[E55l2]

Steve

Your books are bad or you didn't wear your glasses i don't know but the information is very easy to find if you use google.

http://frank.pocnet.net/other/RCA/RCA_Special_Red_Tubes.pdf

The fairytails about Telefunken.....

Btw i use a modern tubetracer (calibrated, stabilased powersuplies) not an old tubetester.

Your first sentence above is absolutely ridiculous except +/- 3% for Thoriated Tunsten filament types, and for pulse types that need proper filament voltage to prevent space charge depletion, and possibly horizontal power output types in TVs. See my previous posts.

Please specify, post links, where you actually get your bits of information as none of my RCA manuals, whether receiving manual or transmitting manual, RCA Radiotron Designers Handbook (26 engineers, 1400 pages on tubes, 1960), Eimac, Radio Amateur Handbook, Radio Handbook (750 pages) etc state such.
By the way, Telefunken was known for getting 100,000 hours on their 12ax7 "family" under typical conditions. Now if someone/company is running the tubes at maximum ratings (some do), life will be shortened.

I also had multiple Tektronix 535A scopes and tubes (50 years old, still have one scope left) that check/measure within normal range, using actual circuitry/typical manual conditions to measure specs, not a "tube tester". So I have trouble believing your information.

Cheers
Steve

[JakeJ]

Man, it sure would be nice if you guys could just have a discussion about tubes without getting emotional and insulting!

OK, so what if someone wants to run a 7DJ8 in place of a 6DJ8?!  It their equipment and their choice.  Yes it will effect performance and tube life, as long as the mfr. says it's OK and they are aware of it then it is all good.

 

[ACHiPo]

Steve

Your books are bad or you didn't wear your glasses i don't know but the information is very easy to find if you use google.

http://frank.pocnet.net/other/RCA/RCA_Special_Red_Tubes.pdf

The fairytails about Telefunken.....

Btw i use a modern tubetracer (calibrated, stabilased powersuplies) not an old tubetester.

E55,
You are out of line, and this discussion has gotten off course.

AC

[Steve]

Hi Jim,

I am sorry I misunderstood your question. I will attempt to reply in order.

Let me try again.
I know if the idle plate/cathode current of each section of a dual triode 6922 tube is not the same or close to the same it is not possible to bias the driver tube in the ARC Vt50 amp per ARC specs.

Correct since there is only one adjustment for both sections.

I think I understand the reason why when looking at the VT50 circuit design of the driver tube. That's why I asked is Ohms Law was at play. E=IxR

VT50 schematic wiring diagram.
http://www.audioresearch.com/ContentsFiles/VT50_SchemPL.pdf

If you look at the VT50 schematic wiring diagram for the driver tube you will see +330Vdc feeds each plate voltage drop 37.5K resistor of the dual triode tube.

Each plate set point plate idle voltage is +170Vdc ideally, best world.

A trim pot, RV3, circuit is used to vary (raise or lower) the negative cathode bias voltage of each section of the triode tube through a 681 ohm cathode resistor connected to each cathode of the tube which raises or lowers the +DC plate voltage measured at each plate. As you can see from the design of the circuit individual control of the +DC plate voltage is not possible. RV3 raises them both up or both down. 

Correct Jim. Just the one control for both sections.

And there is the rub. Unless each section of the tube's plate/cathode current, load, is balanced, (closely matched), you can adjust RV3 that will set the plate voltage of one section of the tube  at +170Vdc but the other plate of the other section of the tube could measure, +190Vdc or +150Vdc, for an extreme example. 

Again correct Jim. You are doing fine.

The closer the match, balance of the plate/cathode current the smaller the spread of the two plate voltages will be. (Note: even in the extreme case above if the section plate voltage was adjusted by RV3 to +170Vdc and then the other ended up +150Vdc it could a problem and take out a 6550 tube or blow a screen resistor because of the design of the VT50. Not to mention the performance of the channel and sound of the channel.)

I would not necessarily say that since coupling capacitors block DC from the output tube grids. However, if a voltage pulse occurred, there could be problems. 

So, where does close matching of GM come into play? If each section of the tube GM are several hundred MICROMHOS apart will this cause/affect the plate bias voltage adjustment of the driver tube in the VT50 as described above? 

Although not perfect indicator, the Gm plays a part when checking the specs because it tells you that if all the voltages applied to both sections are the same the Gm will be identical/the plate current will be identical in both sections.

 

Or was it the used tubes I tested that that were closely matched just better selected for balanced/matched for plate/cathode current for each section and was not the close MICROMHOS readings I tested on the Hickok 6000A tube tester at all that yielded the close differences of the plate voltages of each section of the driver tube? If that is the case then the vendor that sold me the last quad of closely matched low noise tubes did not select/find a closely balanced/matched tube/s with tube sections of each tube with equal plate/cathode current measurements.
Your thoughts?

The Micromhos should have been the same if the sections are matched. It also depends upon the accuracy of the Hickok per section. If the Hickok is off by only 1% between sections, 12,500 umhos would read 12,625 or 12,375. 2% would be twice the error.

I hope I understood your questions accurately. If not, I can give it another try.

Cheers
Steve

 

[Steve]

Steve

Your books are bad or you didn't wear your glasses i don't know but the information is very easy to find if you use google.

http://frank.pocnet.net/other/RCA/RCA_Special_Red_Tubes.pdf

The fairytails about Telefunken.....

Btw i use a modern tubetracer (calibrated, stabilased powersuplies) not an old tubetester.

The only reason I am posting is because when I posted using 6.0 volts in place of 6.3 volts, my competency was questioned.
6.0 is 95.238% of 6.3 volts. Is that not within 5% which you yourself quoted?

Next, the books were by RCA as well as other manufacturers.

Next, I quoted from RCA sources.

From the RCA Radiotron Designers Handbook, 26 engineers, 1960:

Under normal conditions a valve should be operated with its filament or heater at the recommended voltage; in the case of an oxide coated valve it is possible to have fluctuations of the order of 10% up or down without seriously affecting the life or characteristics of the valve........

If the filament or cathode is operated continuously with a higher voltage than that recommended, some of the coating material is evaporated and permanently lost, thus reducing the life of the valve. Moreover, some of this vapour tends to deposit on the grid and give rise to what is known as grid emission when the grid itself emits electrons and draws current commonly known as negative grid current.......

If the filament or heater is operated for long periods at reduced voltages, the effect is a reduction in emission, but no damage is generally done to the valve unless the cathode currents are sufficient to exhaust the "space charge." Low cathode temperature is, therefore, permissible provided that the anode current is reduced in the proper proportion.

As noted above, under normal conditions, +/- 10% will not deplete the space charge (typical plate current is generally listed in the specifications sheet), and with less than typical plate current, 5% is just fine. 17% or more is another matter that must be weighed carefully. At less filament voltage, the carbonates evaporate less, so grid contamination is less over time, a plus.

Directly heated tubes with common cathode/filaments are a different story. Thoriated tungsten filaments with plus 3% increase voltage will increase emission by some 20%, while reducing cathode life by some 50%. Conversely, lowering the filament voltage by 3% will decrease emission by some 20%, but extend cathode life by some 50%. (Eimac Care and Feeding of Power Grid Tubes)

Next, let's check your source for information.

http://frank.pocnet.net/other/RCA/RCA_Special_Red_Tubes.pdf


From the link, pdf:

They are for industrial applications where 10,000 life, rigid construction, uniformity, and stability are paramount.

Included in each tube's discription:

RCA-XXXX is a XXXX-mu twin triode designed and manufactured for critical industrial applications. In such service, it is particularly useful as a voltage amplifier.

Again, from the pdf link:

This type has its heaters for the two triode units connected in series so that failure of either heater in bridge circuits makes both units inoperative.

Notice the extremely critical applications, including uniformity, and voltage amplifier, bridge which includes DC amplifier. Those conditions are extremely critical, and measurements must be absolutely stable if high accuracy is to be obtained. If the plate voltage were to deviate .05 volts, .1 or 1 volt, that could be catastrophic in voltage measurements. However, in an audio amplifier, it be meaningless, unless possibly in a dc coupled as Achipo's schematic presents. Even then, Audio Research used a coupling capacitor between the driver and output stage so little problem.

Secondly, one cannot make the claim that this condition is present with all vacuum tubes. Many of the 5691/92/93 tubes were manufactured, but only a few make the cut as the qualities are so critical. There are many rejects. It also depends upon whether for military vs commercial application. So what might be rejected in one arena is not rejected in another. There is little tolerance for minute changes, as such would eliminate the tube from use.

One must also consider that to keep the tubes matched, the filaments/cathodes need to be consistently and uniformly operated. That means if one tube is operated under different conditions for a period of time, the tubes will not be matched.

Again, from a previous post of mine:

There is no problem supplying 6.0 volts on a 6.3 volt filament in virtually all cases. However, there are extenuating
circumstances involving heavy plate current and increased frequencies into the high MHz, and depending upon the
tube, in which transit time becomes a problem.
 
1) Consider that the plate current draw means electrons leaving the cathode, which cools the cathode.
(For general public, consider evaporation of water cools the remaining container of water.) The lower
the plate current, the less the cathode is cooled by electrons leaving. I run my tubes in the 11A less
than typical plate current.

6.0 volts is 95.238% of 6.3 volts, less than 5%, so why the negative reply?

I am not taking sides, just providing one piece of information that some manufacturers set the filament voltage to 6.0 volts.

I agree with Jake, there is no need for defaming. I am not taking sides on 7.6 volt filaments being used in 6.3 volt circuits. By the way,
1000 hours life is based on maximum tube ratings.

In closing, one must understand all the parameters of how a tube works, and under what conditions before applying
a singular comment from a specification sheet as all encompassing. Otherwise, one will misapply the information.

ps. The 12ax7 can last much longer than a 569x tube because the plate current draw is much less for a given cathode size. Purity of materials
also enhances life. Telefunken was famous for their 12ax7.

Cheers and no hard feelings. 
Steve

[E55l2]

Pardon me?

Someone say it is ridiculous what i say and that no books/ datasheet whould ... Etc. Etc...Don't blame me, I am just the messenger.


Btw, is there any evidence from any tube that last 100000 hours! No! Just 80000 for a thoriated tungsten tube and oxidecoated tubes have shorter life........

E55,
You are out of line, and this discussion has gotten off course.

AC

[FullRangeMan]

Pardon me?

Someone say it is ridiculous what i say and that no books/ datasheet whould ... Etc. Etc...Don't blame me, I am just the messenger.


Btw, is there any evidence from any tube that last 100000 hours! No! Just 80000 for a thoriated tungsten tube and oxidecoated tubes have shorter life........

Steve are great tube expert, but I will agree
w/55, not to mention the equipment fanfare.
This thread is usefull to future tech references even
with occasional raids, what are quite normal among
users on internet free land. I would like specially thanks
55 for be ever ready to generously share his knowledge.

Your first sentence above is absolutely ridiculous except +/- 3% for Thoriated Tunsten filament types, and for pulse types that need proper filament voltage to prevent space charge depletion, and possibly horizontal power output types in TVs. See my previous posts.

Please specify, post links, where you actually get your bits of information as none of my RCA manuals, whether receiving manual or transmitting manual, RCA Radiotron Designers Handbook (26 engineers, 1400 pages on tubes, 1960), Eimac, Radio Amateur Handbook, Radio Handbook (750 pages) etc state such.
By the way, Telefunken was known for getting 100,000 hours on their 12ax7 "family" under typical conditions. Now if someone/company is running the tubes at maximum ratings (some do), life will be shortened.

I also had multiple Tektronix 535A scopes and tubes (50 years old, still have one scope left) that check/measure within normal range, using actual circuitry/typical manual conditions to measure specs, not a "tube tester". So I have trouble believing your information.

Cheers
Steve

[Steve]

I feel for those who have spent hundreds of hard earned dollars on tubes and will be paying an extra price because of unnecessary early tube failure due to those preaching a "message" which they did not understand. -5.0% or more will prolong the life of those expensive tubes, even more with just a few minor precautions listed below. (However, not if one is running the tubes at near maximum plate current ratings like some design around.)

Sometime back I worked on an old tube type Scott receiver and some of the original small signal tube filaments were connected in series. The Scott engineers placed the filaments in the mid 5 volt range by design.

One cannot take a comment/data out of a specific context as I have repeatedly explained in my previous posts, and claim it as fact in other and all applications/conditions. That is not science. The applications mentioned in the pdf on three 569X tubes are specific for extremely tight industrial and military conditions for measurements etc. Back in the day of tubes, critical industrial and military applications required some military grade tubes being removed after 100 hours of use. That is how critical some applications were. Yet those extreme conditions are ignored when it comes to these 569X tubes.

Unfortunately, typical purchasers of manufactured equipment won't be able to measure and alter the circuit context as is. However, diyers may do much more.

As mentioned before, but ignored, is the fact that thoriated tungsten filament voltage dropped by 3% will have a 50% longer tube life. Indirectly heated filaments will also have increased life by:

1) limiting the cathode current, to be determined by each type/tube number
2) reducing the plate voltage when possible so as to limit ion velocity towards the cathode. 
3) reducing the filament voltage, lack of space charge around the cathode being the limiting factor, which is nearly impossible.

As an interesting side note, the Telefunken 12ax7 has many times the cathode unit area/cathode current vs any of the three 569X tubes listed in the pdf. Yet the claim appears to be that the Telefunken 12ax7 life is limited, when the three 569X tubes, each rated 10,000 hours. That does not equate to real science demands.

Another hint is to keep the tube pins as cool as possible to keep the integrity of the seals.

I just hope those diyers who are spending mega bucks on tubes, or any tubes; please design, if possible, using the appropriate voltages on the tubes for maximum life.

Cheers
Pos

[G Georgopoulos]

if you go back in time things get simpler,so tubes being older than solid state are simpler in nature...

[FullRangeMan]

Steve what cathode tension you would suggest on the 6SN7,6SL7 and 6C33(12V parallel)?

[Steve]

Steve what cathode tension you would suggest on the 6SN7,6SL7 and 6C33(12V parallel)?

As I mentioned in my previous posts, it depends upon the design implementation, which includes the plate voltage, plate current, plate dissipation, cooling etc.

I have not used the first two listed tubes for at least 35 years. I have never tested or used the Russian 6C33.

Cheers
Steve

[FullRangeMan]

As I mentioned in my previous posts, it depends upon the design implementation, which includes the plate voltage, plate current, plate dissipation, cooling etc.

I stopped used the first two tubes in decades. I have never tested or used the Russian 6C33.

Cheers
Steve

The schematic is this, seems Bias is 180mA:

The power trafo(1 per channel) details are these:
Ogonowski TS400W EI120/73  core annealed, laminations 0,3mm thick.
7 Kg each, Input 220V.
Secondary are:
0 - 160V  x 1Amp  /
0 -  80v  x 200ma  /
6,3 V x 3Amp   /
0 - 300V  x 200ma /
12,6V x 7Amp

[E55l2]

Give the 6c33 some time to heat up before you switch the hv on. Its a slow starter.

[Steve]

The schematic is this, seems Bias is 180mA:

The power trafo(1 per channel) details are these:
Ogonowski TS400W EI120/73  core annealed, laminations 0,3mm thick.
7 Kg each, Input 220V.
Secondary are:
0 - 160V  x 1Amp  /
0 -  80v  x 200ma  /
6,3 V x 3Amp   /
0 - 300V  x 200ma /
12,6V x 7Amp

Ok, you set the bias for -65 volts, give or take, for 180 ma plate current and probably around 200 actual plate volts. At 225 ma, the bias is approximately -60 volts at approximately 183 plate volts. That is less than 250 volts for above 30 watts plate dissipation, which is good. Maximum plate dissipation is listed as 60 watts. The tube takes a maximum of 2 minutes to warm up at rated filament voltage.

1) I would suggest a regulated dc filament voltage so the AC line voltage variations become a non factor. Adjustable would give flexibility. Of course it is much easier for the 6sl7 than the output tubes.

2) It appears that the maximum cathode current is approximately 600 ma for both sections operating. Let's go ahead and assume maximum to be safe.

3) A good place to start is to set the filament voltage to -5 to -7% of 12.6 volts, or 11.8 to 12 volts DC after filament warm up. Let's leave it there for now. To go any further with the filament voltage, I would have to personally test the tube in my lab. Filaments will take a few seconds longer to completely warm up at lower voltage.

4) One can either use a separate switch to power the high voltage after the filament is warmed, or a simple RC FET relay combination for approximately
2 1/2 minute delay. It is not really critical since the space charge is already setup.

You will increase the tube life vs rated filament voltage.
 
Cheers
Steve

[FullRangeMan]

Give the 6c33 some time to heat up before you switch the hv on. Its a slow starter.

Well said this is the correct way to drive a tube aside with 2 power on switches.
Switch 1 cathode tension
Switch 2 HV-grid & plate

[FullRangeMan]

Thanks Steve for your interesting insights comments,
its a splending idea if we can extend the tube life.
I will copy your comments to forward to my tube builder.

I like this schematic:
-its not a hi power project
-Seems a early (1990s) Jap 6c33 project
-its a minimalist schema w/just 2 tubes
-I already was pointed to interaction problems at the inside 6SL7 triodes with the gain and drive being the same tube, but I dont know what it will do in the sound or in the amp life.

I just remember lower the cathode tension are a usual procedure to ham radios operators after the idle TX moments, as those big tubes are expensive.
These radios have a voltage pot to proper cathode tension.
Regards

[E55l2]

Did you realised that the transconductanece (S) of the 6c33 is very  high?

[FullRangeMan]

Yes I know transcondutance is very hi, but I cant read schematics.
More I dont know what a hi S could do in the power position.
Do you think this schematic did not take it into account?
Fell free to point any prob in this project.
Thanks

[vonnie123]

Unbelievable!! and you are happy with one sole year of life.
This could be as little as 500 to 1000 hours of use.

CD players are know to put very little stress on tubes.
This CDP have soft start circuit?

The AH player smoked itself due to some electrical issues in my system, and the fuse protection in the modification (Marantz CD4000 was the base player for the AH!) was less than "protective."  I now use my 6DJ8 tubes (pair) solely in an Audio Horizons tube buffer.  I think 2500 hours is probably closer to the lifespan of the tubes mutual conductance.  So a year at 6-8 hours a day would probably still hold water.  I listen nearly every night for several hours.  The music vibrations makes my snifter of rye, bourbon, or cognac taste a bit smoother.   Likely to get at least half again as much in hours out of a 6922 (versus a 6DJ8).

I lucked into several pairs of 6DJ8 Bugle Boys for almost nothing.  Some tested almost new, most had moderate life left on them per my 1955 TV7 tube tester.  My brother found them for me in a junk surplus store in a bin after they had been removed from some old textronixs scopes.   $1 each.   

[Steve]

Yes I know transcondutance is very hi, but I cant read schematics.
More I dont know what a hi S could do in the power position.
Do you think this schematic did not take it into account?
Fell free to point any prob in this project.
Thanks

No worries Gustavo, transconductance (Gm) has no bearing on setting either the plate current or filament voltage. The plate current parameters are
established by the load line the designer uses. No worries, the requested critique I gave of the design via PM is accurate and what needs to be taken into account has been taken into account by the designer. 

Gm = Mu/Rp       Mu is the amplication factor
                          Rp is the plate resistance of the tube
                          /  means divide by

The transconductance (Gm) is high because the Mu is low, approximately 2.3, and the plate resistance is very low, only approximately 100 ohms give or take. It means that with a given plate voltage, a change in grid voltage will result in a large change in plate current. It does not influence the peak plate current, or the average plate current. The loadline determines the peak plate current, via the plate voltage, OPT and load. Of course one needs to stay within the plate dissipation and keep harmonic distortion low.
 
In SET operation, the high idle current reduces the primary inductance, so the transformer needs to account for this.

Cheers
Steve

ps. Just remembered another use for Gm, the equation to figure the figure of merit for pentodes (basically most tubes).
Av bandwidth = Gm/2Pi X Cs       where Cs = Cpk1 + Cgk2.

For two tube circuit.
Cpk1 is plate to cathode capacitance of the X tube (previous tube).
Cgk2 is the grid to cathode capacitance of the 2nd tube Y.
Keep stray capacitance to an absolute minimum.