I’m refurbishing a tube-type Hallicrafters shortwave receiver. I was planning on using a solid-state plug-in replacement for the rectifier tube; mostly because I can’t find the tube.
I’ve been told that a solid-state rectifier could result in higher voltage, and may blow the filter capacitors and run the tubes at a non-linear part of their operational curves. Can someone confirm or explain if this is a good idea or not?
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Vacuum tubes are resilient and can withstand over voltage better than solid state parts. Tube characteristics are dependent on the physical construction and don't change with voltage if the circuit is properly designed. If the radio has a power transformer, It certainly has a full wave rectifier. The 5Y3 has a tube drop of 60 volts so if it is replaced by a solid state rectifier, the DC output should be 60 volts higher. This is not significant when the nominal DC output is 300 volts. If you are worried about it, put a resistor in series.
The maximum voltage output from any rectifier is less than the peak voltage, so start from that end by measuring the RMS voltage of the AC power transformer output , and multiply that by 1.4 to get an average peak voltage. If it is less than the filter capacitor DC rating then you are safe to install any solid state rectifier that has a higher voltage and current rating. Anyway, replacing the filter capacitors is always a good idea due to aging.
You might look at tubesandmore.com. They have thousands of vacuum tubes.
Tube guitar amps use solid state rectifiers all the time, check out Mesa Boogie. I suspect they are more efficient, and the supply voltage may increase a bit, but my guess is that it’s not so dramatic as to destroy everything, maybe 10-20% increase.
If you can disconnect the rest of the receiver you can always try it and measure the before and after supply voltage, see if it’s still in a safe range. Be careful doing that, of course, it’s a high voltage power supply.
I think the filter capacitors are at a slight risk during the replacement procedure. But maybe they need replacement anyway.
First you need to know the existing rectifier output voltage, measured from a trial run or read from the instruction manual. Then you use silicon diodes to replace the tube rectifier. Re-measure the voltage and B+ current. Be careful these voltages can give you a memorable shock.
Then use Ohm’s law to calculate a resistor to put in series with the B+ line to return the B+ to near its original value. Don’t forget to calculate the power dropped by the resistor to make sure to get one of sufficient dissipation (watts) rating.
If you are uncomfortable with these steps, get a ham old-timer to help. The good side is you should improve reliability and save the power that went into the rectifier filament.
I would say it's a bad idea. Solid state rectifiers have a much lower internal resistance than tube rectifiers, resulting in significantly higher voltage. I'm surprised you can't find a replacement. What's the tube you need?
A solid state replacement will have a lower forward voltage drop than a tube, typically less than a volt. So, the rest of your circuitry will see a few extra volts. This is not likely to be a problem, however I would have another concern.
Using a solid state rectifier means that it will begin rectifying immediately upon power up, the rest of the tubes will NOT yet be conducting and therefore the entire circuit will be drawing very little current. This means that your power supply caps etc. will probably be seeing close to PEAK voltage until the tubes begin to conduct and draw current.
When using a tube rectifier; it too comes up slowly as the rest of the tubes do, you do not get the PEAK voltage anywhere. I’d stick with the tube rectifier; eBay and lots of other on line suppliers can get you one.
There’s some good material on this subject at www.antiqueradios.com/forums/viewtopic.php?p=973781.
I would also suggest that you look for your tube replacment at https://www.tubesandmore.com.
By the way, the latter site also offers their T-SSR01 solid-state replacement for 5Y3, 5U4 and 5AR4 tube rectifier, but please note the cavaeat in the part description.
In regards to your rectifier tube for the Hallicrafters Radio, I suggest you have a look at this website: https://www.tubesandmore.com/products/vacuum_tubes?filters=Type%3DRectifier
They have all kinds of older tubes, most are what is called New Old Stock. We could have provided more information as to use of a solid state rectifier replacement, but it would have helped to know the radio model number and the tube number you were replacing. Hope this helps you fix it.
A solid state rectifier will have less than a voltage drop, a tube is higher depending upon the tube and the current level, etc. This will result in a somewhat higher voltage to the circuitry, but not an awful lot.
There is, however, another situation to consider. When you power up with all tubes, they all come on slowly. Tubes begin to conduct plate current as the rectifier begins to work, sort of a “soft” start. With a solid state rectifier, it will begin immediately upon application of power, while the tubes are not yet consuming any current as they warm up.
This means that since drain on rectified supply is quite low until the tubes come up to speed, the power supply caps will charge pretty much to peak levels, which may exceed their ratings, or certainly stress them more than if you were using a tube rectifier.
I’d suggest sticking with a tube rectifier. You should be able to get one via eBay, or many other on line sources.
Also, a solid state diode has no warm up period. It means that full power supply voltage instantly is present on a circuit that may not be ready for it yet. It can be done, but replacement of a tube with a solid state diode may need some additional precautions (a heavy resistor in line may be enough). I say may, since there are definitely sets which are capable of absorbing this difference.
Most Hallicrafters sets were not the most rugged out there, many without the safety of power transformers. I’d be careful. On the other hand, I find it hard to think you would not be able to find a suitable tube on FleaBay or some of several tube vendors on line (Try Antique Electronic Supply).
You are correct in that there can be some issues to be considered when replacing a tube rectifier with a solid-state replacement. The primary issue is timing. A solid-state rectifier will provide the full B+ voltage almost immediately, while a tube rectifier needs to have the filament warm up, and the B+ voltage will come up much more slowly. Also, the other tubes which will use the B+ will need to warm up and be able to provide a suitable load for the B+. Without a load, the resistors in the supplies will not drop voltage as intended, and so the voltages at the plates of the consuming tubes will be much higher initially until those tubes warm up.
In addition, the tube rectifier has a “resistance” providing a voltage drop that the solid-state device will not. This means that the output of the power supply section will be at a higher voltage than that with a tube. The capacitors must be rated to handle the increased voltage, and the circuit will need to operate properly with the increased B+ voltage.
If the radio has not been operated in some time, it would be good to replace the filter capacitors at a minimum. Some solid-state replacements will include some internal resistors and thermistors (resistors that change value as they heat up) to approximate the characteristics of a tube rectifier. This type of replacement is usually best.
After installing the replacement, the B+ voltage should be checked against the normal expected voltage, and perhaps some of the resistance values in the power supply can be adjusted if needed.