Multiple identical-looking VTVM's.

Discussion in '"Boat Anchor" & Classic Equipment' started by W1GUH, Sep 16, 2014.

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  1. KB1WSY

    KB1WSY Ham Member QRZ Page

    There are two schools on this, and I alternate between them. In this case, it's a piece of test equipment that I use almost daily and it's frequently being power-cycled on and off. (The main reason I don't leave it on all day is fear that the selenium could blow while I'm out shopping or something, and fill the house with noxious fumes.)

    Put it this way, replacing the rectifier cannot do any harm -- if I do it properly -- and removes a fault source, however minor, in future decades. I'll also, eventually, add the fuse as suggested by N2EY. I'm 57 years old and fully intend to keep using my vintage test equipment for the rest of my life!

    That's what I thought too, it would be very low in normal usage. For some reason I don't have anything bigger than 1/2W in those low values; that would probably be OK at a pinch but might as well super-size it to 2W. After all it's a piece of test equipment, you want your resistor to run cool so that the B+ stays steady.

    Yes of course. Call me strange, but resistors are cheap and I usually prefer to use one rather than several. I often use parallel/series combinations while experimenting; then, when the value is determined, I buy a single resistor of the right value. No logic behind this at all, I agree.
  2. N2EY

    N2EY XML Subscriber QRZ Page

    Yes. In fact the actual current is probably much lower.

    First off, even if you have to drop 30 volts at 35 mA, the total power is only 1 watt. So a 2 watter is plenty, and you probably don't even need that much.

    What I plan to do with my old V-7 is this:

    1) Measure the B+ in stock configuration (using a 20,000 ohms/volt VOM) and write that down.

    2) Disconnect the selenium rectifier and extend the leads, ending in insulated alligator or crocodile clips.

    3) Hook up a temporary silicon rectifier and dropping resistor and measure the B+. Write down results. If the resulting B+ is too high, change to a higher value of resistance, if too low, change to a lower value. Measure again and again write down the results.

    If just right, go to step 5.

    4) Using the two results generated in 3), compute the optimum value of resistor. Linear approximation is OK. Hook up the computed value temporarily and check.

    5) Wire in the correct value permanently.

    There are other methods, of course, but the above gets you there pretty quick, IMHO.


    In some low-power tube designs, the power supply uses a C-R-C filter. This looks like a capacitor-input filter (C-L-C) but with a resistor replacing the choke. In such designs, the effect of the series resistor can sometimes be had by increasing the value of the filter resistor. However, it's important to be sure the input filter capacitor can take the added voltage.
  3. K8ERV

    K8ERV QRZ Member QRZ Page

    I doubt if the voltage difference would be important, especially in a balanced circuit.

    Or you could put several silicons in series if necessary and you don't have the right resistor.

    TOM K8ERV Montrose Colo
  4. AF6LJ

    AF6LJ Ham Member QRZ Page

    Given around 20V reverse breakdown voltage I would think there would be seven or eight cells in series for a 120V supply.
    Assuming 5me load current for the DC electronics 1600 ohms would be reasonable. At 5ma 8V drop 40mw you could use a half watt resistor and that should cover the surge current provided you don't decide to super size the filter cap. .
  5. KB1WSY

    KB1WSY Ham Member QRZ Page

    I don't think I have a VOM with that high a DC resistance; I have a $3 Harbor Freight POS DMM and its instruction manual's listed specs don't even list that parameter. I figured that it doesn't matter if it gives me the wrong B+ voltage because what matters is the relative voltage, not absolute.

    IOW, if I measure the B+ from the selenium as "X" and then install the silicon replacement and resistor, all that matters is to adjust the resistor until I get back to "X" (even if X is wrong by 10 or 20 volts, as measured by the DMM).

    Make sense to you?

    (I could also use my Heath EK-1 VOM but its meter scale is physically small, for these sorts of small voltage differences.)

    Edited to add: Yes, I know, time to get a decent VOM. I've been looking around for a nice old Simpson or something.

    BTW I don't know how many cells there are: it's encapsulated in a little plastic shell.

    Edited to add: The new filter cap I installed is 10uF, 450V -- same capacitance as the original Eico one, much higher voltage rating just for the heck of it.

    I think we are all over-thinking this; action follows shortly. Real results will be fun. Besides, I can play hooky from work.
    Last edited: Sep 17, 2014
  6. N2EY

    N2EY XML Subscriber QRZ Page

    Yes, all that matters is that you get the B+ fairly close to what it was before. 10 or so volts either way won't make any difference.

    As to VOMs, the Simpson 260 is the classic. But don't overlook the AN/USM-223, aka ME-297/U. They are rugged, although they do take odd batteries.
  7. KC9UDX

    KC9UDX Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    I have radios with selenium rectifiers, one of which, is out in the cold year round. I hardly ever turn it on. When I do, I'm always surprised the Se rectifier still works. But, it does.

    When your rectifier fails, it won't fill the house with noxious fumes, nor will it burn the place down, likely. There will be enough odour in the area that you will know what it is, but it's not going to smoke the place up with deadly poison.

    I would install a fuse, but I wouldn't alter the rectifier. It might continue to work for 30 years, who knows?
  8. KB1WSY

    KB1WSY Ham Member QRZ Page

    I appreciate the comments of those who said I shouldn't replace the selenium ... and I can see their points. However I've gone ahead and replaced it with silicon. As I said, there are valid arguments on both sides of the issue.

    So here is the "culprit": a perfectly good selenium rectifier that's I've decided to replace anyway.


    Before disconnecting this old rectifier, I measured the voltages (with my $3 Harbor Freight DMM) and found +56VDC at the positive end of the filter capacitor, and -101VDC at the negative end. This is in accordance with the original Eico schematic, which shows +50V and -100V, respectively (I have edited this sentence later because my initial listing of the voltages was the wrong way round in the sentence).

    I added this terminal strip temporarily, for testing purposes.


    I disconnected the selenium and installed the new rectifier (a 1N4004) on the terminal strip, then re-inserted the tubes and the battery.

    Feeding the output of the silicon rectifier direct to the filter cap, I measured +60VDC and -108 VDC, respective to the measurement points already quoted above. So, we have an "over-voltage" of about 7 percent.

    I started adding resistors in series after the rectifier, starting a low values and eventually found that this 680-ohm resistor did the trick, with voltages of +55.8V and -100.2 volts, which is extremely close to what I had with the selenium rectifier. I was quite surprised at the relatively high resistance value. In fact it isn't 680 ohms, it's even a bit more: it's a 20 percent resistor and I measured it on the DMM at 748 ohms.

    Replace_3.JPG Replace_4.JPG

    So there we are. I will now permanently remove the selenium from the chassis and install a terminal strip in its place, holding the 1N4004 and the "680 ohm" resistor.

    A postscript: immediately after doing all of the above, I moved a few things around on the workbench then decided to check those voltages one last time. Bad idea. The last thing I had done was to measure that resistor, using the ohms scale on the DMM ... and I had left the DMM on the "ohms" scale. I didn't remember to switch it back to the VDC scales. So when I tried to measure the voltages again, I fried the meter. I just knew I was going to do that one day! Just as well it's a $3 impulse buy from Harbor Freight!

    Concerning the resistor power rating. It's "only" a 1W resistor (an old Ohmite carbon comp). After leaving the meter on for more than half an hour, if I grasp the body of the resistor with my fingers it's at room temperature. I think it should be totally safe and see no need to find a higher-power resistor.
    Last edited: Sep 17, 2014
  9. W1GUH

    W1GUH Ham Member QRZ Page


    They do appear to be eternal. And why not? What does the job better and costs so little? Funny thing is, I never saw a VTVM in any of the EE labs I worked in - it was always the Simpson 260.

    I broke one of those 260's once when it fell over on it's back (wasn't dropped, just tipped over) - the meter got zues'ed. I figured that the meter was already broken but functional, and that spill was the last straw. Anyone know if this was a common occurrence?

    OTOH - a small, cheap VOM from RS also does the job in a lot of situations.
  10. KC9UDX

    KC9UDX Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    The only bad experiences I've had with 260's were

    1) Getting one stolen

    2) Buying broken ones (which turns out to be good when they spontaneously come back to life).
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