6L6EH Bias for SE amp

[chunkitup]

I cant find any tube data info on the Electro Harmonix 6L6EH and I biased my Single Ender to 70mA. Sounds fine just wondering what the max bias current is for Class A 6L6.

[chunkitup]

Check my math. I have a plate voltage of 377 and I am biasing for 30w diss. so that gives me up to 79mA. If my math is off let me know.

[stokes]

Depends on your plate volts.You dont want max current,you want about 90% of the max plate watt dissipation.You have to know the plate volts to figure the dissipation.Multiply plate volts times current- for Class A you want to be around 26 watts idle for a 6L6GC.Be aware that a 6L6GC has a max output of 30 watts where other 6L6 types only have a max of 19 watts.Make sure you know which tube you have.

[stokes]

I was typing while you posted the second time,you have to back off some.

[chunkitup]

Ok so I have 70mA with 377 Plate voltage which gives me 26W plate diss. You think I should roll off a bit more? Also, the electro harmonix 6L6EH I am almost sure is similar to the 6L6GC but not totally sure. Their website gives no info.

[MWJB]

Have you subtracted your cathode voltage from your plate voltage to arrive at 377v? The tube only sees the difference between plate and cathode voltages.

I assume 25W plate dissipation for a current production 6L6, unless manufacturers figures say otherwise.

The truth is that the absolute limit will vary from tube to tube & amp to amp. If your plates glow or you have audible problems, you have too much current.

It doesn't really make sense to bias so hot that you can only use a few brands. The only way to get it "right" is to experiment and see what you like the sound of, it doesn't take long to sub out cathode resistors. That 90% is a good "rule of thumb"/starting point, but your amp is primarily a musical instrument, not a maths project. Having said that, I do agree with stokes that coming down a shade would be a sensible.

Being single ended, you don't really have any choice as to whether your amp is class A or not.

[stokes]

"Being single ended, you don't really have any choice as to whether your amp is class A or not."
Nothing could be further from the truth.Merely being single ended does not make an amp Class A.Nor does the fact that it is single ended automatically make it cathode biased.If it is not cathode biased,there are no volts on the cathode.Biasing the tube to idle at about 90% of the max plate dissipation gets you into "Class A territory",but if you read up on true Class A,there are many different opinions on what constitutes true Class A but the most common attribute has to do with the voltage swing when a signal is applied.
Chunkitup,when changing the bias whether it is cathode biased or fixed bias you must check your plate volts as you change current.If you had 377v's at 79ma's,when you lower the current to 70ma's your plate volts will rise,so it is a balancing act of sorts.
"I assume 25W plate dissipation for a current production 6L6, unless manufacturers figures say otherwise."
This is safe if you know you are using a 6L6GC or equivalent,but if it is one of the other 6L6 types that have a max plate dissipation of 19 watts you will be in trouble.Also going by this assumption you wont get near Class A since a 6L6GC has a max plate dissipation of 30watts,90% of 30 is 27,90% of 25 is 22.You do however have to subtract the cathode volts,as MWJB says,if it is cathode biased.I do agree with the fact that "what sounds best is the proper idle current,as long as you dont exceed the 90% rule of thumb,and the plates dont glow,but you must check for redplating while there is a signal applied and the volume is turned up,not just at idle.

[chunkitup]

OK, here is the stats and math. Vk= 29v, Vp=383v, I=70mA Now the math:
Voltage across the 6L6=354v so 354v x .07 = 24.78w
My tube is 6L6EH / 6L6GC with a max plate diss of 30w.
And the amp sounds great!!

[MWJB]

Indeed, I assumed that the amp was cathode biased, but without specific information to the contrary I think most people would. Normally, SE amps are conceived as budget amps hence the expense of a fixed bias circuit is deemed unwelcome, despite being a great idea for the hobbyist/more esoteric amp builder.

Yes, there are conflicting ideas as to what constitutes "real/proper/true" class A. So if my SE 6L6 amp with a JJ 30W tube amp runs at 60mA & 370v, what class is it? What class is it if I replace the JJ with a 25W or 19W tube? What does it actually matter? Are you going to rebias every time you sub a different 6L6, wouldn't you rather make music? Does anyone currently make a 19W 6L6 and would one be guaranteed to go up in smoke if biased to 25W? What if can switch in/out global NFB? Are worried about grid conductance or plate conductance? You see what I'm getting at, there's a lot of considerations that might unnecessarily keep a novice awake at night?

By some definitions, some of the most well known and loved SE "class A" amps were never class A. Which is why I'm not convinced that it is always useful, as a musician, to get too carried away with the subject of class. OK, hands up, I should have pre-empted my statement getting shot down, I don't really want to escalate the debate but I do feel that the day to day practicalities of biasing such an amp should be foremost.

Bear in mind that biasing a 6L6 to halfway between cut off and saturation may well exceed a realistic plate dissipation. For a SF Champ 6V6, for example, that might be 22W plate dissipation.

All the best, Mark.

[stokes]

"Which is why I'm not convinced that it is always useful, as a musician, to get too carried away with the subject of class."
If you are going to bias your own amp as a musician it is very important to know what you are doing.If you bias a 6l6GC at 60ma&370v's it would be considered a cold bias if you are looking for class A.If you change to a 19watt 6L6 you had better rebias.I dont know if anybody currently makes a 19 or lower than 30 watt 6l6,I dont use current production tubes at all,but there are a lot of people relabeling tubes and the chances of coming across one is very possible.Many people think a 5881 is a drop in replacement for any 6L6,if it is a true 5881 (Sovteks apparently arent) it wont take the higher watts of a 6L6GC.If you bias a 19watt tube at 25w of course it will go up in smoke,maybe not immediately but it will.I put a pair of Nos Tung-sol 5881's in a Boogie Mkii just to see once and within an hour the flashing turned brown, testing for about an hour a day in two days the tubes where cooked.
"Bear in mind that biasing a 6L6 to halfway between cut off and saturation may well exceed a realistic plate dissipation".Not so,if you bias to the halfway point you will be in class A which is about 90% max plate dissipation.My point is this,if you are a musician who wants to service his own amp and bias your tubes it is very important to know what you are doing,whether you call it Class A or Class AB may not be critical,but knowing what tubes you have and what the acceptable current draw for them is.Look at his first post,this guy was shooting for a 30watt idle,if left there those tubes wouldnt last a month.There are a lot of considerations to keep one up at night,and the bias is one of the basic considerations.Better to be kept up than to fry your amp or your self,no?If you are going to poke around in the amp you better know what is going on,you could get dead.Oh and by the way if you bias a 6V6 at 22watts,as you said,it will fry.The 6V6's max dissipation is 14watts idle.Your cutoff and saturation points are determined by the voltage swing with a signal applied,not just by the idle current alone.So it is true that merely picking 90% watt dissipation wont necessarilly get you to "true" class A,it will most likely get you close enough for guitar amp purposes.

[MWJB]

OK, I still don't want to get into it about what determines class (I'd love to see that SE class AB amp though) but you are overstating certain points. Chunkitup's amp was running 25W plate dissipation, once he had allowed for the volage drop accross the cathode resistor...perfect! He could probably run a 19W tube in there for years without a problem.

Whyever not use current production tubes?

Your example of what happens to 5881's in a Boogie is not relevant in this instance. The Boogie undoubtedly ran more than 354v and is fixed bias. A real 5881 is a 24W tube, again, fine in a cathode-biased, SE amp biased to 25W.

SF champ failures, of the catastrophic kind, are pretty rare. They soldier on for years with the same 6V6, typically running 50mA+ at 400v (corrected). The owners/players don't know that they have a "problem", they just love their killer sounding champs. Now, I'm not suggesting everyone rush out and rebias their cathode biased SE amps to 150% plate dissipation (or more than 100% for that matter), but this just goes to show what the reality of the situation is.

[stokes]

"Check my math. I have a plate voltage of 377 and I am biasing for 30w diss. so that gives me up to 79mA."Note where he states he is biasing for 30w diss.,his words not mine.Biasing even a 23 watt 5881 for 30watt idle is bound to fail.Advising someone that it is okay to do so is bad advice.
"OK, I still don't want to get into it about what determines class (I'd love to see that SE class AB amp though)".Yet you go on about it.I am using the term AB here as a reference.If you bias your SE amp to 70% you are not in class A territory anymore.If you are not going to check the voltage swing with a signal applied the easiest way to get into class A is to use the 90% rule,most players dont own a scope so 90% would be the way to go.The example of the 5881's in the boogie is relevant,I dont care if it is SE,PP fixed or cathode biased,if you bias your tubes to draw more than their rated dissipation or above the rated plate v's they will fry sooner than if you stay within the design max,if you have some magic trick that will make a 19watt tube work at 30watts idle,patent it,you will make a lot of money.As for the 6v6 biased at 50ma and 400v that is 20watts idle.The best 6V6 is rated for 14watts and some are only 12 watts.Some tubes will last but most will not.If you think that is safe,then we just have to agree to disagree.I prefer to err on the side of safety.
"Whyever not use current production tubes?"Because I have yet to find a current production tube I like.I have 19 amps in my own collection,all have NOS tubes,I also have amassed a stock of over 200 NOS tubes in the years I have been involved with amps,and continue to add to them,I have no need for current production tubes.If a customer wants them that is what he gets,but I personnally have no use for them and dont use them in my amps.If they work for you that is fine, it is all subjective anyway.