[corndog71]

The instructions that came with my amp (not one of Roger's) recommend setting the bias at .55VDC for KT88, 6550 and even KT120's.  Some run their KT120's between .60-.65VDC.

I've been running my 6550's lower at .40VDC with no problems.  I tried searching this forum for clarity on where the ideal bias setting is but am having some trouble.  I saw one post where Roger recommended .30VDC for EL34 type tubes.

So my main question is what is the proper bias voltage for: 6550, KT88, KT120 type tubes? 

[Ericus Rex]

I believe Roger's bias numbers were for current (mA) not voltage.  I seem to remember him recommending 30mA for EL34 and 40mA for KT88/6550.  My memory could be faulty.

[rbwalt]

i have a music reference RM9SE. it has test points on the top of the plate. these allow me to measure in mV the bias of the tubes( each one).  they should measure around 15 mV  each with a range of 12-20 mV. i run EL34's but the amp can use 6550's, KT77's, KT88's. so in pairs the measure is around 30mV with a range of 24-40 mV. if you run high bias then you will have a shorter tube life. i run my tubes at around 28 mV for each pair. i can adjust for balance and bias of the output tubes.

now roger states for my amp i can run 6550's and KT88's at 40 t0 60 Ma per tube for more of a class A operation. Multiply the current by 500 volts to get dissipation. max for EL34's are 25 watts, 6550's are 30 and KT 88's are 35 if they are good ones. full dissipation limits tube life to  about 1000- 2000 hrs. half dissipation 5000 to 10,000. life vs. dissipation is not at all linear. i think you can use this infor as rough guide for your application.


hope this is of some help.

[Captainhemo]

Isn't 15 ma (mv from ohm's law)  kind of low for an EL34 ?  At a plate  voltage of  500V that would  give you  only 28% disapation .   30ma / tube would give you 56% which would seem  more "normal " .

I'm probably missing something here , just need a bit of clarification ...


-jay

[rbwalt]

mV x 2 gives mA.. easier to test mV than mA.

[rbwalt]

here is the dissipation and power values for a RM9MK-ll and a RM9SE.

MKll:

power:                                Dissipation

8 ohms- 128W                         100W
4    "   - 65W                            156W
2    "   -  40w                           200W





RM9SE:


power:                                  Dissipation:

8 ohms - 162W                           88W
4   "     - 200W                           95W
2    "    - 128W                           110W

[corndog71]

I don't have a Music Reference amp.  I have a stereo version of the Dynaco MkIII.  VTA ST120 to be exact.  In the manual it recommends .55VDC measured from the cathode through a 10R resistor to ground.  This is for 6550/KT88.

I thought I read somewhere that Roger recommends a lower bias setting for power tubes.  He also mentioned a lot of people/companies run their tubes hot which shortens their life.

[Captainhemo]

mV x 2 gives mA.. easier to test mV than mA.

would the mV not have to measured across a .5 ohm resistor for thie above to be true ?
I=V/R

or am I missing something very obvious

[rbwalt]

correct on the .5 ohm resistor.

[Roger A. Modjeski]

I don't have a Music Reference amp.  I have a stereo version of the Dynaco MkIII.  VTA ST120 to be exact.  In the manual it recommends .55VDC measured from the cathode through a 10R resistor to ground.  This is for 6550/KT88.

I thought I read somewhere that Roger recommends a lower bias setting for power tubes.  He also mentioned a lot of people/companies run their tubes hot which shortens their life.

There is a good discussion about bias going on now.  http://www.audiocircle.com/index.php?topic=126026.0

In reading these posts it is apparent that we have to know the value of the resistor that the current is flowing through and use Ohm's law to calculate the idle current.  Most manufacturers don't put these details in their manuals, they often just tell you what to measure in mV and I hope they tell you to set your meter to mV, not mA as some owners have done. In the RM-200 the measuring resistor is 1 ohm, in the RM-10 it is 10 ohms, in the RM-9 is it 15 ohms and in the RM-9SE there are a combination of 0.5 ohm resistors that allow measuring each tube and also setting the balance.

Therefore, in the future we should be discussing the idle currents for our amps if we want to discuss this properly. Also keep in mind that if one wants to calculate dissipation one needs to know the plate voltage which varies from amp to amp. Although many amps including the RM-9 and 9SE run at 400-475 volts the RM-10 and RM-200 run near 700 V.

Although the industry and users have adopted an incorrect language for bias, we should try to do it properly. Once terms begin to be used casually they lose their meaning and cause confusion. I invite everyone here to start using the correct language and teach others to do so also. Using the correct terms demonstrated a higher level of understanding.

The correct terms are:

Bias is the negative voltage we put on the control grid of a tube to make the tube conduct the current we desire. Withous this negative bias the tube will conduct its full possible current and burn up very fast. Typically this is negative 12volts for a EL84, -35 for a EL34 and -50 volts for a KT 88. The new KT-150 appears to be a low bias tube but the specs are very short and unclear as they only list one application.

By the way the bias we put on a transistor is about 0.6 volts and on a MOSFET about 4 volts, both positive.

Idle current is the current we desire to flow in the tube or transistor with no signal.

[corndog71]

I've been following these recent posts and have learned a lot.  I appreciate your efforts with using the proper vocabulary.   I understand different amps create different variables with respect to idle current.  That said there still seems to be a consistent range of idle current for each power tube. 

I understand that some manufacturers run their idle current high because it imparts a particular sonic quality.  Your amps conversely run with a lower idle current for the purpose of longer life of the tubes. 

My amp kit is essentially an oversized ST70 built to use 6550/KT88 tubes.  (thus named the ST120). It uses 10ohm resistors on each tube for calculating idle current.  In the kit instructions the idle current should measure .550VDC for each tube.  Does that seem high to you? 

[Roger A. Modjeski]

I've been following these recent posts and have learned a lot.  I appreciate your efforts with using the proper vocabulary.   I understand different amps create different variables with respect to idle current.  That said there still seems to be a consistent range of idle current for each power tube. 

I understand that some manufacturers run their idle current high because it imparts a particular sonic quality.  Your amps conversely run with a lower idle current for the purpose of longer life of the tubes. 

My amp kit is essentially an oversized ST70 built to use 6550/KT88 tubes.  (thus named the ST120). It uses 10ohm resistors on each tube for calculating idle current.  In the kit instructions the idle current should measure .550VDC for each tube.  Does that seem high to you?

Thanks for your compliments on teaching this topic. It is one of the most mis-understood of the popular discussions I see on many forums.

I looked up the ST-120 and found the  tubes4hifi amp and I assume that is the one you have.

Your idle current works out to 55 mA.450-500V.  Converting 55 mA to amps we have 485 V x 0.055 amps = 26.7 watts, well within the 35-40 watt rating of the original Genelex KT-88.  You should be just fine with that operating point.  What brand tubes are you using? Some modern KT-88s are not reliable above 30 watts and should be down-rated. I have done in depth research and re-rated several tubes to where I think they are safe. A 30 watt KT-88 should really be run at 15 watts if you want the long life. Not everyone cares about long life, we should just keep that in mind. Many modern KT-88s don't deserve the 40 watt rating. Some manufacturers say "if it looks like a KT-88 then it must also have the rating of a KT-88" However they don't consider the fact that the originator of that tube did some very special things and used some special materials to get that performance.

One of the things I would like to do in my school, if I could find a student to do it, is to do the research on a sample of modern tubes and publish the results. There are so many factors that influence life that warrant a study. Although Sylvania, RCA, and GE did life studies I doubt any of the current manufactures does. In the heyday of tubes, which was entirely driven by color TV (re my discussion with Sylvania) the tube makers were dealing with the TV manufacturers not the end user. The TV maker wanted a quality TV (Zenith and RCA leading that) that needed only occasional servicing. That's why those companies really worked on tube life. When the transistor took over color TV that goal was lost.

A few things about ratings. Very few, if any, tube manufacturers stated the life of the tubes they made at full dissipation. When I visited the Sylvania factory I asked the question "how long will your tubes run at full dissipation" the chief engineer of 25 years said "Well you aren't supposed to run them at full dissipation because they will only last about 1000 hours. If you run them at half dissipation they will run 10,000 hours." Now Sylvania made some of the best American tubes and he also told me that their goal was to make a color TV run 10 years at 3 hours a day with their tubes and that they had pretty much achieved that if the designer of the set ran things properly. Unfortunately life vs dissipation is never given in any data book I have seen. I was lucky to have the Sylvania enginieer share that information early in my career so I didnt make the mistake some designers make and run the tubes full out.

This is just a thought but many modern tube amplifiers are designed by people who have not thoroughly investigated tubes. Perhaps they think that tubes are like transistors. In essence transistors, properly heat sunk, can be run at full dissipation and not impact their much longer life. Without mentioning names, there are several modern amps of all nature (Transformer coupled, OTL, Single Ended) by many first time makers that have run tubes at full dissipation and learned their lesson, at the owner's expense in many cases. that full dissipation is a short road to unreliability. I have worked on many amps where only the strongest power tubes will make it past burn in on my bench.

This is why I run my tubes around 1/2 rated dissipation. I recently replaced one driver tube in an amp I had sold locally and serviced through its entire 25 year owner ship by a friend. In that period we only replaced one of the eight Siemens EL-34's and this driver tube only because it became noisy. Those remaining tubes 10 out of 12 in the amp have at least 20,000 hours on them. Compare that to ARC who recommends replacing output tubes every 1000-2000 hours.

[corndog71]

I looked up the ST-120 and found the  tubes4hifi amp and I assume that is the one you have.

Your idle current works out to 55 mA.450-500V.  Converting 55 mA to amps we have 485 V x 0.055 amps = 26.7 watts, well within the 35-40 watt rating of the original Genelex KT-88.  You should be just fine with that operating point.  What brand tubes are you using? Some modern KT-88s are not reliable above 30 watts and should be down-rated. I have done in depth research and re-rated several tubes to where I think they are safe. A 30 watt KT-88 should really be run at 15 watts if you want the long life. Not everyone cares about long life, we should just keep that in mind. Many modern KT-88s don't deserve the 40 watt rating. Some manufacturers say "if it looks like a KT-88 then it must also have the rating of a KT-88" However they don't consider the fact that the originator of that tube did some very special things and used some special materials to get that performance.

That's the one!  Although I built mine a little differently. 



Currently I'm using a trio of RAM 12BH7 driver tubes and a set of new issue Tung Sol 6550s from Upscale Audio.  So far I'm pretty happy with their performance.  Sometime this year I plan to get a set of RAM KT120s as I've heard a lot of good things about that tube.

I have a better understanding now of how to calculate dissipation.  Though if I'm understanding what you wrote it seems the specified dissipation rating of a given tube may not be accurate due to manufacturing variables. 

Just to clear up one more point.  Is that 450-500V measured at the plate?  If so then perhaps having a test point tied to the plate would be beneficial to better calculate dissipation.  It seems knowing the actual dissipation would be better than the idle current.

[Roger A. Modjeski]

Nice job on the amplifier. It appears you have test points for each tube. How many bias pots are there? Was the tube matching good?

The B+ at plate is nearly the same as the B+ at the center tap of the output transformer, the main B+. Once known it would only change if your line voltage changed. It's more important to know how your idle current varies with line voltage, in some amps it varies markedly as I stated before.  You would need a variac to vary the line to see what happens.

[corndog71]

There's a pot for each tube.  I haven't checked the dissipation yet but the idle current has been holding steady for several months now. 

Since I changed the amp to solid state rectification my B+ ended up being higher than that listed in my kit instructions.  So I've actually been using a variac to drop my AC from the 125 it usually is down to about 117VAC.  That brought the numbers in the right ballpark.

[Roger A. Modjeski]

With ultralinear amps  you really have to respect the B+. I find above 470 V is pushing it. Do check your B+. Good that you have a variac. I wish everyone did.