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Author Topic: Tubes going soft - Again  (Read 18814 times)

Posts: 5096

« Reply #90 on: April 19, 2010, 05:43:00 PM »

Couple of points:

1) I agree 100% with what W8JI says about the capacitor input supply. I should have emphasized more that the buck-boost transformer, if used, has to be able to handle the non-resistive load of the amp plate supply, which has a high peak-to-average ratio.

2) IMHO the next step is *not* to buy a Variac or bucking transformer. The next step is to determine if one is needed by measuring the filament voltage at the tube pins with an accurate, true-RMS voltmeter. If the voltage is OK, the problem is elsewhere, and won't be fixed by lowering the supply voltage.

3) IIRC, thoriated-tungsten filaments are supposed to be operated within 5% of rated voltage. Svetlana 572B data sheet and my old RCA Transmitting Tube manuals say so.  For a 6.3 volt tube, that's 6.0 to 6.6 volts. So if the applied filament voltage really is 6.9 volts, it could be the problem.  (But until it is measured with a voltmeter of known accuracy, you can't be sure).

4) If line voltage reduction turns out to be necessary, I'd use a low-voltage transformer of adequate rating, connected as an autotransformer, rather than a Variac. Personal preference, really, plus lower cost and harder to misadjust. The filament-resistor idea proposed earlier also has a lot of merit, because it adds a small amount of soft-start to the filament supply.

Example: Suppose it is determined that the supply voltage should come down about 5%. Assuming the supply is 125 volts, 5% is a reduction of about 6.25 volts.

So we take a transformer with 6 volt secondary and ~120 volt primary and connect primary and secondary in series, phased the same way. The supply connects to the two windings in series while the load connects across only the primary winding. The result is a reduction of 120/126, which will reduce the 125 volts to about 119 volts.

Of course the big question is "how many amps?".

73 de Jim, N2EY

Posts: 279

« Reply #91 on: April 19, 2010, 06:44:12 PM »

Thanks, Jim... and you're absolutely convincingly right about "knowing for sure" what the filament voltage is. I have, as mentioned, ordered and will receive a .04 ohm 25 watt mounted resistor...  Yet, I'm not that close to finding a known good DVM.  I'll be persuing that next.


Brian K7ZRZ

Posts: 9748


« Reply #92 on: April 20, 2010, 04:31:38 AM »

Thanks, Jim... and you're absolutely convincingly right about "knowing for sure" what the filament voltage is. I have, as mentioned, ordered and will receive a .04 ohm 25 watt mounted resistor...  Yet, I'm not that close to finding a known good DVM.  I'll be persuing that next.



It is always more cost effective to order the parts after we know the exact values needed.   Of course I do it the wrong way myself all the time, that's why I have three big storage buildings full of parts.  :-)

You will never regret having a good meter. It will be with you many years, and pay for itself over and over again.

73 Tom


Posts: 279

« Reply #93 on: April 21, 2010, 02:02:34 PM »

After having received the power resistor (.04 ohm @ 25 watt mounted) and very carefully hooking it up, I turned the amp on very briefly to find no filaments... just a little fssst and I got it off immediately. The resistor looks completely inadequate (especially those tiny leads).  So I fetched out a piece of #14 solid copper wire I had and decided to splice some in just for the heck of it.  About 6 feet brought the voltage down to 6.6 V.  About 15 or more feet brought it down to 6 volts.  So the in between amount has it at exactly 6.3 volts.  I coiled it up neatly, got that inside neatly, and let it sit on for a while. The wire gets just warm and not at all hot.

So I have stuck in the new tubes, buttoned up the amp, and loaded it up. The loading is much higher than the tube taken out. I made some benchmark readings... thusly:  Into the dummy load - 65 watts drive = 1000 watts amp output, with 675 mils plate current... and idle current is between 120 and 130 mils. (hard to read exact numbers on that meter).

I'm going to go on the assumption that with proper filament voltage (as confident as I can be about what it really is with my current meter - a Sperry), very careful tuning, and very conservative operating practices, that if the tubes go bad again, it will be because I have very bad karma or something else.

I'm going to be selecting a better meter (Fluke, probably in the $200 range) and also presume that the name and $$$ will get me pretty close to being able to make accurate measurements. I'm also going to be trying to single out a tube or tubes from the last batch by using three good ones and substituting old tubes in the other hole.. making a reading at a set value of tune conditions.  Hope this seems logical to others.

Thanks for all the help and discussion on this. I know it has been a long thread, and I also hope something will come of it for others (except I REALLY hope no one needs that kind of help).

Cheers all,

Brian K7ZRZ

Brian K7ZRZ

Posts: 7996

« Reply #94 on: April 21, 2010, 03:21:47 PM »

The 0.6 volt drop for 15' of #14 AWG wire tells us the current is 15 amps. That sounds right.

The cold filament current must have cooked the 0.04 ohm resistor. That one is good for steady state 25 amps but must not like the 100+ amp surge at power-up.

In handheld DMMs the best buy for an industrial grade meter is the Agilent U1242A. About $200. A Fluke is very nice of course but the equivalent of the U1242A costs more. The U1242A comes with a cal certificate at no extra charge and so your measurements are NIST traceable for what that's worth.

The worry with a non true RMS DMM is the error caused by third harmonic distortion. That will cause the AC waveform to be flattened and it will read low.

An accurate AC voltage measurement can be conducted with a cheap DVM but it takes some doing. Two 12 volt lamps, a variable DC power supply, and a DC DVM are required.
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