The AM Forum

I have a piece of equipment that uses two Amperite 3H-11 ballast tubes in a VFO supply circuit. I'm having trouble finding replacements in the event the current ones goes belly up. I could measure the resistance on them but I'm sure that wouldn't do me any good just to replace them with the cold resistance value. Is there a replacement circuit like maybe using zeners that I could substitute?
 
Any help greatly appreciated.
Tom  N5AMA 

   My Clegg Zeus uses two of those as well, and yes, they've pretty much become unicornium. Zeners wouldn't work, since we're talking about AC, although you could rectify that filament supply and then regulate it in the conventional manner.
   I suspect, though, that the solution might be a bit simpler, although it would require some experimentation, measurements, and math. Those ballast tubes are essentially PTC devices: resistors with exaggerated positive temperature coefficients. Some people have simply substituted another vacuum tube for the ballast tube, letting the new tube's filament stand in for the ballast tube's PTC resistive element. This works because all "hot wire" elements, whether in incandescent light bulbs or vacuum tube filaments/heaters, have huge positive thermal coefficients.
   You'd need to find the current requirement for your VFO tube's filament, easily found in the tube specs, and then measure the voltage drop across your 3H-11 ballast. The voltage drop and the known current will give you the resistance at that current. Experimentation with a filament transformer, a variac, and some assorted tubes should get you close to what you need. With experimentation, a light bulb, like those used in automotive brake lights, might even work.
   Or, failing that, you could simply replace the ballast tube with an appropriate resistor, following the math again, and be done with it, since the effect of normal variation in line voltage on your VFO's frequency stability is probably very small.
   Good luck; I'll be interested to see what you come up with.

on page 2, 2nd graph of the Amperite ballast datasheet (link below), it shows the I/V curve for a typical ballast tube. Also included are some curves of other types of materials. None of those come close to the ballast tube. I thought maybe a small light bulb could be a substitute but even that may not be close enough. I think Bill may be onto something with using a vacuum tube.  maybe a LM317  voltage regulator with temperature compensation??


https://www.technicalaudio.com/pdf/Electronics_Catalog_Extracts/Amperite_ballast_tube_regulator_1960_REM_24.pdf

National used a 4H4C ballast tube in a number of their receivers back in the day. They recommended a 6V6 tube replacement:
http://emmittsfixitshop.com/files/Download/Projects_HRO_4H4C_3.jpg
Or build a solid-state replacement:
http://emmittsfixitshop.com/files/Projects_HRO_4H4C_4.jpg

Probably something similar could be applied to the 3H-11 ballast tube. Time to get creative.
Some info: https://threeneurons.wordpress.com/wp-content/uploads/2015/05/amperite_ballast.pdf

Here's a link for a bigger image of the thumbnail in Pete's post.

Or build a solid-state replacement:
http://emmittsfixitshop.com/files/Download/Projects_HRO_4H4C_4.pdf

Wow, lots of possible fixes!!! I?ll keep my fingers crossed that the ballast tubes hold , at least until I can find a fix.

I may find something that looks reasonably close

The VFO uses a 6BK7B, so I?ll look it up and play with the drop across the parallel ballasts. It shouldn?t hurt to plug n play different things and see what effect it has on the VFO.

Thanks,
Tom N5AMA

Is there a reference that would give the specs on ballast tubes that might be used to get close or maybe another maker?s ballast that would work?

Tom

A 3-11 or 4-11 would be close electrical alternatives. Not sure about physical size and pinout.  they're probably unobtanium as well.

Where to get ballast tubes:

https://vacuumtubesinc.com/index.php/ballast-tubes.html

https://vacuumtubes.net/Prices%20Ballasts.htm

https://tubeworldexpress.com/collections/ballast-tubes/ballast?srsltid=AfmBOoo6JLXmFV0mpz-4QP0Vhv5TMc0iOlvC0fZliYpzTxhcZE35fBt_

https://www.tubedepot.com
Search for ballast

https://www.ebay.com/b/radio-ballast-tube/bn_7024792139

I measured the resistance on the 3H-11 and it was 12.5 ohms cold. What about finding an octal tube with that filament resistance and using it?

Tom N5AMA

   The VFO filament winding on the transformer supplies 20 volts, I believe (I don't have my good copy of the schematic in front of me right now; just a blurry PDF). The 6BK7B, according to the RCA Receiving Tube Manual, will draw 0.45 amps at 6.3V. So, you'll need a resistance that will drop 13.7 volts at 0.45 amps. Easy math gives a resistance of 30.44 ohms (actually, 60.88 ohms, since there are two ballasts in parallel), but the tricky part is that you need more than just that static value; you need the resistance/current curve. The cold value doesn't even get you close to that.
   As the Amperite document that Bob provided a link to shows, those ballast tubes have resistance/current curves that, rather than being linear, are highly tailored to provide proper regulation. Using a series element with the wrong curve would likely be worse than simply using a fixed resistor.
   It is a bit of a conundrum. Experimentation might reveal a tube with a filament that comes close to the curve of the ballast tube, though it's very unlikely to duplicate it. The LM317 circuit that Pete linked to would probably provide the best starting point for getting yourself free of the ballast issue altogether. If I ever need to replace my 3H-11's, I'm thinking I'll go with the LM317 circuit. Or, maybe you'll find the 3H-11 somewhere.

Pete, could you provide the component values to your SS regulator? I?ll make up one of those and put it aside in the Clegg spares box for future emergencies.

In the meantime, I?ll look for Zeus power supplies to purchase for spare 3H-11 ballasts.LOL
Tom N5AMA

I think you should just put in an appropriate series resistor value and prove whether that is good enough or not before going fanatical on this.

That would certainly be done initially. I?m just trying to cover my bases in the event the simpler things don?t work.
Thanks,
Tom

   I wish you luck in finding a Zeus power supply/modulator. I've got several examples of the Zeus, but only two supply/modulators, and I've only seen a few at hamfests. The problem is that when a ham goes SK, and his family or non-ham friends dispose of his gear, the impressive-looking stuff like transmitters and receivers, with high-tech looking meters and dials, are given priority, while the big, ugly, and very unattractive power supplies sitting in the dust under the bench often wind up in the scrap metal bin. The Gonset G-76, Clegg Thor-Six, and Multi-Elmac AF-67 external supplies often meet the same fate, which is why you find so many of the rigs at hamfests, orphaned from their power supplies.
   Zeus supply/modulators sometimes turn up on eBay, but shipping costs can be discouraging.

   I agree with Tom: do the math, solder the correct resistor into an octal plug, and plug it in. I commend Clegg for their diligence in keeping VFO drift to an absolute minimum, but in my experience with my own Zeus, the drift due to the natural aging of the VFO components over their 60+ years of life will surely render any drift due to line-voltage fluctuations insignificant. As an exercise in engineering for its own sake, finding a modern approach to regulating the filament voltage could be an enjoyable rabbit hole to go down, but if you just want to enjoy running the rig, you might want to avoid the rabbit hole.

What SS regulator?

The one you posted a link to.

Bottom of this screenshot.

The link to a much larger schematic is:  http://emmittsfixitshop.com/files/Download/Projects_HRO_4H4C_4.pdf


--Shane
WP2ASS / ex KD6VXI

  The values are all in the schematic that Pete posted the link to. I didn't get the impression that he was claiming it was his design.
   If you do decide to use that approach, I would suggest ensuring that both of those regulators, the LM317 and LM337, are well heat-sunk, and you might want to put a resistor...20 ohms or thereabouts and rated for at least 5 watts...in series with the input, to lessen the total input/output differential, since your input will be 20vac and your output only 6.3 vac. They will handle that differential, but it's always better to let a dumb resistor be the beast-of-burden, rather than your solid-state parts.
   I also noted that the schematic says that the output is 6.3vac RMS, but then states that the output "approximates a square wave." RMS values for sine waves are calculated differently from those for square waves, but the end result should be within the tolerance of the tube. If you're concerned about that, you could make the circuit adjustable; the app-notes for the LM317 and 337 will show you how. Just make sure your digital voltmeter is capable of displaying the RMS value of waveforms other than sine.

I?m sorry, SS= solid state versus my generation of vacuum tube technology.

One post down from Pete's, Bob, W1RKW posted a link to a larger version that will probably print better.

Duh! Who's On First?

Slowly I turned...step by step...inch by inch...

I really like the electronic one idea but I wonder if the RMS current is 1A as it would be with a clean sine wave and a vintage ballast, because I think I see the current waveform being clipped and no longer able to be measured the same way by a  ammeter. or is the math and waveform wrong in my head?

When I first saw the topic, envisioned a bridge rectifier with a regulator across it but also a large filter cap there, to convert the AC voltage to smooth DC and then regulate the current to the exact value the tube would want. Theoretically it would therefore be 6.3V. I hope. after midnight thinking I guess.

Does the waveform really matter?

It's the temperature of the cathode we really care about.

Wondering if the clipped waveform matters.....  Or if its the ability to keep the cathode at the dame temperature key up vs key down Vdrop....

Sounds like a glorified lamp dimmer from the 80s.

--Shane
WP2ASS / ex KD6VXI

I'm with Mr. KLR and just sub a resistor in place and see how it behaves. It may be tolerable or it may not be.

The sky's the limit on how to control it if it is not tolerable. if you really want to go extreme, a  properly coded and integrated Arduino would give rock solid stability.

   While I like that circuit as a way of regulating AC, I'm thinking that, if you're going through the trouble to build that circuit, why not go all the way and rectify, filter, and regulate? The filament isn't going to care whether AC or DC heats it, and any worry over sinewave RMS vs. squarewave RMS values is eliminated.
   Many modern DMM's will accurately measure RMS for non-sinusoidal waveforms. Some not-so-modern ones, as well; my Fluke 8050's and 8010's will, and they're 80's vintage, I believe.

   I'm thinking it would be perfectly tolerable. Clegg's engineers, I suspect, were being overly cautious. Most, I'm sure, vacuum tube AM transmitters simply fed non-regulated AC right to their VFO tube filaments, with fine results.
   But, we can't discount the fun of deliberately over-engineering something just as an exercise in creativity. I'm thinking that maybe I'll employ an Arduino to regulate the AC current to my toaster to within 0.1%. Perfectly and consistently toasted English muffins every time! ;D

The waveform does not matter directly. My concern is only the ability to know the true current, regardless of the waveform. I don't have a True RMS meter, which I would assume doesn't care about the waveform.  

If I want to use the solid state circuit there, I want to first see it work and accurately know the power going into the heater, by measuring it. I'm kind of a skeptic, no disrespect intended.

I guess a series resistor is OK, but in cases where the mains is badly regulated as it is at my home in summer, then one has other choices, like an AVR UPS, ferroresonant unit like a Sola, a Stabiline unit, or an Inductrol.

Any of those could be considered pre-regulators to the individual equipment's own internal heater regulator, whether it be a resistor, The Circuit, or an Arduino. But I think one is treading very near to blasphemy when adding somthing like an Arduino-based regulator to a vintage radio set.

Many comments and suggestions have been offered regarding stabilization of the local oscillator heater element voltage.  Some are simple, some seemingly overly complex.  Due to the late hour, my comments may not be as concise as expected, but I will give it a shot nonetheless.

It has been said that the first five days of the work week are the hardest.  And here we are at Saturday night, the first of two days to recover from a stressful work week.  After over 19 years of retirement, I have not yet adjusted to destressing on Saturday, for me it still takes all weekend; my apologies in advance.

When it comes to oscillator stability, we normally think of physical structure to avoid variations with vibration or component movement, compensation components in the frequency determining tank circuit, temperature stabilization via proper ventilation, and regulated plate and screen supply voltages; however, filament voltage regulation is rather far down on the priority list.  Minor variations in filament voltage will cause negligible variations in the oscillator frequency, but these variations may be objectionable if the oscillator frequency is multiplied several times, as in VHF and UHF equipment.  A series ballast is designed to increase its resistance with an increase in voltage or current, thus absorbing the variation and stabilizing the heater which is in series with the ballast.  Whether the mains line voltage is stable enough for satisfactory oscillator performance is a decision the local OP must make on his own.  A series resistor replacement for the ballast offers the simplest and most practical alternative, albeit offering no regulation.

My personal opinion is that a stable voltage on the heater is more important than achieving the exact voltage specified by the tube data sheet, for example, 6.3 volts AC RMS.  Running slightly lower or higher should not present an issue, so long as the voltage is unvarying.   The heater supply should be either a clean sine-wave AC or a pure DC source.  Providing a square wave or pulsed waveform may create serious issues with oscillator performance, whether employed as a receiver local oscillator or a transmitter VFO.  Undesirable signals or waveforms applied to the heater may be coupled to the oscillator cathode with detrimental results in oscillator signal quality.   Case in point - many high quality  low level audio amplifiers apply a positive DC bias offset to the filament circuit, raising the positive DC filament potential above that of the cathode, such that the filament does not function as the cathode of a diode rectifier, while the vacuum tube cathode functions as the plate of the diode, resulting in a current that couples hum or noise from the filament to the cathode through diode conduction.  This biasing technique is used in addition to hum balance pots across the filament transformer secondary winding.  One must be aware that any extraneous signal applied to the heater may be coupled to the cathode if steps are not taken to understand all the "potential" issues and avoid this occurrence.

If a simple ballast-replacement series resistor proves to be insufficient, and improved regulation is desired, the simplest method , without incurring a corruption of oscillator signal quality due to noise applied to the heater, would be a simple rectifier, filter, and regulator to supply pure DC at the approximate filament voltage required.

Either a single diode or a bridge rectifier, followed by a 1000 to 3000 uF filter capacitor may be used to feed a garden variety LM-7805 regulator, with a .1 uF bypass cap at the output to avoid any tendency to oscillate.  The output voltage may be raised from 5 VDC to 5.7 VDC or 6.4 VDC by adding one or two forward biased diodes, respectively, between the negative supply lead and the common regulator terminal, and a resistor from the output of the regulator to the common regulator terminal to cause these diodes to conduct, adding either 0.7 or 1.4 to the regulator's specified 5 volt output. Schottky diodes may be used if a different forward drop is desired.  Using this approach, regulated pure DC will power the VFO heater at minimal expense.   This regulator could be built on a small perf board, attached to a recycled octal tube base; plugged into the ballast socket, minimizing the alterations needed to the classic amateur rig.

Bottom line, none of these gyrations should be attempted before determining whether a 6V6 or similar substitute tube for the ballast provides sufficient oscillator stability for the desired operation.

   Excellent comments all the way around, as usual, Rick, but particularly the quoted portion below. I'd never heard of or seen that approach before, and honestly, if I'd ever come across a circuit like that, I'd have been cudgeling my brains (apologies to Shakespeare) trying to figure out why anyone would apply a DC offset to a tube's indirect heater.
   I hope I never learn so much or become so complacent that coming across new revelations like this ceases to be a joy.

Bill, based upon your comments, I started to wonder just how common that technique of avoiding hum and noise injection via the heater biasing might be.  I decided to have a quick look through some of the archives. 

I figured a good shot might be "Audio" magazine, perhaps sometime in the '50s.  I visited the repository at WorldRadioHistory.com, formerly known as AmericanRadioHistory.com.
I pulled up an issue at random, starting with the September 1954 issue, available here:
https://www.worldradiohistory.com/Archive-All-Audio/Archive-Audio/50s/Audio-1954-Sep.pdf

Skimming through the schematics, I stopped at page 46, which revealed a power amplifier manufactured by none other than the "National Company", purveyor of fine amateur equipment for as long as I can remember....  Lo and behold, THEY DID IT!

A glance at the tap on the B+ bleeder string R17 and R18 reveals an example of the circuit to which I was alluding.  So it appears the technique has been in existence for quite some time.  I will attempt to attach that schematic page to the thread.

   Interesting circuit; thanks for posting the schematic. It's got me wondering, though: raising the heater above the cathode would certainly eliminate any possibility of diode action between heater and cathode, but assuming a filament transformer with no center-tap, and neither side of the heater grounded, the heater essentially has no ground reference to the cathode, so I'd think that no current should flow. The cathode is referenced entirely to ground, through its bias supply or grid-leak; its return path is to ground, while the heater is floating regarding the cathode, so where can the current go? The filament of a vacuum rectifier like a 5U4 doesn't need a center-tap, because the load is connected between the filament and the plate, establishing its reference to the plate. Does this make any sense, or am I getting lost in an electron fog?
   

Most of this is off-topic with regard to the stabilization of the filament voltage for oscillators.  But a couple more posts should finish the hum and noise issues.

There are many ways to wire the filament supply circuit.  Most common for many rigs is to tie one side of the filament to the chassis, saving wiring and providing a low RF impedance to ground, while bypassing hot filament lead with a disc cap.  This is fine for RF circuits, but it invites hum and noise for audio sections, especially the low-level stages.  And this issue does not apply only to audio, but it is also relevant if a low-noise VFO or local oscillator is of importance.

For quality tube audio, the chassis should never be used as either a ground return or a filament/heater current path.  Induced currents in the chassis will produce different voltage potentials at other points on the chassis, adding hum and noise to the desired signal.  A common bus wire, perhaps #12, is grounded at the lowest signal level point in the circuit, say a mic input connector, then left floating as it traverses the signal path components, with the power supply negative connected at the far end.  Bypass and decoupling caps are then grounded to this bus wire, very close to the grid and cathode return resistors of each stage, minimizing noise between ground points. 

But let's get back to the filament issue.  Normally, the filaments are wired with a twisted pair, starting at the power output stage, and working back to the low-level stages.  Then either the filament secondary center tap is grounded, or a pot is placed across the filament secondary, and the variable pot wiper is grounded, allowing the hum induced by the filament to be minimized.

Since many tubes use cathode bias, the cathode is naturally positive with respect to the negative supply lead and ground, and this invites the diode noise and hum conduction problem.  While negligible for the power output stages, this problem, however, becomes increasingly noticeable on the lower level stages. As an alternative, tying the pot to a positive bias, greater in potential than any of the cathodes, prevents any diode current flow.

 It is true that no diode effect would result if the filament bus is left floating, but unfortunately this normally results in even greater hum and noise induced by the filament, which may propagate any unwanted signal back to the low level input amplifier stages by capacitive coupling.   Often, in some ham transmitters, this problem is circumvented by the use of "contact potential" bias, whereby the first audio stage runs with the cathode grounded, and the miniscule grid bias required is generated via grid current on positive input peaks, charging the input coupling capacitor to provide the required negative grid bias.

In the case of the 5U4 rectifier, it is possible to take the pulsating DC from one side of the filament, without the need for a center tap on the 5 volt supply, but this creates an imbalance where half of the filament supply voltage is coupled to the filter circuit, increasing the hum that must be subsequently removed.  For very high voltage supplies, this is probably inconsequential, but for low voltage supplies, especially in quality audio circuits, every possible step must be taken to deliver noise and hum free DC power to each stage, so the center-tapped filament on the rectifier tube is truly an advantage.

Hopefully, some of this theory and best-practice information will "filter back" to the problem at hand, that of having a stable and clean oscillator.  Often we over-engineer as solution, trying to make it better, but then our solution may induce other unexpected problems.  In the case of the ballast tube, it did not introduce any other noise or high-frequency switching grunge, so if the ballast fails, either a resistor or a 6V6 tube will likely provide stable enough filament voltage, which would likely be superior to a more modern switching regulator which may easily corrupt an otherwise clean oscillator output.  Apologies for going so far off-topic; the audio references more clearly illustrate the issues at hand, which also directly apply to low-level oscillators where any PARD may cause undesirable effects in the overall system.  (PARD is a Hewlett Packard acronym for "periodic and random disturbances", which encompass all sorts of undesirable signals, including hum, static noise, parasitic oscillations, overshoot, etc. which are all enemies of accurate measurement and circuit operation.)