Creating bias sheets could use help with voltages to include.

[Mike_J]

I find it slightly unenjoyable to measure plate and cathode voltages, pick up my calculator to calculate dissipation, then go adjust the bias pot and it is all over again. So I decided to create some dissipation charts to expedite the process. Have a nice leather binder to put it in with nice plastic inserts.


The first thing in it that I put in it many years ago is Hoffman's explanation as to why he increased the bias ecap value and voltage to 47uF/100v. Probably should have read that yesterday. It is in a very nice leather binder in a nice plastic insert just apparently the binder doesn't get opened much.


My latest expedition into trying to simplify amp building is the creation of a dissipation chart by voltage at 50, 60 and 70% dissipation. I thought I would do charts for the 5881 (see attached picture), 6L6GC, 6V6GT and EL34 since they seem to be the power tubes I use almost all the time. What I need help with is the voltage range to include. The purpose of these charts is to give a quick glance idea of what the mA reading should be for the tube at the voltage it is showing.


I can do the math of plate voltage minus cathode voltage in my head on a good day. However, don't want to include voltages that are unrealistic because they make the graph a little harder to read. If anyone could share what they think are good voltage ranges for the various tubes I would appreciate the help.


Thanks
Mike

[pdf64]

I think you’d be wasting your time, it seems something of a nonsense to me.
I’m not aware of any technical imperative or even benefit for idling different power valve type to a particular target % of their anode’s limiting dissipation.
The way it’s presented on some sites gives the impression of a scientific methodology that doesn’t exist.
The 3 different rating systems used for deriving anode dissipation limits, at least 2 of which are used for the types noted, somewhat makes a mockery of the concept.

5881 is a 6L6, all 6L6 types have the same characteristics, so what’s the reason for idling one variant hotter than another, other than ‘because a higher dissipation rating allows me to do so’.

It seems to me it would be a more valid comparison to idle all types at the same anode current?

Anode dissipation limits are just that, a limit; in and of itself, some or other particular % of the limits doesn’t typically constitute the basis of a power stage operating point.

[2deaf]

What about the screen voltages?

[Mike_J]

I think you’d be wasting your time, it seems something of a nonsense to me.
I’m not aware of any technical imperative or even benefit for idling different power valve type to a particular target % of their anode’s limiting dissipation.
The way it’s presented on some sites gives the impression of a scientific methodology that doesn’t exist.
The 3 different rating systems used for deriving anode dissipation limits, at least 2 of which are used for the types noted, somewhat makes a mockery of the concept.

5881 is a 6L6, all 6L6 types have the same characteristics, so what’s the reason for idling one variant hotter than another, other than ‘because a higher dissipation rating allows me to do so’.

It seems to me it would be a more valid comparison to idle all types at the same anode current?

Anode dissipation limits are just that, a limit; in and of itself, some or other particular % of the limits doesn’t typically constitute the basis of a power stage operating point.

It has been my experience that putting a pair of power tubes in an amp and starting to play can result in an undesirable outcome like smoke rolling out of the amp. It sounds very nice for a short while until the tube plates melt. I am not a tech or an ee but I do know enough that it is advisable to try to make sure the tubes aren't melting shortly after turning on a new build.


As such, a method should be established to ensure that this undesirable event does not occur. This is the method I know of for doing same. Just hoping for some ranges by tube.


A 5881 has a wattage rating of 23 watts whereas a 6L6GC has a rating of 30 watts. That is 76.7% of the wattage of a 6L6GC. Merlin suggests that tubes can be biased up to 85%. However, if I biased my 5881s at 70% dissipation and 420 net volts for a 6L6 it would be 50mA which is in excess of 90% of a 5881s dissipation capability. Seems a little on the edge to me.


It is not nonsense when a brand new set of matched NOS Tungsol 5881s go up in smoke. It is a bad experience. One I don't want to have again.

[Mike_J]

What about the screen voltages?

Huh? Could you elucidate me as to why I should be concerned about screen voltages while I am biasing my amp?

[2deaf]

Huh? Could you elucidate me as to why I should be concerned about screen voltages while I am biasing my amp?

The screen voltage has a dramatic effect on how much bias voltage is required to achieve a given idle dissipation when the anode-to-cathode voltage is held constant.

[66Strat]

Plate current to a great extent is a function of screen voltage as well as control grid voltage. As screen voltage is increased, plate current increases. Refer to pages 5 and 8 in the attached.

[Mike_J]

Plate current to a great extent is a function of screen voltage as well as control grid voltage. As screen voltage is increased, plate current increases. Refer to pages 5 and 8 in the attached.

Uncle, those charts look like lines on a page. How am I supposed to bias my amp? Doing nothing doesn't work well often.


Thanks
Mike

[Mike_J]

Huh? Could you elucidate me as to why I should be concerned about screen voltages while I am biasing my amp?

The screen voltage has a dramatic effect on how much bias voltage is required to achieve a given idle dissipation when the anode-to-cathode voltage is held constant.

Is the anode to cathode voltage held constant though? Been quite some time since I biased an amp but I seem to remember when the bias is adjusted the plate voltage changes. Could be misremembering but I don't think so.


Thanks
Mike

[pdf64]

...It has been my experience that putting a pair of power tubes in an amp and starting to play can result in an undesirable outcome like smoke rolling out of the amp. It sounds very nice for a short while until the tube plates melt....

It is not nonsense when a brand new set of matched NOS Tungsol 5881s go up in smoke. It is a bad experience. One I don't want to have again.

So idle them at a reasonable anode or cathode current.
When fitting valves to an amp, adjust the bias voltage to its greatest magnitude before powering up. When they’re conducting happily and a few high volume test strums don’t cause issues, remove your light bulb limiter from circuit and adjust bias to a reasonable level.

Your valves had a catastrophic failure incident because they were either bad or because they were idling ridiculously hot.
Not because they were idling 62.369% or whatever.
I suspect you may not have checked their idle anode or cathode current, but rather got stuck into enjoying your amp?
That’s a hard lesson to learn, but flipping from nooby negligence to this dissipation chart idea seems something of a baseless overreaction.

A valve type’s anode dissipation may be rated under (decreasing order) absolute max, design max, or design centre systems. Unfortunately only one of those ratings tends to get published, but which that is varies depending on era and region of manufacture.
eg 6V6GTA Aug 56 - 12W design centre https://tubedata.altanatubes.com.br/sheets/127/6/6V6GTA.pdf

6V6GTA Nov 67 - 14W design max https://tubedata.altanatubes.com.br/sheets/127/6/6V6GTA.pdf

There’s explanation of the different systems on the 1st couple of pages of http://www.one-electron.com/Archives/RCA/RCA-AppNotes/RCA%201958%20AN-174%20Design-Maximum%20System%20for%20Rating%20Electron%20Tubes.pdf

[Mike_J]

I will tell you exactly what happened to me. I put them in and tried a few strums and liked it and then started playing like I was playing a stadium. They weren't bad at all. As a matter of fact for a short period of time they were very good. You are right it taught me a lesson. Also taught my daughter a lesson. If I got excited about the tone an amp was making she would rush into the room with the fire extinguisher certain her father was trying to burn the house down again.


Suppose it is time for me to create a light bulb limiter. Been able to get away with the yellow highlighter twice through the schematic method which has worked for me for ten builds and I think is essential. Enough people talk about light bulb limiters that I probably should build one. Still think the charts are a good idea. Take very little time to create and are easy to look at and determine where I am at the time. Part of my CPA background was working with charts and graphs. Just seems natural. Otherwise I could be fiddling with a calculator while the plates are melting, again.

[2deaf]

Is the anode to cathode voltage held constant though?

It is for this example so that the effect is solely the result of screen voltage differences.

[Mike_J]

Is the anode to cathode voltage held constant though?

It is for this example so that the effect is solely the result of screen voltage differences.

I still am not sure what to do. How am I supposed to factor in the screen voltage while I am biasing my amp? Have always checked the screen voltage after I biased the amp to make sure it is less than the plate voltage which it should be but that is all I did with the screen voltage.


Thanks
Mike

[Shack]

maybe I am wrong, but I dont put much thought into it, I set the negative voltage on the grid in the area that is around right for tube type...and if all is good, I try to check dissipation the uncle Doug way....if its in the right area im happy, so most of my amps might run on the cool side....and sound good, so I just play.

[Mike_J]

This light bulb limiter looks as easy as taking an extension cord and cutting it in two pieces connect the hot wires from both ends to the light fixture and connect the ground a neutral wires and placing them in a secure electrical box. Anyone think the plan is what is needed? Have attached front and back pictures of what I found for the job. Only question left is it asks for a 75 to 100 watt bulb. Can you still buy those?


Thanks
Mike

[Mike_J]

maybe I am wrong, but I dont put much thought into it, I set the negative voltage on the grid in the area that is around right for tube type...and if all is good, I try to check dissipation the uncle Doug way....if its in the right area im happy, so most of my amps might run on the cool side....and sound good, so I just play.

I think you are right but I am still going to make my graphs. Will let me know if I am in the middle of where I need to be immediately. No calculator necessary.


Thanks
Mike

[PRR]

> What about the screen voltages?

That's not the question he is trying to answer! Note that G1 voltage is not on there either.

He want to avoid rapid melt-down. OK, do it like a dummy. See 440V no-load and expect that to sag under load, pencil 400V. Tube data says 23 Watts, which is an ugly number, round to 20W. 20W/400V is 50mA. Verify less than 50mA at start-up and through smoke test. If tested properly, VERY cold, you might expect under 20mA. But if it looks like 50mA or more, you gotta problem.

Tubes CAN stand 150% for many minutes. You keep an eye on it, but you don't go to jail for 24 in a 23 zone.

Once the initial excitement is passed, measure actual volts and current. In self-bias, 100% is proper. In fixed bias, anything goes; but generally <70% Pdiss will be hoarse at low levels, and >80% Pdiss idles HOT and runs some risk of redplating in extended half-power passages. There's no real need of a graph, IMHO.

[HotBluePlates]

... I decided to create some dissipation charts to expedite the process. ...

Whether or not you need them, kudos for taking the time to put them together!  If they make life easier for you, then it was time well spent.

It's worth noting at this point a lot of tube-math was done graphically in the old days, as it was sometimes faster, sometimes less error-prone, and very often easier to notice "optimum conditions" than with a table of numerical data.

I have to admit, PRR does mention some good shortcuts, though.  Probably a good idea to note those down, as I find myself "rediscovering" them as I learn to see the bigger picture.

[2deaf]

That's not the question he is trying to answer! Note that G1 voltage is not on there either.

Yes, you are correct.  I just glanced at it and totally misinterpreted it. 

[2deaf]

I still am not sure what to do. How am I supposed to factor in the screen voltage while I am biasing my amp? Have always checked the screen voltage after I biased the amp to make sure it is less than the plate voltage which it should be but that is all I did with the screen voltage.

I thought those numbers on the curves were bias voltages, but PRR set me straight.  I apologize for any inconvenience that I may have caused you.

[Mike_J]

... I decided to create some dissipation charts to expedite the process. ...

Whether or not you need them, kudos for taking the time to put them together!  If they make life easier for you, then it was time well spent.

It's worth noting at this point a lot of tube-math was done graphically in the old days, as it was sometimes faster, sometimes less error-prone, and very often easier to notice "optimum conditions" than with a table of numerical data.

I have to admit, PRR does mention some good shortcuts, though.  Probably a good idea to note those down, as I find myself "rediscovering" them as I learn to see the bigger picture.

Added 85% and 100% dissipation to my graph. All it does is tell me real fast if there is a problem. Problem may be the bias range resistor needs to be changed. This will tell me almost instantly if that is the case without having to calculate anything. Just the way I think. I am not a trained EE so what PRR is saying does not come naturally to me. Have to find a way that makes sense to me and the graph does. I will undoubtedly bias by ear and use the graph to let me know if I am getting into dangerous territory. Will just set my multimeter to DC and clip it across one of the one ohm resistors coming off the power tube cathodes. Will have a general idea of what the plate and cathode voltages are which will hopefully keep me out of a danger zone from a mV, really mA point of view.


Thanks
Mike

[Mike_J]

I still am not sure what to do. How am I supposed to factor in the screen voltage while I am biasing my amp? Have always checked the screen voltage after I biased the amp to make sure it is less than the plate voltage which it should be but that is all I did with the screen voltage.

I thought those numbers on the curves were bias voltages, but PRR set me straight.  I apologize for any inconvenience that I may have caused you.

You and pdf64 have always been very helpful to me and I very much appreciate it. This is a method that will work for me. Someone with a lot of experience in amp repair would have most of this in their head and wouldn't see a need for a graph like this. I don't so I am using it as a cheat sheet of sorts. Again thank you for all the help you have given me in the past.


Thanks
Mike

[Mike_J]

> What about the screen voltages?

That's not the question he is trying to answer! Note that G1 voltage is not on there either.

He want to avoid rapid melt-down. OK, do it like a dummy. See 440V no-load and expect that to sag under load, pencil 400V. Tube data says 23 Watts, which is an ugly number, round to 20W. 20W/400V is 50mA. Verify less than 50mA at start-up and through smoke test. If tested properly, VERY cold, you might expect under 20mA. But if it looks like 50mA or more, you gotta problem.

Tubes CAN stand 150% for many minutes. You keep an eye on it, but you don't go to jail for 24 in a 23 zone.

Once the initial excitement is passed, measure actual volts and current. In self-bias, 100% is proper. In fixed bias, anything goes; but generally <70% Pdiss will be hoarse at low levels, and >80% Pdiss idles HOT and runs some risk of redplating in extended half-power passages. There's no real need of a graph, IMHO.

Thanks PRR. Your explanations are complex but at the same time you make them easy for me to understand. I very much appreciate that.


Thanks
Mike

[brewdude]

I like charts and graphs.  They are like pictures—worth a thousand words... And, it’s usually much easier to find the detail I need at a glance through a chart than reading through paragraphs of sentences formed from strings of words that get lost on a page full of letters.  Soon, I forget what detail I’m looking for. 

[pdf64]

That's not the question he is trying to answer! Note that G1 voltage is not on there either.

Yes, you are correct.  I just glanced at it and totally misinterpreted it.

I’m confused - the upper charts on p5 and 8 have g1 voltage (ie bias) on the x axis 

Though not stated, it seems implicit that the anode voltage is being held at Eb, 250V on p5 and 400V on p8.
Then the traces would have been plotted by holding the screen grid voltage (Ec2) at the fixed levels in 50V increments, whilst varying the control grid voltage (Grid-Number 1 x axis) and noting the corresponding anode current (y axis).
Have I got myself into a muddle with the above?

Whatever, for the purposes of setting an appropriate bias, I agree that the above charts aren’t super pertinent.
Whereas (for typical valve guitar amps in which the idle screen grid and anode voltages are near as dammit equal), the triode connected chart on p3 is very handy for setting an initial bias voltage. And for identifying an appropriate range of adjustment available on a bias trimmer, eg +/-20% of the above.

[66Strat]

What about the screen voltages?

Huh? Could you elucidate me as to why I should be concerned about screen voltages while I am biasing my amp?

I also misinterpreted the initial question and graph. Sorry. My bad. My references to the transfer charts were to illustrate the effect that screen voltage has on plate current.

[Mike_J]

I like charts and graphs.  They are like pictures—worth a thousand words... And, it’s usually much easier to find the detail I need at a glance through a chart than reading through paragraphs of sentences formed from strings of words that get lost on a page full of letters.  Soon, I forget what detail I’m looking for.

There is no way I could have said it better. What works best for one may not be best for all. Key is to know what works best for you.


Thanks
Mike

[Mike_J]

That's not the question he is trying to answer! Note that G1 voltage is not on there either.

Yes, you are correct.  I just glanced at it and totally misinterpreted it.

I’m confused - the upper charts on p5 and 8 have g1 voltage (ie bias) on the x axis 
Though not stated, it seems implicit that the anode voltage is being held at Eb, 250V on p5 and 400V on p8.
Then the traces would have been plotted by holding the screen grid voltage (Ec2) at the fixed levels in 50V increments, whilst varying the control grid voltage (Grid-Number 1) and noting the corresponding anode current.
Have I got myself into a muddle with the above?

Whatever, for the purposes of setting an appropriate bias, I agree that the above charts aren’t super pertinent.
Whereas (for typical valve guitar amps in which the idle screen grid and anode voltages are near as dammit equal), the triode connected chart on p3 is very handy for setting an initial bias voltage. And for identifying an appropriate range of adjustment available on a bias trimmer, eg +/-20% of the above.

I didn't look at page three only pages five and eight which might as well have been spaghetti in a bowl for the level of comprehension I have of the subject. Would love to know more about it but I am just trying to bias my amplifier within an acceptable range that I find audibly satisfying and as PRR identified trying not to fry the tubes before I get the amp to that point. Pretty sure you aren't in a muddle though.


Started a lamp limiter, picture attached, based on what you were saying. Never have used one but enough people have that it seemed like an easy thing to do and I could make one with parts in an electrical bin I have my electrical parts in.

[Mike_J]

Have attached a picture of the finished lamp thing. Would someone be so kind as to explain what it does or how it is used so I can write it on the wood with a black sharpie. May be six years until I use it again like it has been six years since I biased an amp. Anything that helps with the memory is good.


This is what I know so far. Plug the AC cord into an AC receptacle. Plug the other side into the amp or I suppose I could use my Variac as a degree of caution. Turn on the switch. Now what?


Thanks
Mike

[pdf64]

...Started a lamp limiter, picture attached, based on what you were saying. Never have used one but enough people have that it seemed like an easy thing to do and I could make one with parts in an electrical bin I have my electrical parts in.

Good job 
Keep it handy and make sure to use it when powering up new projects, fitting new / unknown power or rectifier valves, after repairs etc.
Dunno if it works the same in 120V regions, but I use a 40W bulb for small amps or for first power up on any amp with no valves fitted (or just the rectifier). Then a selection of higher wattage bulbs as I gain confidence that the amp is running ok, eg 60, 100, up to 150W for 100W or solid state amps only.

...Plug the AC cord into an AC receptacle. Plug the other side into the amp or I suppose I could use my Variac as a degree of caution. Turn on the switch. Now what?

The bulb may light fairly brightly for a few seconds, then go dim, may look completely off.
For AB amps, it will get brighter when putting power out.
Class A amps may not go as dim as AB ones do.
All depends on the bulb wattage and the amps idle and dynamic ac current draw.
The best way to learn is to experiment with good amps.
Make a video to remind yourself 

[Mike_J]

...Started a lamp limiter, picture attached, based on what you were saying. Never have used one but enough people have that it seemed like an easy thing to do and I could make one with parts in an electrical bin I have my electrical parts in.

Good job 
Keep it handy and make sure to use it when powering up new projects, fitting new / unknown power or rectifier valves, after repairs etc.
Dunno if it works the same in 120V regions, but I use a 40W bulb for small amps or for first power up on any amp with no valves fitted (or just the rectifier). Then a selection of higher wattage bulbs as I gain confidence that the amp is running ok, eg 60, 100, up to 150W for 100W or solid state amps only.

...Plug the AC cord into an AC receptacle. Plug the other side into the amp or I suppose I could use my Variac as a degree of caution. Turn on the switch. Now what?

The bulb may light fairly brightly for a few seconds, then go dim, may look completely off.
For AB amps, it will get brighter when putting power out.
Class A amps may not go as dim as AB ones do.
All depends on the bulb wattage and the amps idle and dynamic ac current draw.
The best way to learn is to experiment with good amps.
Make a video to remind yourself 

So I plug in two of my amps excluding the one I am currently working on and observe what happens with the bulb. I got a 100 watt bulb that is supposed to draw like a 75 watt whatever that means. Read your explanation a few times now. Starting to make sense. Thanks.


Have also attached a picture of the 6L6GC bias graph. It is based on the 30 watts maximum plate dissipation listed in the RCA 6L6GC datasheet. The 5881 is based on the TungSol datasheet and is 23 watts maximum. Added two more categories to the graph 85% (Merlin maximum, although he may be getting an override on power tube sales) and 100%.



Thanks
Mike

[acheld]

Your light bulb is functioning as a current limiter.  When you plug your amp into the limiter, the amp can draw ONLY as much current as the light bulb will pass.  It's different from a Variac, of course, which allows you to control the incoming voltage.   The current limiter allows you to control the current.

Assuming your wall voltage is 120VAC (rms):

40W --> .33A
100W --> .83A, and so on.

[pdf64]

...I suppose I could use my Variac as a degree of caution...

It’s fine to use them both together, but it’s best for the variac to go first, ie plug that into the mains outlet, plug the LBL into the variac’s output, then the amp.

[Mike_J]

Your light bulb is functioning as a current limiter.  When you plug your amp into the limiter, the amp can draw ONLY as much current as the light bulb will pass.  It's different from a Variac, of course, which allows you to control the incoming voltage.   The current limiter allows you to control the current.

Assuming your wall voltage is 120VAC (rms):

40W --> .33A
100W --> .83A, and so on.

So you are saying a 40W lightbulb only allows .33A to flow through it if I understand you correctly? That certainly wouldn't be enough to operate a Bassman I wouldn't think but at the same time the amp isn't going to be as apt to destroy itself if there is only .33A flowing through it and 50% of that if the Variac was set to 60VAC. So couldn't I control the the amperage with a Variac by in this example reducing the AC voltage from 120 to around 48VAC? That way I could use just one light bulb. Just set it lower and wouldn't I do the same thing with a Variac as pdf64 is by changing bulbs?

[Mike_J]

...I suppose I could use my Variac as a degree of caution...

It’s fine to use them both together, but it’s best for the variac to go first, ie plug that into the mains outlet, plug the LBL into the variac’s output, then the amp.

The only problem with putting the Variac first is it cuts down on the coolness factor of having the light switch. The Variac has an AC switch as well so my new machine is rendered in effect only a light bulb. However, I very much appreciate the heads up about putting the Variac first in line.

I guess what was being discussed in the topic before this one couldn't happen because you want the light bulb to limit current if I am understanding this correctly.

Thanks
Mike

[PRR]

I’m confused - the upper charts on p5 and 8 have g1 voltage (ie bias) on the x axis  .................

"p5" of what? The 6L6 datasheet?

I was looking at the 'bias sheet' in the FIRST post. A possibly useful tool, but not tube-specific for electrode voltages, only gross power.

I agree we may all be off the rails.

[Mike_J]

How would I figure out how much a 5f6a Bassman should draw at idle? My guess is this light bulb limiter must be being used to show if the amp is drawing excess current because of a problem, correct and the bulb is limiting the damage that can happen because it is limiting the current that can flow through it? But in order for that to be effective the idle current of a properly operating amp would need to be less than the current that can flow through the bulb wouldn't it? That way if it is operating properly the light bulb would be a little less or a little more bright?

[Mike_J]

I’m confused - the upper charts on p5 and 8 have g1 voltage (ie bias) on the x axis  .................

"p5" of what? The 6L6 datasheet?

I was looking at the 'bias sheet' in the FIRST post. A possibly useful tool, but not tube-specific for electrode voltages, only gross power.
I agree we may all be off the rails.

Don't think I am off the rails. As long as the maximum plate dissipation is the same for any 6L6GC as it is for an RCA 6L6GC. Where I can see a problem with my method is if something is calling it a 6L6GC and it is not even close. Especially if the maximum plate dissipation is much lower. Could be frying the plates when you think you are at a 60% dissipation rate. Don't know enough about different tubes to know if this is even an issue.


Thanks
Mike

[Mike_J]

Here is an example of a pair of tubes I would like to try. How I ever got them I will never know. There was an amp Fender was making 20 years ago that I played and liked that supposedly used these power tubes. However, what is a GT6L6GE and what is the maximum plate dissipation for the tube. Have had trouble getting much information on the tube let alone a datasheet. So it is a crap shoot as to the mA that is safe.

[Mike_J]

Figured out how I got these tubes. They came with my Fender Bassman LTD. When I played it at the store the amp sounded really good so I bought it. Of course I was convinced it needed NOS TungSol 5881s and GE preamp and PI tubes so these tubes were probably yanked five minutes after I got the amp home. Still does not answer the question about maximum plate dissipation for these made in China tubes. Hopefully the tubes will sound good biased cold and it won't be an issue.

[bmccowan]

I'm staying away from the bias discussion to protect my sanity. But lightbulbs:

I got a 100 watt bulb that is supposed to draw like a 75 watt whatever that means.

This refers to the idiocy of the lamp industry. Years ago people started to think that light output is measured in watts. It is not. It is measured in Lumens or Lux. Watts is the power it consumes, some goes to heat and some goes to light. What you have is a 75 watt lightbulb that produces as many lumens as some other standard 100 watt lamp. Its a lamp with higher lumen per watts, which the lighting industry terms efficacy. This is why you see 9 watt LED lamps as having the equivalent lumen output of a 60 watt lamp. Its a 9 watt lamp, not a 60 watt lamp. Stupid, eh? Anyways, you also asked if you could still buy incandescent lamps - the answer is yes, but the standard Edison base A lamp we are all used to is disappearing. Specialty lamps are available - rough service, appliance lamps, etc. Or visit a crappy dollar store and you are likely to find some. Sorry for going on - I used to be a lighting designer. Now I'm a price checker at a dollar store.

[pdf64]

I’m confused - the upper charts on p5 and 8 have g1 voltage (ie bias) on the x axis  .................

"p5" of what? The 6L6 datasheet?

I was looking at the 'bias sheet' in the FIRST post. A possibly useful tool, but not tube-specific for electrode voltages, only gross power.

I agree we may all be off the rails.

Haha, crossed wires, post #7 by 66Strat had the GE 6L6GC info
sheet attached 

[Mike_J]

I’m confused - the upper charts on p5 and 8 have g1 voltage (ie bias) on the x axis  .................

"p5" of what? The 6L6 datasheet?

I was looking at the 'bias sheet' in the FIRST post. A possibly useful tool, but not tube-specific for electrode voltages, only gross power.

I agree we may all be off the rails.

Haha, crossed wires, post #7 by 66Strat had the GE 6L6GC info
sheet attached 

Thanks pdf64. The maximum plate dissipation for a GE 6L6GC was 30 watts as well. There was a theory that these tubes were made by the Chinese from GE parts which from my reading of the story has pretty much been debunked. Have no idea if the Chinese tubes can handle the same plate dissipation. Probably doesn't matter. Already have the tubes might as well use them or will put the original TungSol power tubes and GE preamp tubes back in. Want to try both and will need to just jump in I guess.


Thanks
Mike

[pdf64]

Just to note that the 23W rating of the 5881 is design centre, whereas the 30W of the 6L6GC is design max. A design centre limit for the 6L6GC would probably be about 26W.
Hence the types aren’t really that far apart, certainly the differing rating systems used act to exaggerated it.

[Mike_J]

I'm staying away from the bias discussion to protect my sanity. But lightbulbs:

I got a 100 watt bulb that is supposed to draw like a 75 watt whatever that means.

This refers to the idiocy of the lamp industry. Years ago people started to think that light output is measured in watts. It is not. It is measured in Lumens or Lux. Watts is the power it consumes, some goes to heat and some goes to light. What you have is a 75 watt lightbulb that produces as many lumens as some other standard 100 watt lamp. Its a lamp with higher lumen per watts, which the lighting industry terms efficacy. This is why you see 9 watt LED lamps as having the equivalent lumen output of a 60 watt lamp. Its a 9 watt lamp, not a 60 watt lamp. Stupid, eh? Anyways, you also asked if you could still buy incandescent lamps - the answer is yes, but the standard Edison base A lamp we are all used to is disappearing. Specialty lamps are available - rough service, appliance lamps, etc. Or visit a crappy dollar store and you are likely to find some. Sorry for going on - I used to be a lighting designer. Now I'm a price checker at a dollar store.

Glad you cleared that up because it sure confused me. I was going for a 75 to 100 watt bulb but they sure do make it confusing. Glad to know it is really a 75 watt bulb.


Thanks
Mike

[Mike_J]

Just to note that the 23W rating of the 5881 is design centre, whereas the 30W of the 6L6GC is design max. A design centre limit for the 6L6GC would probably be about 26W.
Hence the types aren’t really that far apart, certainly the differing rating systems used act to exaggerated it.

That is probably why so many of you don't think it is that big of an issue. To me it is because when they say 23 watts is max then I want to believe them. It seems to be getting harder and harder to find numbers you can believe and rely on. Makes it a little bit frustrating.


Thanks
Mike

[bmccowan]

Whether its light bulbs or tubes or EPA vehicle MPG, the way numbers are used can be very confusing - I agree. There are many threads on this Forum trying to talk people down from the ledge of obsessing on numbers. Trying to hit some exact power tube plate voltage from a schematic is a good example. Even the best manufacturers were very inconsistent. I always try to remember that I am trying to produce likable sound, not accurate numbers. If I have a fixed/adjustable bias amp I get the bias in a safe range and then adjust it for what I think is the best sound for those tubes. If its still in the safe range, I consider it good and done.

[Mike_J]

I always try to remember that I am trying to produce likable sound, not accurate numbers. If I have a fixed/adjustable bias amp I get the bias in a safe range and then adjust it for what I think is the best sound for those tubes. If its still in the safe range, I consider it good and done.

Wise comments. The reason I want the graphs is to know what the safe range is. If the bias pot ranges within the safe range then all I would need to do is find the best sound for the tubes.


Thanks
Mike

[Mike_J]

...Plug the AC cord into an AC receptacle. Plug the other side into the amp or I suppose I could use my Variac as a degree of caution. Turn on the switch. Now what?

The bulb may light fairly brightly for a few seconds, then go dim, may look completely off.
For AB amps, it will get brighter when putting power out.
The best way to learn is to experiment with good amps.
Make a video to remind yourself 
Forgot to make the video but it did exactly what you said. Hooked the Fender LTD to the contraption and the light was moderately bright for a very short period of time and then got real dim, then went back to being moderately bright. Maybe like a 25 watt bulb. Does that mean the amp is idling at somewhere near 20 mA?

[pdf64]

Not sure if you’re aware of it yet, but the very useful characteristic of a filament bulb is that its resistance is low when cold, high when hot. So it behaves as a high power negative temperature coefficient NTC thermistor.
Which is perfect for our application, ie protecting equipment from fault current.
We choose the bulb power rating such that normal / idle operation doesn’t draw enough current to heat the filament, so it stays cold, low resistance, and doesn’t drop much voltage, which allows the equipment to idle a close on full mains voltage.
But if the equipment’s current draw increases, the filament heats up, increases resistance and drops the mains voltage received by the equipment. Thereby reducing the current it can draw, and so protecting it from damaging levels of fault current.