

But that is just how I feel about it, others may feel differently.

[reeferman]

www.dialcover.com/tubes lists a 5U4 solid state replacement for $7.50. Less heat, cooler chassis. Better. Plug-in, un-plug, no problem.

[Big Dave]

I run the risk of the "slippery road." I remove all selenium diodes and replace them with silicone. As far as a 5U4 (and others), if I didn't have spares (I need to get some), I would rig up a bridge for temporary use. As soon as I were to get the right rectifier tube, then I would remove the bridge. As far as sockets, I would remove one off a known dud of little value. I will modify my hot chassis sets so they are safe. From a historical standpoint, I will use original tubes. I will, however, sacrifice history for safety. I don't restuff the old shells when I recap, nor do I restuff the lytics, unless there is no other way to replace the caps. If I were to sell a restored set that I made modifications on, I would state it and note that what I did is reversable and include a 5U4 if the buyer wanted it.

[peverett]

By replacing the 5U4 with solid state rectifiers, you are lowering the foward resistance of the recitifiers. This raises the B+ voltage, placing not designed for strain on old components.

If you must replace the 5U4s, at least add series resistance to bring the B+ back in line. Otherwise, your restoration may not last long.

As far as I am concerned, 5U4s are common enough that I leave them in. Also, these do not tend to fail shorted as solid state diodes do.

[RetroHacker]

I agree with Dave - certain things should be changed, and a certain amount of modifications are necessary to keep the set operational and safe. Historically accurate is important to some degree, i.e., you wouldn't rip out the guts of an old console and stuff a modern solid state set in there, but replacing components is unavoidable. Capacitors have changed a lot, but new units need to be used, since almost all the old ones (NOS or otherwise) will have aged poorly. Selenium rectifiers are notoriously unreliable, and die slowly. When they do fail, they typically create a nasty smelling smoke, and could even start a fire. They also starve the set of power as they fail, becoming less and less efficient. I always replace them with modern rectifier diodes, disconnecting the original selenium stacks and leaving them in place. I'll typically use one of the terminals on the old selenium as a mounting point for the new diode. These kinds of modifications are easily reversed, simply by unsoldering the new diode and reconnecting the selenium rectifier.

As for recycling tube bases to create plug-in 5U4 replacements, I don't see much of a problem here - the 5U4 is an octal tube, which are very, very, very common. There were many different octal tubes made, and rectifiers are pretty common. Dead rectifier tubes are useless. And so are most failed tubes, with the exception of tubes that can be 'rejuvinated', or tubes that have collectible value because they were so unusual or early. 5U4's are much too common to become valuble, ever. OK, maybe in 300 years, but not any time soon. And there are LOTS of other tubes out there with octal bases, not to mention all those relays with octal bases, etc. If you _really_ wanted to be sure not to destroy a tube, you could always break up a relay, or find an octal tube with cracked or broken glass, or even just make something with a hunk of wood and some heavy gauge wire.

Basically, what I'm trying to say, is that there's a certain amount of historical inaccuracy with any repair or restoration. Every time you repair something, you have to modify it, melt the old solder off, remove a component, something. Modern components are much safer, more reliable, and can make a set preform as good as, or better, than when it was new. A certain amount of component replacing will be necessary, and you'll have to use new components if you want the thing to work, and work for a long time. Safety upgrades are important - silicon rectifiers, line fuses, polarized line cords, and making sure it's the hot line that's switched with the power switch and not the neutral - these are all important to ensure the set will function properly and safely, and prevent nasty accidents that could possibly destroy it. Just think of how many power transformers have been destroyed by shorted electrolytics, and how much damage can be caused by a shorted selenium stack. How many deadly fires were started because of a burning electronic component, and how many people were electrocuted by touching the metal shaft of a tuning knob on a hot chassis set?

-Ian

[peverett]

I would agree with the above poster except for one area. In a lot of the sets built in the 1950s, fuses were not present. In these sets, I would leave the tube rectifier in place. As solid state diodes tend to fail shorted, they would actually be less safe in an older set unless a fuse is added. All it takes for a solid state diode to fail is a power surge from a nearby lightning strike, I have seen it happen.

In this case, the older device (5U4) is the safer.

I do agree that replacing the Selenium rectifiers is in order. When I replace these, I add both series power resistors to bring the B+ in line and a fuse. Not original, but safe.

[reeferman]

Uncle, Uncle. But it got some good discussion going. Didn't it?

[rca2000]

If you MUST replace the rectifier tubes in a color(or BW )set, receiver, amp, radio, or anything else, at LEAST leave the filament ckt. connected to the tube. That way, it will "look" like the rectifier tube is powerng the set, as it will be lighted, BUT, the heat will be a LOT less. HOWEVER... I would not recommend replacing a tube rectifier, wiht a SS one,unless there is no reasonable alternative.I, for one, find color sets using rectifier tubes in the power supply, to be the MOST desirable, for collecting, IMHO. (not that many did, and they are not that easy to find, such as RCA ctc 5 and early 7, Zenith 29jc20, Admiral 31z1, etc.)

[RetroHacker]

Wether or not to replace the rectifier tube is really a matter of personal tastes and the situation at hand. I for one, really like the tube rectifiers - I'd never replace one with solid state unless I had to, like if I didn't have another tube rectifier handy, or if heat was really going to be a problem in the cabinet. Some sets don't have the ventallation that they should have, such as the smaller metal portables. If a set with poor ventalation used a rectifier tube, and you were planning on using it a lot, solid state may be the way to go to cut down on the heat. _Personally_ I like the way rectifier tubes look, and they tend to warm up more slowly, whereas solid state is instant. But the point I was trying to make was that a plug in replacement doesn't count as a 'modification' in my book - since you can just swap the tube back in, and it would be all tube again. I would never replace a tube rectifier with solid state under the chassis. Not when it's so simple to make an easily removable replacement.

Elevated B+ will be a problem any time you replace an old rectifier with a modern silicon diode. Those old selenium stacks weren't all that efficient, and the diode will let a higher voltage through. Depending on the set/situation, this may or may not be a problem. With a cheap console or table radio, the voltages are rarely critical to performance, and the extra 10 volts won't hurt - might actually make it run better. In something like a television, a complicated one, the extra juice may be a problem. A power resistor may be necessary.

Aack. I did it again, rambling on and on when there's not really a lot to say. I read my last post again today, and it reminded me that I probably shouldn't be posting when I'm really tired... Ahem. Oops.

-Ian

[Chad Hauris]

5U4's are pretty reliable...have had hardly any failures from them. Some have been destroyed by shorted caps further down the circuit, though. Don't really feel there is a need to replace rectifier tubes with solid state.

Actually 5U4 and other filament type rectifier tubes are pretty "instant on"...as soon as you can see the filament lit, it's working (usually within 5 seconds). 5AR4 and similar types with a cathode sleeve warm up as the same rate as amplifier tubes.

[David Roper]

Project includes a dummy load consisting of three 7-watt bulbs in series from B+ to ground (around 300V). With an 80 rectifier, maximum brightness of bulbs reached in .75-1 second.

This discussion is akin to old car semi-restorations. Many people will install disc brakes, seat belts, and electronic ignition to upgrade within reason (safety and reliability). It's when you get into changing engines and suspension that the slope gets extra slippery! (like installing a modern TV inside an old cabinet)

Anthony

[peverett]

This is off the subject, but the previous post reminded me of an article I read. The article said that some 1930s Fords are hard to find now in stock or near stock form. Almost all of them have been made into hot rods or custom cars with modern drive trains. Kind of sad.

[frenchy]

On my CTC-5, I opted to only replace the 3A3 rectifier tube with a solid state plug-in. That way there is no filament for that tube (which is drawn off of the flyback itself), figured it might reduce the stress on the flyback. Conceivably could allow for brightness to be turned up higher before blooming creeps in but I didn't really notice a significant difference. (These very early color sets don't really have any anti-blooming circuitry in them so you can't just twist the brightness/contrast up as high as you want but you guys know that.) Well all that and...it was just something to do, I'm not necessarily recommending it.
I didn't want to mess with going solid state on the power rectifiers as al it would do is reduce the stress on the transformer a little and that's not something I'm going to worry about like I would a flyback. Plus from what I heard from others, sounded like it might not be good for the rest of the set for the power tubes to come up immediately upon turn-on.....Frenchy