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 Post subject: BK 650/675 Mutual Conductance
PostPosted: Oct Sat 24, 2020 4:06 am 
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Hi All

I have basically finished with my BK 675 (which shares the circuitry with the 650 largely), and have calibrated the piece for balance, bias, signal strength, and gas.

Most all different tubes test somewhat low....while it is difficult to compare Gm readings across platforms, whether it is set for quality readings or Gm (done by the sensitivity pot...R 16 on the schematic), the readings are in the neighborhood of 10-20% low, often bringing a known good tube into the top of the questionable range.

I have attached the schematic for those who might be interested in offering ideas.

Of course, all caps, resistors have been tested and/or replaced for accuracy.

I actually tried both a native 83 tube as well as a solid state, and after balancing, results were identical.

BTW, the plates of 83 tube serve as the bridge driving the panel meter.

Thanks!

Steve


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675 like 650 GM 6L6 .jpg
675 like 650 GM 6L6 .jpg [ 321.83 KiB | Viewed 292 times ]
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 Post subject: Re: BK 650/675 Mutual Conductance
PostPosted: Oct Sat 24, 2020 4:14 am 
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I should add that I have maximized the signal strength through the pot adjustment...improved the Gm readings, but not sufficiently...


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 Post subject: Re: BK 650/675 Mutual Conductance
PostPosted: Oct Sat 24, 2020 4:35 am 
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Have you checked that the meter itself meets it’s sensitivity spec?

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 Post subject: Re: BK 650/675 Mutual Conductance
PostPosted: Oct Sat 24, 2020 5:06 am 
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I am ashamed to admit that I did not compute the FS parameters whwen I checked the meter for full movement...

One of the reasons that I didn't (and I actually thought of that...) is that there are no specs available....

Having said that, it would either be likely 200µA or 1mA FS (though it could be in the middle).

However, if the internal resistance of the panel meter has increased requiring lets say 10% more current to reach FS (ie, 220µA or 1.1mA), I am not sure how I could corect for that...

I will give those parameters in the morning.

Steve


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 Post subject: Re: BK 650/675 Mutual Conductance
PostPosted: Oct Sat 24, 2020 5:13 am 
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Increase in meter resistance is highly unlikely and, even if it happens, is unlikely to affect full scale current. Full scale current is determined by the rotary force from the reaction of the magnet and the coil fields vs. the spring tension. Resistance doesn't enter into the equation.


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 Post subject: Re: BK 650/675 Mutual Conductance
PostPosted: Oct Sat 24, 2020 5:24 am 
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Is this the same tester?
viewtopic.php?f=8&t=385305&p=3279249#p3279249

DM


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 Post subject: Re: BK 650/675 Mutual Conductance
PostPosted: Oct Sat 24, 2020 5:31 am 
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yep...just posted this question as a separate topic because it deals with a different issue...

And, Bob...I didn't know that...I guess I always assumed that it was resistance that changed rather than mechanical...

Steve


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 Post subject: Re: BK 650/675 Mutual Conductance
PostPosted: Oct Sat 24, 2020 3:14 pm 
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FS= 990µA, 107mV...so, likely very close to specs....meaning it is likely a 1mA movement....actually, I did measure it when I first started working on it, just forgot...

Not enough to account for the decrease. I think.

Steve


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 Post subject: Re: BK 650/675 Mutual Conductance
PostPosted: Oct Sat 24, 2020 3:52 pm 
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It could still be a linearity problem in the meter. I’ve had that issue from corrosion in the pivots. It might be worth checking four or five points on the scale, especially around where you suspect low readings.

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 Post subject: Re: BK 650/675 Mutual Conductance
PostPosted: Oct Sat 24, 2020 8:47 pm 
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If the meter is linear, then 1/2 FS should be 500µA...I will check...

Steve


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 Post subject: Re: BK 650/675 Mutual Conductance
PostPosted: Oct Sat 24, 2020 9:15 pm 
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If it's got a spot of corrosion on the pivot, it could be linear everywhere except right around where it rides over the corroded spot. Try just slowing ramping the current up to full scale and then back down, and see if the pointer hesitates anywhere along the way.

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 Post subject: Re: BK 650/675 Mutual Conductance
PostPosted: Oct Sun 25, 2020 2:21 am 
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For some weird reason some tubes just test at 20% (like domino plated tubes), and some 12ax7 tubes... BK hasn't tried to fix this known issue I don't think ever.


https://www.youtube.com/watch?v=KpVwfq2 ... 40&t=1103s


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 Post subject: Re: BK 650/675 Mutual Conductance
PostPosted: Oct Sun 25, 2020 9:16 pm 
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So, the meter is smooth through its arc.

In checking voltages (again), I noted that there was some white residue over much of the metal pieces, and got me thinking about potential ground issues.

It turned out that there were issues with adequate grounding of all points...once these were cleaned, and the instrument re-calibrated, things look much better.

So, it was a chore, but the unit looks done. I sent Ed at BAMA my notes as well to help the next person.

For those of you that don't know this tester (it was not used a lot), the engineer that designed the card system must have been a new grad...The cards have holes, which when a lever is activated, pushes 1/4" steel balls on springs through the card to make contact in the mating matrix. If one moves the machine too much without a card locked in, the 1/4" steel balls will fall out and roll around in the case.....yikes.

Steve


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 Post subject: Re: BK 650/675 Mutual Conductance
PostPosted: Oct Mon 26, 2020 12:01 am 
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Part of your low test readings may be caused by the resistor you used to drop the screen voltage after replacing the rectifiers with silicon. As soon as you put a load on the screen circuit that resistor may be letting the voltage fall.

DM


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 Post subject: Re: BK 650/675 Mutual Conductance
PostPosted: Oct Mon 26, 2020 1:55 am 
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Thanks!

After ensuring the ground contacts were clean, the tester stabilized with repeat setting of the signal and bias, and now appears to be accurate.

I probably didn't need to replace the seleniums, but I was ruling out all causes.... BTW, 300Ω dropping resistors did the trick.

A final note for the other less experienced folks (like myself)...I only found the ground issues when I went back to check the AC signal voltage, and used a different ground connection point than previously used, and found markedly different voltages...

I had previously just checked simply continuity, but the voltages didn't lie...after cleaning the same signal voltage at all ground points.

So, tester works well.

Interestingly, a note that everyone here already knows....in testing a 6AG7, I noted that one of the batch I had shown a bright shorts light...I re-checked it on my BK 707...sure enough shorts light on..

Not to be obsessive, I tested it on my TV-7 and Precision 10-12...no shorts....

The answer came when I pulled out my 666...excessive H-K current! (it was the only 6AG7 in the batch where this happened...)

So, as many of likely know, it is function of what to set the H-K leakage threshold at...the 675 is set at 1MΩ fro all shorts, the TV-7 is around 100KΩ.

See what one can learn!

Steve


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 Post subject: Re: BK 650/675 Mutual Conductance
PostPosted: Oct Mon 26, 2020 4:54 am 
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drworry wrote:
Thanks!

After ensuring the ground contacts were clean, the tester stabilized with repeat setting of the signal and bias, and now appears to be accurate.

I probably didn't need to replace the seleniums, but I was ruling out all causes.... BTW, 300Ω dropping resistors did the trick.

A final note for the other less experienced folks (like myself)...I only found the ground issues when I went back to check the AC signal voltage, and used a different ground connection point than previously used, and found markedly different voltages...

I had previously just checked simply continuity, but the voltages didn't lie...after cleaning the same signal voltage at all ground points.

So, tester works well.

Interestingly, a note that everyone here already knows....in testing a 6AG7, I noted that one of the batch I had shown a bright shorts light...I re-checked it on my BK 707...sure enough shorts light on..

Not to be obsessive, I tested it on my TV-7 and Precision 10-12...no shorts....

The answer came when I pulled out my 666...excessive H-K current! (it was the only 6AG7 in the batch where this happened...)

So, as many of likely know, it is function of what to set the H-K leakage threshold at...the 675 is set at 1MΩ fro all shorts, the TV-7 is around 100KΩ.

See what one can learn!

Steve

I had the exact same thing happen with a GEC 6as7g last week and wanting to keep it I sent it out for a check in my buddy's Hickok's and whatever else he had...it passed, no shorts, so whatever that H-K deal is, maybe the same for me, I copy and pasted what you posted for him to translate in layman's terms. :D


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 Post subject: Re: BK 650/675 Mutual Conductance
PostPosted: Oct Mon 26, 2020 5:08 am 
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There are people here WAY smarter than I about his topic, but current flowing between the heater and the cathode happens (as a result of the effects of heating and proximity, as wella s capacitance).

My understanding is that the resistance necessary to prevent the current flowing between the K and H is set by the tester manufacturer...In the case of the TV-7 it is somewhere around 100kΩ, and teh 675 is adjustable and I set it at the recommendations of the maker at 1MΩ,

So, the 675 will light with far less K-H leakage than the TV-7. The 666 reads the actual resistance.

But, it doesn't mean a dead short exists.

I was just wondering what the "bar" is...would hate to toss a tube that doesn't need to be tossed...

Steve


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 Post subject: Re: BK 650/675 Mutual Conductance
PostPosted: Oct Mon 26, 2020 6:27 pm 
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drworry wrote:
There are people here WAY smarter than I about his topic, but current flowing between the heater and the cathode happens (as a result of the effects of heating and proximity, as wella s capacitance).

My understanding is that the resistance necessary to prevent the current flowing between the K and H is set by the tester manufacturer...In the case of the TV-7 it is somewhere around 100kΩ, and teh 675 is adjustable and I set it at the recommendations of the maker at 1MΩ,

So, the 675 will light with far less K-H leakage than the TV-7. The 666 reads the actual resistance.

But, it doesn't mean a dead short exists.

I was just wondering what the "bar" is...would hate to toss a tube that doesn't need to be tossed...

Steve

I'm told Hickok consider 2 megohms to be ok, so that might be the limit, some guys don't use tubes below 3 megohms. My GEC tested at 5 Megohms so not high enough of a leakage. So my question is if BK measure a 1 Megohms resistance as fail... Isn't that still lower than 5 Megohms? O.o


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 Post subject: Re: BK 650/675 Mutual Conductance
PostPosted: Oct Mon 26, 2020 7:02 pm 
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drworry wrote:
There are people here WAY smarter than I about his topic, but current flowing between the heater and the cathode happens (as a result of the effects of heating and proximity, as wella s capacitance).

My understanding is that the resistance necessary to prevent the current flowing between the K and H is set by the tester manufacturer...In the case of the TV-7 it is somewhere around 100kΩ, and teh 675 is adjustable and I set it at the recommendations of the maker at 1MΩ,

So, the 675 will light with far less K-H leakage than the TV-7. The 666 reads the actual resistance.

But, it doesn't mean a dead short exists.

I was just wondering what the "bar" is...would hate to toss a tube that doesn't need to be tossed...

Steve

Was told that 2 Megohms is the bar with Hickok. Some say 3 megohms, would it hurt the BK to calibrate to 3 Megohms instead of the 1k seems awfully low.. don't you agree??


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 Post subject: Re: BK 650/675 Mutual Conductance
PostPosted: Oct Mon 26, 2020 7:05 pm 
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Don't miss the point here. Testing for leakage may make rejection or acceptance easy but the bottom line depends on the circuit. If the circuit doesn't present much voltage between cathode and heater, the leakage is unimportant. For more sensitive applications this may not be true.

I never saw a specification for leakage so I assume this is more of a rule of thumb than a true test of a tube. I have found, in an AA5, hum caused by leakage. That's one reason all the circuits place the detector/first audio heater at the ground end of the string.

Bob


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