Replacing rectifier and VR tubes witih SS

[KE7OAY]

I know this is sacrilegious to even suggest such a thing, but there are times when you might consider it.

The rectifier tube, such as a 5U4, makes a lot of heat, and is quite expensive to replace. Most of these tubes use the heater as the cathode, so what ends up happening is that you have B+ applied to the 5v winding in the power transformer. There are many boat anchors that have a problem with power transformer failure. One of these is the National NC183D and others in that series. Often what brings on transformer failure is this 5v winding getting compromised in one way or another and shorting out the B+ supply. The 183D I just restored had this problem. I replaced the tube with a SS rectifier I made up.

You can buy these already made up on a plug in octal base. However they can lead to additional problems, because there is no voltage drop built in, and your B+ will go up by 30-40 volts. The 5U4 typically develops that drop across the plate/cathode, which accounts for a lot of it's heat. You can build up a 5U4 replacement on an octal base using two 1N7001 diodes and a 150-250 ohm 5 watt resistor in series with the diode output. You may have to experiment with the resistor to get your B+ where you want it, but it definitely works. In my case, my NC183D is now working WITHOUT having to replace the power transformer. I was ecstatic about that.

If you do this, it is a good idea to power up with the unit in standby, so the B+ does not get applied instantly to cold tubes. In my 183D, only some of the tubes have the b+ interrupted by the standby switch, so I moved that switch to the center tap B+ ground return of the transformer, so as to completely switch off the B+. In that same vein, if you want to increase tube life, you can also add an appropriately rated thermistor in the AC line that will cause the AC to come up slowly, over a period of 2-3 seconds, and lessen the thermal shock to the heater elements. In most cases the thermistor will develop a 3v drop on the AC voltage, which is often ideal. Modern mains AC are typically 120v or more, and getting back to the original spec of 117v can only be a good thing. I certainly don't know, but many tube gurus claim that if you do these tricks, tubes will last almost indefinitely.

Most of these old receivers also have a voltage regulator tube. That tube is simply a high voltage zener diode. SS zeners are available in voltages high enough to completely replace the tube, and give better regulation by far, to boot. Cost from Mouser is less than $0.50. You just might get some better stability out of one of these old drifters. The OB2 in my 183D works at about 105-115v, the 0D3 in my SX-42 at about 150v. These are expensive tubes, and sometimes hard to find. The zeners are generally available in 3 watt versions, which should be more than enough. Again, you could put it on a plug in tube base, but some of those are getting very hard to find also, and expensive. The only source I know of for a 9 pin plug is $14, more than the tube. You can always just break a defective tube and solder to the pins on the base, though. Or you can just solder in, underneath.

Hope this might help out for some of us Boat Anchor lovers. The transformer problem is very real, and sometimes we just don't have the money for an expensive rectifier tube.

[K7AV]

Thanks for all the info, it's much appreciated!

[W2WDX]

One note though:

I wouldn't suggest breaking open a regulator tube to use the base, unless you want radioactive material present in your home from that day onward. Some of the VR tubes contained radioactive isotopes. Which is fine if the bottle isn't broken. Once you break it though those material become part of your home forever. Another reason to use a Zener.

John W2WDX

[KM1H]

I know this is sacrilegious to even suggest such a thing, but there are times when you might consider it.

It takes a lot more consideration than just swapping things but there are times when it is required. A NC183D is not one of them. OTOH its almost mandatory for the HT-32 series, HT-37 and a few others that have poor insulation designs between the HV and 5V windings.

The rectifier tube, such as a 5U4, makes a lot of heat, and is quite expensive to replace. Most of these tubes use the heater as the cathode, so what ends up happening is that you have B+ applied to the 5v winding in the power transformer. There are many boat anchors that have a problem with power transformer failure. One of these is the National NC183D and others in that series. Often what brings on transformer failure is this 5v winding getting compromised in one way or another and shorting out the B+ supply. The 183D I just restored had this problem. I replaced the tube with a SS rectifier I made up.

First of all the 5U4 only takes 15W from the transformer filament winding which contributes very little to the transformer heating Most of the tubes heat is actually due to the tubes internal voltage drop. Remember Ohms Law?
The NC-183D transformer was a high failure item from new since it was underated and the wire and paper insulation failed. A different design was used in late production and offered to all other owners. This has been discussed for over a decade on several Internet forums.

The normal failure mode of a 5U4 is decreased filament emission resulting in less B+. OTOH excessive starting or operating current due to leaky electrolytics can cause an internal short which will take out the unprotected transformer. A fuse in one of the B+ AC leads will save a future transformer.

With todays high AC line voltages in the 120-125V range even the good transformer runs hotter than I like and a 10-13V bucking transformer is the only viable solution to get both the B+ and filament voltages down. Radio Shack has a 12.6V@ 3A transformer which is more than adequate for the job. It can be mounted internally or externally or a larger transformer used to power all the vintage gear.
http://www.allaboutcircuits.com/vol_2/chpt_9/5.html

http://sound.westhost.com/articles/buck-xfmr.htm

You can buy these already made up on a plug in octal base. However they can lead to additional problems, because there is no voltage drop built in, and your B+ will go up by 30-40 volts. The 5U4 typically develops that drop across the plate/cathode, which accounts for a lot of it's heat. You can build up a 5U4 replacement on an octal base using two 1N7001 diodes and a 150-250 ohm 5 watt resistor in series with the diode output. You may have to experiment with the resistor to get your B+ where you want it, but it definitely works. In my case, my NC183D is now working WITHOUT having to replace the power transformer. I was ecstatic about that.

The last thing I would do is SS the 183D B+, especially with only a pair of 600V diodes! Even using the much more common 1000V 1N4007 I would use a pair in series for each leg as a single doesnt even come to 5U4 PIV specs. There are other reasons to not go SS and here is one of the better explanations.
http://www.w8ji.com/power_transformer_stress.htm

In addition adding resistors to reduce the new B+ does very little for transformer heat as its still supplying that current to simply waste heat in the resistor. Remember Ohms Law?

Your transformer, if it really has a interwinding short is living on borrowed time, a bucking transformer might extend it.

If you do this, it is a good idea to power up with the unit in standby, so the B+ does not get applied instantly to cold tubes. In my 183D, only some of the tubes have the b+ interrupted by the standby switch, so I moved that switch to the center tap B+ ground return of the transformer, so as to completely switch off the B+. In that same vein, if you want to increase tube life, you can also add an appropriately rated thermistor in the AC line that will cause the AC to come up slowly, over a period of 2-3 seconds, and lessen the thermal shock to the heater elements. In most cases the thermistor will develop a 3v drop on the AC voltage, which is often ideal. Modern mains AC are typically 120v or more, and getting back to the original spec of 117v can only be a good thing. I certainly don't know, but many tube gurus claim that if you do these tricks, tubes will last almost indefinitely.

Thats another bad idea as it now allows the full B+ voltage spike across the input filter cap for the first few cycles which will be above that caps voltage rating by a little bit. A thermistor is not the answer here as the AC is still excessive for satisfactory transformer temperature and besides, they are just another source of heat and eventual short circuit as they have a finite life as shown in the spec sheets.

The only way tubes will have a long life is to never shut the radio off as proved by the military many decades ago, it is the filament cycling that is harmful. If you monitor the B+ at turn on with the set wired as designed the B+ comes up slowly and no tubes are harmed in the process. Cathode stripping and other excuses is strictly a myth brought on by the uninformed over many decades.

Most of these old receivers also have a voltage regulator tube. That tube is simply a high voltage zener diode. SS zeners are available in voltages high enough to completely replace the tube, and give better regulation by far, to boot. Cost from Mouser is less than $0.50. You just might get some better stability out of one of these old drifters. The OB2 in my 183D works at about 105-115v, the 0D3 in my SX-42 at about 150v. These are expensive tubes, and sometimes hard to find. The zeners are generally available in 3 watt versions, which should be more than enough. Again, you could put it on a plug in tube base, but some of those are getting very hard to find also, and expensive. The only source I know of for a 9 pin plug is $14, more than the tube. You can always just break a defective tube and solder to the pins on the base, though. Or you can just solder in, underneath.

The zener is a good way to go for both the HFO and BFO circuits. The standard size is 5W in the 1N53nn series at Mouser, etc. The 7 pin VR tubes rare gases deterioate over time, even NIB, and its not at any given rate or brand related. Ive had OB2's from roughly 103-140V. The larger octal versions seem to be much better.ation


Carl
KM1H
National Radio 1963-69
Service Tech, Service Manager
USN ET

[KB4QAA]

The rectifier tube, such as a 5U4,....... and is quite expensive to replace.

Expensive? On what basis?

A check of Ebay from a partial search shows 623 5U4 returns. Dozens and dozens of these tubes are available at fixed price "Buy it now" offers for ten dollars or less.

What do you think these tubes cost? What do you think is expensive?

Bill

[KE7OAY]

Hi Carl,
Great post. I really appreciate hearing from someone who was actually there. First of all, I made a mistake in my original post, I did use 1N4007's. I caught it, but too late to be able to go back and edit it.

I don't understand what you mean when you say that a 183D is not one of those times when you should use a SS rectifier. The rest of your post describes different ways to get the voltage and heat out of the transformer and keep it going. Since finding a new "used" transformer for a 183D would be difficult at best, isn't this exactly when you should use a SS rectifier? Especially when combined with your recommendations and others that I came up with below?

You recommend adding a 12v bucking transformer. I don't quite understand as my line voltage here is about 120v and the specs for the 183 say 110/120v AC. Won't this take me down to 108v? That's not very far off, but still it seems to be out of range. You're saying 108v is ok?

I appreciate your recomendation to leave the heaters on, that's what I learned when I was a young ham of 12. I had an SX-99 by my bed that I left on for years. However, a few years ago, I talked with another engineer who had worked in the tube division at RCA back when, and he said that by actual test, there was nothing to be gained by doing it. Normal use on and off would last just as long. You get more hours of heater life by leaving them on, but the hours that you actually use the tube are not any more. It depends on your average use percentage, or duty cycle. He also said best heater life was at 6.3v, and it was worth it to adjust line voltage, to get exactly on 6.3v. He did say that soft start up, with a thermistor or variac would be good for the filaments.

I want this 183D to hang together for as long as possible. What about doing this-- Add the bucking transformer, AND, add a 6.3v transformer for the filaments, taking the filament load out of the big power transformer. If I were to do that, I could reduce the line voltage going into the main transformer by an even greater amount. There's plenty of room left with the SS rectifier/resistor combination to run the B+ back up to where it should be by making the resistor smaller, or taking it out all together. The original 5U4 has a drop across it of 30v or more. I could probably put in an 18v bucking transformer, use a much smaller or no resistor in the SS rectifier, and still get 255 volts B+. What do you think of that? There's loads of room underneath the chassis. That would reduce the voltage on the windings and I wouldn't be wasting the watts to drop the voltage in the rectifier resistor.

And yes, I understand ohms law. LOL And how it affects heating. I'm also adding holes to the bottom cover and a fan hanging on the back of the unit.

thanks,
KE7OAY

[W2WDX]

Carl you are right on about all that.

However, one other way to help extend tube life by the use of tube heat sink shields, like those found in the R390a. The IERC types. Also light air circulation in the area around the radios helps in lowering ambient temperature allowing heat exchange to happen more readily.

However, while not reducing heat the thermistor idea, if placed on the primary of the B+ transformer, will help reduce those peaks for the first couple of cycles. I have not had one fail and some have been in use here for at least a decade and a half. But yes they do produce quite a bit of heat themselves.

I do think the best solution for transformer life is to reduce AC voltage below 117VAC, or use different more substantial transformers.

Also I like adding separate filament transformers. Many of the receivers of this era used B+ transformers that had filament taps for 6V as well as the 5V for the rectifiers. Filament supplies are usually the highest current draw, so getting that out of the main transformer will reduce heat for the HV transformer. Your total heat will be the same within the radio, but the transformer for the HV will be a little cooler.

[KE7OAY]

Hi Bill,
You're right, they are only $10. To me, any tube over $2-3 is expensive. An expensive transistor is $2-3. It's all relative, I guess. A receiver that has 17 tubes in it, well.. it makes a difference when they are all over $6-7 vs only $2-3, if you have to buy all new tubes.

Jim

Expensive? On what basis?

A check of Ebay from a partial search shows 623 5U4 returns. Dozens and dozens of these tubes are available at fixed price "Buy it now" offers for ten dollars or less.

What do you think these tubes cost? What do you think is expensive?

Bill

[N2EY]

The rectifier tube, such as a 5U4, makes a lot of heat, and is quite expensive to replace.

Not for me. I've got lots of them. 5Y3s and 6X5s too.

Most of these tubes use the heater as the cathode, so what ends up happening is that you have B+ applied to the 5v winding in the power transformer. There are many boat anchors that have a problem with power transformer failure. One of these is the National NC183D and others in that series. Often what brings on transformer failure is this 5v winding getting compromised in one way or another and shorting out the B+ supply. The 183D I just restored had this problem. I replaced the tube with a SS rectifier I made up.

That's certainly a valid fix for that rx.

But I've never had a transformer failure from B+ on the 5 V rectifier winding.

Some (but by no means all!) BAs have relatively-weak iron. The Heathkit DX-35 and DX-40 are one example.

If you do this, it is a good idea to power up with the unit in standby, so the B+ does not get applied instantly to cold tubes. In my 183D, only some of the tubes have the b+ interrupted by the standby switch, so I moved that switch to the center tap B+ ground return of the transformer, so as to completely switch off the B+.

Not a bad idea IF the switch can take the voltage and the surge.

In that same vein, if you want to increase tube life, you can also add an appropriately rated thermistor in the AC line that will cause the AC to come up slowly, over a period of 2-3 seconds, and lessen the thermal shock to the heater elements. In most cases the thermistor will develop a 3v drop on the AC voltage, which is often ideal. Modern mains AC are typically 120v or more, and getting back to the original spec of 117v can only be a good thing. I certainly don't know, but many tube gurus claim that if you do these tricks, tubes will last almost indefinitely.

I disagree. If you're worried about higher line voltages, a tapped autotransformer is the way to go. Easily made from a heater transformer.

Most of these old receivers also have a voltage regulator tube. That tube is simply a high voltage zener diode. SS zeners are available in voltages high enough to completely replace the tube, and give better regulation by far, to boot. Cost from Mouser is less than $0.50. You just might get some better stability out of one of these old drifters. The OB2 in my 183D works at about 105-115v, the 0D3 in my SX-42 at about 150v. These are expensive tubes, and sometimes hard to find. The zeners are generally available in 3 watt versions, which should be more than enough.

Let's do the math!

The highest-power common VR bottle is a 0D3, which can run at 150 volts 40 mA. That's 6 watts. (!) The lowest-power common VR bottle is a 0C2, which can run at 75 volts 30 mA. That's 2.25 watts. IMHO a 3 watt zener isn't adequate to replace a VR tube!

As for being a better regulator, I'm not so sure about that.

Years ago I built a simple VR tube tester, because I'd accumulated a lot of VR tubes and the usual tester doesn't tell you much.

My tester consists of a stepup transformer and diode rectifier, with simple capacitor filter, to provide 0 to 200 volts or so. A variac adjusts the input voltage, and a 10 K series resistor limits current. The VR tube under test has a milliameter in series and a voltmeter in parallel, so it's easy to watch the regulating action.

To test, I slowly raise the supply voltage until the tube fires, then adjust for 5 mA current and measure the voltage. Then I increase the supply voltage until the tube is drawing maximum rated current (30 mA for miniatures and VRxx/30 octals, 40 mA for the 0A3-to-0D3 octals). A good VR will have almost no voltage variation across the current range. That's all we need.

This process weeded out some bad ones but also confirmed the good ones. After testing, each tube gets a label. My current rig has 6 VR tubes, all octals, and they've been in use for years with no trouble.

The ONLY reason I can see to replace tube rectifiers and regulators is if you really can't get good ones at a decent price. Such substitutions can always be made in a reversible way, too.

while we're at it, let's do the math on the rectifiers...

A 5U4 that drops 40 volts at 200 mA is dissipating 8 watts in the plates and 15 watts in the filament. So most of the heat is from the filament, not the forward voltage drop of the diodes.

--

Personally, I prefer not to go down the slippery slope. All my hollow-state stuff is junction-free.

Sure, I could replace the rectifiers with 1N5408s and the VRs with zeners.

But where does it stop? Pretty soon it would be replacing the #47 pilot lights with LEDs. Then the audio stages could be solid-stated with FETs. The oscillators could be made solid-state too, the mixers, the IF amplification done by ICs. Pretty soon there'd only be the finals - then nothing.

If I want sandstate I'll go that way from the start.

73 de Jim, N2EY

[WA9SVD]

One note though:

I wouldn't suggest breaking open a regulator tube to use the base, unless you want radioactive material present in your home from that day onward. Some of the VR tubes contained radioactive isotopes. Which is fine if the bottle isn't broken. Once you break it though those material become part of your home forever. Another reason to use a Zener.

John W2WDX

You have to be more specific. The Voltage REGULATOR tubes, AFAIK, used Neon, Argon, or Xenon gas to provide regulated voltages. There WERE some tubes involved in transmit/receive switching circuits, particularly at microwave frequencies, that had some slight amounts of radioactive compounds. I can't find any simple voltage regulator tubes that used radioactive materials. Neither Neon, Argon, nor Xenon are considered radioactive materials. There were no common (or even uncommon) regulator tubes that used Radon, which is a radioactive gas, but seldom forms compounds, and as a gas, would dissipate from the environment in a short period of time. A single vacuum tube damage would NOT contaminate an area "from that day forward," or "forever." Many homes in areas of the U.S. have higher levels of Radon gas in their basements (and sometimes habitable areas) than would be contained in any usual vacuum tube device. The real danger from some devices would be from direct ingestion or through a cut or would, so the warning against simply breaking an unknown vacuum device is warranted, even if just to prevent cuts from sharp edges. That can often pose a danger when the time comes to remove or replace a device that was constructed from a tube no longer functioning, and the base used for the replacement can contain unexpected sharp points or edges.