6L6GC Power Tube Matching

[noddyhead]

I have just fitted a pair of nos GE 6L6GC in my Fender 57 Twin. I have checked the biasing
Tube 1 = 33 ma
Tube 2 = 37 ma

Is this a close enough match, I usually bias my 6L6GC to 36 ma

[pompeiisneaks]

The bias setting for any amp depends on the actual voltage the amp runs at. I don't know what your B+ Plate voltages are, so I can only speculate, but any of the calculators out there will tell you if a bias point is good enough or not.

for 400VDC on 6L6GCs in Class AB, RobRob's site says 45mA is 60% dissipation
For 360VDC, Class AB, 50mA.

So saying you bias at 36, if near those voltages means you're below 50% dissipation, this means the tubes are running very cool, and will give you a very long life, but also could sound a bit sterile. You can and should find the right bias point for your B+ and amp. As for the 4mA difference, I doubt that's much of a big deal, minor imbalance in current isn't a big deal, you could have a pretty imbalanced PI pushing one side of the pair harder than the other.

I'm not 100% sure of what is considered the 'imbalanced' point, but I've never seen my matched burned in tubes bought from apex closer than about 3mA if I recall.

~Phil

[Kagliostro]

I'll consider a perfect match within 5mA

K

[Stevem]

A 57 twin original had 405 volts on the output tube plates with the lower ac wall voltage of the day , which back then to have the needed 60% disappation rate would call for the tubes to idle at 45 ma.
With today's wall voltage those tubes may be seing close to 425 volts, which in that case I would idle them at 42 ma.

Perfectly balanced output tube's don't mean dick in terms of the overall amps sound if the amps PI section is not perfectly balanced itself , both circuit wise and tube wise!

I have have many pairs of output tubes that where mismatched by 10 to 12 ma, yet the output wave form on my scope was still very close to being balanced due to the drive signal from the PI section!

If you bias your tubes at 60% and the tubes are mismatched even by 10 numbers then you should still be safe and not burn up the better performing output tube.

[pdf64]

I don’t see how it's ‘needed’ to idle the tubes at 60%? Plate dissipation is a limit, not a target, doesn’t affect transfer characteristics, tubes don’t work differently according to how hot their plates are.
Provided the idle condition doesn’t cause average plate dissipation to exceed its limit under load, all that’s needed is to keep the conduction angle reasonable, away from class B.
Balance, whether idle dc, small signal ac or large signal ac is a roll of the dice, having one doesn’t mean all are good.

[Stevem]

The difference in the amount of punch a class A/B amp has at 45% dissappation and 60 is quite large to my ears!

[martin manning]

I don’t see how it's ‘needed’ to idle the tubes at 60%? Plate dissipation is a limit, not a target, doesn’t affect transfer characteristics, tubes don’t work differently according to how hot their plates are.

But they do! The hotter the bias the larger the signal size where the power amp remains in class A.

[Stevem]

Also 60% is not the limit, 70 is on a proper spec OT and output tube along with the plate V+ and even more so the screen voltage !

[pdf64]

As plate current rises, so does stage gain. Human perception is such that a slight gain increase sounds ‘better’.
It’s as well to be careful to avoid mixing up what’s ‘better’ with what’s ’needed’?
Bear in mind that the 57 Twin cited was designed for 5881s 23watt plates; no need for a beefier 6L6 type tube (by definition all have the same bogey characteristics) to be biased for a wider conduction angle than would be appropriate for 5881 when fitted in there. The benefit of the ‘GC’ here should really be longer tube life, though bias them hotter if that’s your preference and the transformers are happy with it.
Also bear in mind that the plate dissipation limit is kinda arbitary; an absolute maxis higher than a design max is higher than a design centre.
Aiken’s ‘idle no higher than 70% of plate limit’ is clearly based on EL34, whose limits are design centre; perhaps his guideline would be lower if he’d been thinking about design max tubes, eg 6L6GC?

[pompeiisneaks]

I'm the one that brought up 60% because the OP's mention of a value of 32mA which would have been WAY COLD. I was trying to imply that even if they don't like to bias hot, that 60% was adequate and still not too hot. But I guess I didn't really say that outright, was just suggesting they move upwards in their bias.

~Phil

[pdf64]

I'm the one that brought up 60% because the OP's mention of a value of 32mA which would have been WAY COLD. I was trying to imply that even if they don't like to bias hot, that 60% was adequate...

I don’t get why you think 32mA would be way cold? They should be well away from cut off, right up the signal level range, ie taking into account some degree of HT sag at higher signal levels. And as we don’t know the rest of the OP’s idle conditions, we don’t know that isn’t equivalent to 60%
Plus modern Fender 6L6 amps tend to specify an idle cathode current of 30mA.
The big downside of the whole ‘idle at some particular % of plate dissipation limit’ is that it seems to lead to a heat based analysis of operating conditions, akin to looking down the wrong end of a telescope.
Plate dissipation limit is just a number that shouldn’t be exceeded, beyond that it (and hence x% or y% of it) has no significance.

[pompeiisneaks]

I'm the one that brought up 60% because the OP's mention of a value of 32mA which would have been WAY COLD. I was trying to imply that even if they don't like to bias hot, that 60% was adequate...

I don’t get why you think 32mA would be way cold? They should be well away from cut off, right up the signal level range, ie taking into account some degree of HT sag at higher signal levels. And as we don’t know the rest of the OP’s idle conditions, we don’t know that isn’t equivalent to 60%
Plus modern Fender 6L6 amps tend to specify an idle cathode current of 30mA.
The big downside of the whole ‘idle at some particular % of plate dissipation limit’ is that it seems to lead to a heat based analysis of operating conditions, akin to looking down the wrong end of a telescope.
Plate dissipation limit is just a number that shouldn’t be exceeded, beyond that it (and hence x% or y% of it) has no significance.

I thought it was pretty important to understand that this point was calculated at idle current meaning the value spikes up and down depending on actual operation. I also don't know how to calculate what is too cold, but I have see what was too cold on some amps I was working on, and it wasn't very low below the 70% I bias at, but I didn't calculate the % diss at that point. It seemed like, as i recall whatever the numbers were for mA, I was say at 40mA well biased at about 70% (I'm making these numbers up to try and recall 'roughly' what I experienced in person with cutoff due to cold bias) and at about 25mA it was basically at cutoff. something just above half of the 70% diss point.

I definitely don't doubt my full understanding of 'where cutoff lives' is, I thought that even if signal output is operating fully, part of that sterility can be caused by one side of the output being barely into the almost or somewhat cutoff range and losing some of the signal. Or maybe that's not sterility but just plain old bad tone? Either way, part of the dissipation calculations are to ensure you get the right sweet spot on the voltage to current balance where you get as much of the signal swing as possible while also not over doing the tubes and burning them up. No? or am I confusing two different things? I've read and played with cold bias vs hot bias and know 2 things I found:

1. colder bias means lower output wattage but longer tube life - and I personally didn't like the sound of a cooler biased amp
2. hotter bias means higher output wattage but shorter tube life - and I prefer it... (but around that 70% number for class AB to avoid tube abuse)

Maybe I need to dig into the math and logic of this deeper for a better understanding, because if your statement above is right (and it probably is) I'm a bit confused about this area still.

Also my estimate of 'WAY COLD" was that the amp was a Fender 57 Twin. I looked up the amp and it didn't have voltages, but my best guess due to seeing voltage data on many fenders was that it was 'likely' that it ran somewhere between 350 and 400 VDC at the anodes. I may be wrong... I very clearly stated I didn't know the actual amp voltage and this was mandatory to get real, important, valuable data for biasing it, but that I just chose a few random values to explain the point I was making, and those points, if the amp fits more common fender setups, is biased quite colder than I'm used to seeing. My experience is much more limited than many here, though, so maybe I should just refrain from input until I get that knowledge...

Sorry for any confusion. I didn't mean to mislead anyone.

~Phil

[sluckey]

(I'm making these numbers up to try and recall 'roughly' what I experienced in person with cutoff due to cold bias) and at about 25mA it was basically at cutoff. something just above half of the 70% diss point.

Cutoff means NO current is flowing through the tube, just as if you flipped a switch and killed the B+. There will be no sound when the tubes are cutoff.

[pompeiisneaks]

(I'm making these numbers up to try and recall 'roughly' what I experienced in person with cutoff due to cold bias) and at about 25mA it was basically at cutoff. something just above half of the 70% diss point.

Cutoff means NO current is flowing through the tube, just as if you flipped a switch and killed the B+. There will be no sound when the tubes are cutoff.

Oh that makes sense, maybe that was a point the signal just started getting much quieter before cutoff then? One I specifically recall was one I was troubleshooting with your help sluckey, and it was really thin sounding and horrid, and you told me to increase the bias current. I don't recall the exact details, but I think the bias circuit was not set with the right resistor for that setup and I was getting too little current. Then I swapped a resistor, and was able to adjust it to a level that was producing real sound.

Or am I just remembering even that wrong? does it just go from 'normal clean tone' to 'nothing' in one fell swoop? how do you know the point where it is 'too cold' but 'not yet near cutoff' or is there no such thing? If so then there should never be a debate about this, you'd just set the bias slightly to the point where 'current is flowing' and forget it, no?

Just trying to make sure I get a better understanding of this concept...

~Phil

[tubeswell]

The main benefit of matching output tubes for current draw in a guitar amp, is to minimise hum noise when the amp is idle. A bit of mismatch won’t harm the tubes (provided they aren’t drastically mismatched).