Some help? Rectifier:power tube relationship vs. Bias

[Mango Moon]

Hello Everyone,

I'm admittedly a relative newb with amp service and repair, but I've got a a seemingly reasonable question regarding the relationship of the power tubes and bias adjustment to the rectifier tube.

I recently wanted to swap-out some older Fender Groove tube 6L6GCs with some comparable SED Winged C's (=C=), and before doing so, noted that the existing (Groove Tube) bias was about 31-32 mA with a plate voltage of about 450VDC. But when I installed the replacement SED Winged C's, with no other changes (i.e., same rectifier, same everything, without any adjustments), the bias reading was now only 21-22 mA with plate voltage appeared to be about 460VDC.

For grins, I swapped-out the Ruby 5AR4 rectifier for a comparable Sovtek 5AR4, subsequently repeating the entire process in terms of alternate power tubes, and the results were exactly the same (i.e., 31-32 mA with Groove Tube 6L6's and 21-22 mA with Winged C's).

Please forgive my ignorance here, and if you can, kindly resist slamming me for being inexperienced, yada-yada, but is this normal behavior for different power tubes? In other words, do differ brands of the very same power tube design (6L6GC's) draw as much as 40% greater or lesser current than another brand, even with the exact same rectifier in place? And if so, do I simply (safely?) adjust the bias pot upwards until the new (Winged C) tubes are also reading 31-32 mA like the Groove tubes before them?

Thanks ahead-of-time for your insights, your patience and your self-control.

Cheers.

[R.G.]

Hello Everyone,
... is this normal behavior for different power tubes? In other words, do differ brands of the very same power tube design (6L6GC's) draw as much as 40% greater or lesser current than another brand, even with the exact same rectifier in place? And if so, do I simply (safely?) adjust the bias pot upwards until the new (Winged C) tubes are also reading 31-32 mA like the Groove tubes before them?

Yep, it's a reasonable first-timer question. Yes, it's to be expected.

Each maker tries to get his tubes to fit the (now archaic) specs for what a 6L6 or whatever used to be. And do it at a profitable cost.

Tubes have a transconductance. This is the ratio of how much current flow changes in the plate for each volt of change in the grid voltage. Transconductance is one of those things you get by adjusting the spacing of the grid wires, the distance from grid to cathode and grid to plate, and the other mechanical innards of the tube. Does it vary? You bet.

And yes, you can simply adjust the bias. Back in the Golden Age, the big name electronics firms GE, Sylvania (when they existed), RCA (when they existed) Mullard (when they existed... do you see a trend here?) all competed head to head for the best, most consistent, most reliable tubes to sell, so they were very uniform. Fender could build amps and not worry about matching or independent biasing of tubes. Today - well, your tubes probably came from some place you could not reach by phone, and probably couldn't find someone there who speake English. Not a lot of competition, profits are not high enough to make them do a lot of research and trouble to make them high gain and uniform. At least by USA 1950s and 1960s standards.

[Mango Moon]

Many thanks R.G.!

Clearly, I've got a long-long way to go on all of this stuff, and I had no idea that the various makes/brands of the very same tube-type could/would vary this much, even by today's (lesser) industry standards, but its comforting to know that its alright to simply hit the Winged C's a bit harder without any concerns.

Thanks for taking the time to respond and for providing the necessary guidance I needed. I greatly appreciate it.

[Enzo]

Not only would WInged C 6L6s come up with a different current than whatever brand those Groove Tubes were, but two different sets from the same manufacturer can have widely different currents. That is the point of buying matched sets. If tubes all had about the same current flow, then they would all match and there would be no need to make an effort to match them.

or put another way, look at Groove Tubes. They label each tube on their 0-10 scale (or whatever they use these days) so you can chose. The tubes of each rating are not special made that way, they just buy tubes in bulk, stick them in a big tube tester, measure their current and gm, and sort them by the results.

[Mango Moon]

That is the point of buying matched sets. If tubes all had about the same current flow, then they would all match and there would be no need to make an effort to match them.

Yup, good point Enzo! I hadn't thought about it in those terms. I just presumed that the matched pair approach was intended to eliminate any disparity (or differential) in total output provided by the pair, rather than to account for individual variations between manufacturers or production runs (if that makes sense?).

The other thing that strikes me is, wouldn't you eventually reach some sort of terminal or maximum ceiling to the bias pot adjustment? For example, if one manufacturers tube(s) are as widely different as 35-40% (e.g., 21 mA vs. 32 mA), and the bias pot has to be dialed up accordingly (40% higher) just to account for the difference, what's going to happen when another pair of tubes prove to be yet another 40% off? Wouldn't the bias pot have a limit at some point in terms of how high it could be adjusted upwards?

[Prairie Dawg]

Yup, good point Enzo! I hadn't thought about it in those terms. I just presumed that the matched pair approach was intended to eliminate any disparity (or differential) in total output provided by the pair, rather than to account for individual variations between manufacturers or production runs (if that makes sense?).

The other thing that strikes me is, wouldn't you eventually reach some sort of terminal or maximum ceiling to the bias pot adjustment? For example, if one manufacturers tube(s) are as widely different as 35-40% (e.g., 21 mA vs. 32 mA), and the bias pot has to be dialed up accordingly (40% higher) just to account for the difference, what's going to happen when another pair of tubes prove to be yet another 40% off? Wouldn't the bias pot have a limit at some point in terms of how high it could be adjusted upwards?

You're correct. That's when you gotta start changing resistors.

[Mango Moon]

O.K., I'm no candidate for the Nobel Prize in physics, but something just doesn't seem to jive here, at least not from my understanding of the relationship between plate voltage (B+), bias current and max dissipation (i.e., Ohm's Law).

Ohm's Law tells us that I = P/E where I = bias current, P = max (or safe) plate dissipation (usually 60% of max which is 60% of 30w for a standard 6L6 tube), and E = Plate Voltage or B+. Right?

Now, the matched pair of SED Winged-C (6L6GC's) that I recently tested here were rated by the merchant (pre shipment) at roughly 14 mA each with test voltage at 400 VDC, yet they read 21-22 mA each in my given amplifier at 450-460 VDC.

Not only does that appear to deviate from the inverse relationship of Ohm's Law (i.e., with fixed numerator, as the denominator increases, the quotient should DECREASE), it also raises the question of relative "breakup" point between the original Groove Tubes and the replacement Winged-C's. In other words, with all things being equal (same rectifier, same plate voltage, no alteration of existing bias adjustment), the Groove Tubes read 31-32 mA (near what the manufacturer specifies), but the Winged-C's read only 21-22mA.

So, if I adjust the bias pot of the amp upward to hit the Winged-C's harder at 31-32 mA (instead of 21-22 mA), won't they then breakup earlier than the original Groove Tubes? Or, put somewhat differently, if I prefer a cleaner (no breakup) sound (which I do with this particular amp), shouldn't I seek a different matched pair with higher demonstrated plate current?

Thanks for your patience with me on this stuff folks!

[Enzo]

The current readings on the matching label are the readings from the matching machine, nothing more. They may have been matched using 400v B+, OK so far. What voltage was on their screens? what voltage was on their controls grids - the bias in other words? What load were they working into? That reading ONLY matters to the tube tester they used. None of those other factors are likely to be the same in your amp.

YOur amp has 50v higher B+, so I might expect higher current to flow, if all else were equal..

Yes, P=IxE. YOu start with two of those numbers, you can get the third. When they were being matched, then apparently 14ma at 400v was flowing, meaning the tubes were dissipating 5.6 watts IN THAT TESTER.

Now you have 450v and 21ma flowing in your amp. So in your amp, the tubes are dissipating 9.5 watts.

You seem to expect - if I undertand your post - that Power will be a given. It is not. It is merely the result of the voltage and current. If somehow we could force dissipation in your amp to remain at the 5.6 watts of the tube tester, then indeed the numbers would be funny. But there is no specification that power will be anything other than the product of voltage and current. Turn the bias control and see that in action.

There are various rules of thumb when it comes to biasing tubes. I hear 70% all the time, and you want to go for 60%. ALL fine settings, and all pretty arbitrary, for that matter. But that is idle current IN YOUR AMP. The idea being that at that setting the dynamic performance will be desirable. But clearly the tube tester was not setting them up for that level of dissipation, no reason to assume it did.

If you want your tubes biased to 60% of max, 18 watts for a 30w tube, and you have 450v across the tube, then I = P/E = 18/450 = 40ma.

[Mango Moon]

Yup, I follow all of that just fine Enzo. The amp manufacturer calls for 35mA and 40mA would provide 60% dissipation at 450V, but I'm still seeking an answer to the central question about breakup and the fundamental difference between these two (SED & Groove Tube) sets of 6L6GC tubes . . .

"If I adjust the bias pot of the amp upward to hit the Winged-C's harder at 31-32 mA (instead of 21-22 mA), won't they then breakup earlier than the original Groove Tubes"?

[Enzo]

Can't tell you. Unless the Grooves were in fact Winged C under the paint - always a possibility with Groove - it is hard to compare one brand to another in those terms.

[Mango Moon]

Enzo,

You're a good man, and I greatly appreciate your continued contributions here, but I'm not sure that I've understood you correctly and I want to make sure . . .

Are you saying that regardless of the vendors published pre-shipment testing specs on a given set of power tubes (i.e., plate current, voltage, transconductance, etc.), that there is no means of determining ahead of time (prior to purchase), which set of tubes would be more likely to breakup earlier or later at a given bias setting?

And similarly, are you saying regardless of the fact that when two different sets of power tubes yield widely different plate currents (empirically) in the very same amp (all other things like rectifier and bias adjustment being equal), that there is no means of determining ahead of time (prior to purchase), which set of tubes would be more likely to breakup earlier or later at a given bias setting?

If so, I need to seriously consider a change in my choice of hobby! No joke!

[nickb]

"So, if I adjust the bias pot of the amp upward to hit the Winged-C's harder at 31-32 mA (instead of 21-22 mA), won't they then breakup earlier than the original Groove Tubes?"

This is how I see it:
No. All things being equal, if you adjust the bias current to the same as the original then you will have the same maximum change in plate current and thus the same headroom. I'm assuming that the driver stage has enough capability to cope with a range of tubes.

On determining the performance of a particular tube prior to purchase, in theory you can make some predictions provided you have acess to all the data. And that is the rub. The data on current production tubes tends to be incomplete and that can make it hard to interpret.

[Mango Moon]

Honestly, its been my experience in talking with many boutique amp manufacturers out there, that they will often pre-select power tube sets for their amps based on their desired performance characteristics (i.e., clean or early breakup) using pre-shipment test data provided by their tube vendor or tube manufacturer.

In other words, there are numerous vendors of power tubes out there who clearly advertise that they will pre-select 'cold' or 'hot' tubes (late vs. early breakup) based on their pre-shipment bench testing results of said tubes.

Thus, it makes perfect sense to me that in the same way that pre-amp tubes can be (and are) pre-tested and classified as having balanced triodes or low noise and low microphonics, power tubes can (and I believe are) similarly tested and classified as being either "early or late breakup".

However, if that's not the case (i.e., if that's just some pointless marketing ploy), and I've somehow failed to grasp something, then I stand corrected.

[Enzo]

OK, within a given make of tube, the hotter ones - the ones with naturally higher currents - will be easier to break up. But comparing a Groove Tube "7" with something from Sovtek won;t yield reliable comparisons, that is all I wanted to say. Not suggesting one cannot select tubes, you can. But within the context of looking at matching numbers, unless they were done on the same machine under identical conditions, we don;t know much about the tube. SO I can say that one tube du jur from Groove might be hotter than another of the same type because I know they tested them all the same. But when Groove tests their tubes on their equipment, and SED tests their tubes on their gear, we have no real basis for comparison.


yes, you can order hotter tubes from SED, or Sovtek, or whomever you like, and maybe get closer to the breakup you want. All I am warning against is looking at numbers from one vendor and comparing to the numbers of another vendor. It would be a darn large coincidence to find both companies used the same testing gear at the same test parameters to generate the matching numbers. It is not enough to spec the plate voltage. We also would need to know the voltages on the grids, the load impedances, and so on.

SO if Tube Timmy sells tubes with 14ma on the match label, and Vacuum Vern sells tubes that say 18ma on the match label, we don;t know which tube will be hotter in your amp, because we don;t really know all the test parameters. On the other hand if Vern sells identical tubes measured at 14ma and 18ma, then yes, the 18ma ones will be hotter in your amp. You won;t likely run them at 18ma, of course, you;d set your own bias point.

You can say that these Sovtek 6L6ADCDEF tubes will break up sooner than these other Sovtek 6L6ABCDEF tubes from the same vendor. And we can say these =C= 6L6 from some other vendor will break up earlier than these other =C= 6L6 from that vendor. What we can;t really say with any certainty is that these Sovtek tubes will break up easier than those =C= tubes

Kinda like trying to determine the difference in sound between 6L6 and EL34, and we test all the 6L6s in a Peavey amp and all the EL34 in a Music Man. We can determine that we like some 6L6s better than others in the Peavey, and same for EL34s in the Music Man, but we can't really state what the difference between the two tubes might be from that.


I think the breakdown in communication here is that I am focusing on your earlier assertion that =C= tubes might be hotter than your Grooves, and you are more concerned with if you turn up the current will the tube break up sooner. Is that fair? I feel like we are somehow speaking to crossed purposes. If i am confusing the issues I am sorry.

[Steve Conner]

Tubes are complicated non-linear devices. (Non-linear means, among other things, that you can't rely on Ohm's Law to work around them, as you've noticed.)

If your tube vendor matches/grades them for idle current, that's a single point on the characteristic curve specified, but you don't really know what the rest of the curve will be like. (You hope that, from a reputable tube vendor, it'll be such as to deliver your amp's rated power and make some sweet-sounding harmonic distortion.)

If he also grades them for transconductance, then that's a second point. (Well, it's the slope of a straight line approximation to the curve, but same thing.) So you know one more thing about the tube, but again everything else is unspecified.


I've always thought the "hotter tubes break up earlier" thing was a little suspect on theoretical grounds. Surely if the amp breaks up noticeably earlier, that means that it's failing to make its rated power, so the tubes are no good?

Changing out the rectifier tube for a 5AR4 probably raised all of the voltages in the amp a little. That would include the screen voltage of the power tubes, and screen voltage affects idle current just the same as bias does.



I think the answers to your two original questions are:

Yes it's normal to have that much variation. A few thousandths of an inch difference in the spacing between grid and cathode could account for the 40% difference in idle current.

And therefore, yes just twiddle that bias pot, play, be happy. (Hopefully )