6L6 OT in a 6V6 Amp

[minions]

If I were to take an output transformer found in an amp equipped with 6L6s and install it into an amp with 6V6s (like a Bassman transformer into a Deluxe Reverb, for instance), I realize the affect this will have on the tone, but are there any technical issues that will arise?

Thanks.

[sumgai]

minnie,

If you're talking about an older Bassman that had 50 watts, then you're good to go. If you're looking at something like the 100 watter, or even the Bassman 135, then you need to be aware of a few things.

6L6's and 6V6's have only a small difference between them in the plate's rated load resistance, so they expect to see similar loads, i.e. the impedance of the OT's primary winding. Two of either tube type will be close enough, or 4 of either tube type, that'll work too..... but taking an OT from a 4-tube layout, and using it with 2 tubes, that'll cause your secondary outputs to be off by a factor of 2 - the 4Ω output will be wanting to see an 8Ω load, in order to sound out correctly. Sorta like pulling two out of four tubes from a Marshall plexi or a FTR.

As to "tone".... that's up to the individual. I can't say as I've ever heard a difference between one OT and another, but there are those who claim (without ever putting their money where their mouth is, I might add) that they can tell a difference.

The one upside here is that a pair of 6V6's is not very likely to stress a Bassman's OT at all. Even under a fault condition, it should not suffer any damage. (But a speaker that's shorted out can still bring it down, so don't think that it's invincible. )

HTH




sumgai

[minions]

Okay, so if I were (hypothetically, I doubt I will need to do this) to put a 4-tube OT into a 2-tube amp, the only effect is on the impedance?

I read in one of Dave Hunter's books that the more iron in a particular OT (i.e. the bigger it is), the tighter the bass response.

[minions]

Wait, someone on another forum told me:

"Short and sweet: The impedence of a 6V6 is twice that of a 6L6. The impedence of a pair of 6V6s is twice that of a pair of 6L6s. The impedence of a pair of 6V6s is quadruple the impedence of a quartet of 6L6s.

and on it goes."

[sumgai]

minions,

That person would be incorrect. Since you're getting your internet legs, doing some research, I'll give you a sizable clue: google for simply "6V6" and then do that again for "6L6". Within the first few results, you'll find numerous data sheets. Thereupon, you will note that the customary load impedance for a 6V6 operating in typical Class A₁ or AB₁ is in the vicinity of 5KΩ. For a 6L6, it's about 10% higher.

But don't take my word for it, check it out for yourself, please.


As to "more iron means tighter bass", I would rank that right up there with "solid state amps sound like crap". Both statements can be true for specific instances, but neither of them are true in a general sense. Like all else, how a transformer "sounds" is more dependent of the quality of the build, and not the quantity of any particular material contained therein.

That is, if said transformer were to have some contribution to the sound in the first place.

However, I do admit that a poorly-built transformer, or one that is chosen for use when it is completely inappropriate (different specs, etc.), such cases might well sound bad to most people, not just the "golden ear". I have a hard time believing that such would be the case for most main-stream manufacturers such as Fender, Marshall, etc.

HTH




sumgai

[minions]

Hmm... yeah I checked it out and you're right. Why would they say that it is twice the impedance of a 6V6 then?

[sumgai]

No, in fact I have more to say on the topic. Stand by......


OK, here's the real-meal-deal, as unveiled for you by some guy on the innerwebs.

Most, oh let's be realistic, all but 2 or 3 other people fail to realize what I've known for years. It's not any one part of the signal chain that conjures up "that tone", it's all of them put together. I'm talking about the power supply, the tube(s), the output transformer, and indeed, the speaker(s).

What nearly everyone misses, and this goes 100% for those who use a 12AX7 in a stompbox, is that the OT plays a crucial part, via the phenomena known as "magnetics". Now the term "magnetics" encompansses many properties, and all of them are germane here, but we'll deal with only a few of them for now.

First, it is the action of a current alternating directions through a coil that creates the magnetic field within a transformer. Obviously that describes the primary winding, and the secondary winding observes the same thing, only operating in reverse. But what happens if the magnetic field is not "perfect"? Good question.

In essence, the rate at which the field builds up and collapses is not a perfect reproduction of the sine wave (or any waveform, for that matter). Thanks to inductive lag, there is a moment in time where the current was flowing, but the magnetic field was static - not changing in anyway. But that's pretty small, and for our purposes, we can ignore it. Or can we......

It turns out that if the transformer is not designed correctly, an effect called 'eddy current" will develop within the field. Now this happens in both air core and iron core transformers, but it's most noticeable in the latter. What happens is that if X amount of current is flowing through the winding, then there comes a point where more than X just saturates the core, and any more than that is actually harmful, to our way of thinking. It means that as the incoming current is heading in the downward direction (thus letting the field collapse), the eddy current does not collapse along with it. Instead, it perpetuates the field for longer than intended, when we compare it to the input signal. For most folks, engineers or otherwise, this is known as "distortion".

And boys and girls, I'm here to tell you, THAT's the root cause of nearly all the "smooth" distortion that you consider to be "the magic tone". Blown speakers mean only one thing - bad juju for the transformer itself. A ripped cone, that can be good or bad, depending on your point of view, but consider - would you purposely do what past Guitar Greats have been reputed to do, punch a pencil through the cone a couple of times? And if you did do that, where would you place the pencil? And do you push it through gently, so as not to tear the surrounding paper, or do you jab it with your full strength, and smile in glee as the hole shows several splits in different directions. Oh, and how many times to you do this? The list goes on and on.....

There's no science here, only art. If you've got the moolah, try it with many speakers (that must all be the same kind, so as to not introduce new variables into the experiment), but me, I wouldn't do that. I'd just get an under-powered transformer, rated for say 75% to 90% of the expected output power, and let it "cook" a bit. On the way, it's gonna distort the signal just fine, thanks to those eddy currents (and other magnetic stuff). And if I keep an eye on things, it shouldn't actually burn down or anything. Remember, some heat is expected. After all, a transformer is not one, but two (count 'em, two) transducers in one package. (Come on, think about it.......) And since no transduction is 100% perfect, there is always a bit of heat anyways, right? So we live with it, and carry on.

Now, tubes are also reputed to be the magic panacea of Tone. OK, I'll let that one slide. I've used tubes throughout most of my life, and I can tell when they're biased incorrectly (too sterile, or too hot, as in glowing orange/red), but other than that, I can't tell the difference. Using completely different tube types, yeah, I can hear that, but that's as much the fact that the tube's spec's call for one thing, and the rest of the amp's circuit is delivering something else. Drive current, bias voltage, cathode current, screen differentials, any and all of these can be changed in such a way as to distort the signal. (IOW, force the tube to not reproduce the signal linearly.)

So yeah, the tube contributes to the overall tone in that it might not reproduce the incoming signal in complete faith, but for the most part, it is transparent, and adds nothing to the tone. Or at least, this is the case where the designer was not sleeping during duty hours that day.

Speakers and power supplies has been discussed to death, both here and abroad, so I'll leave them alone for this thread. The power supply does play a small part in getting "the tone", but it's not nearly as important as the OT and the speakers.

Or so I'm of the opinion.

HTH



sumgai

[sumgai]

Apr 9, 2010 17:04:34 GMT -5 minions said:

Hmm... yeah I checked it out and you're right. Why would they say that it is twice the impedance of a 6V6 then?

Because as Chris or Unklmickey would say, they either have a hidden agenda, or else they slept through class that day.

[minions]

Wow that's a lot to mull over. So you're saying, it would be better to get an OT that is doing more work (i.e. one that's a bit too small) than one that isn't doing enough work (one that's too big, a 6L6 OT in a 6V6 amp) because it'll produce more distortion. But what if I want a fairly clean amp? Would I go with a bigger OT?

---

Haha, well I asked them again and they told me to look under the class AB1 section, and it turns out, the load resistance for a 6V6 is around 10k and for a 6L6 it's about 6.6k. Now I'm really confused because that means that the impedance of a 6V6 is about double of a 6L6.

[sumgai]

minions,

Are you looking at "triode" or "pentode" figures? Unless the datasheet is very specific, you should not assume anything but the basics. Both tubes are meant to be used as beam power pentodes, but they can also be wired up as triodes. And that changes the numbers right there. Go back and look again, I think you'll find the numbers are bit closer than 10K to 6.6K.

Also, be aware that you're looking for "Typical" or average values. Sometimes, not always but sometimes, all you can get are the "Maximum" values, not the Typical values. Do note that if you can get "Minimum" and Maximum values, the range will be nearly 2:1 - that's quite a tolerance range, eh? Most tubes will fall somewhere within towards the center of that range, but transformers have to be able to deal with the whole enchilada. Which means, since you're not swapping out transformers that perfectly match the tubes each time, you're dealing with impedance mismatches every time you change tubes. This can't be helped.

But, and here's the kicker, sometimes you get it right on, and it doesn't sound any better than usual. Sometimes you get it way out towards the end of the spectrum, and it sounds really, really, I mean damned good. How do we account for that, eh? No way. We just let 'er ride, and take advantage of what the Gawds gave us that day. There's no way to predict the outcome, and there's no way to guarantee reproducibility - you simply hope for the best. (Although, biasing does play an important part in all this, and adjustments here can make things at least acceptably close, if not "oh yeah, that's it baby!".)

Ready for more? I can keep this up all day (providing the other half don't find out!). After all, I'm drawing on over 50 years of personal experience here.... ;D




sumgai

[minions]

Haha okay I think I got a little sidetracked ;D On the other forum, someone else (not the person who said the impedances were 2:1) told me something similar to what you just told me, but in different words. That the tube datasheets are only hypothetical and from an engineer's perspective. But that is ultimately another issue from the one I needed to be addressed, although I feel I've learned an important lesson in the process!

Now someone else told me that when using an OT typically found in an amp with 6L6s in an amp equipped with 6V6s, you double the output impedance. In other words, if the amp normally has a 4 ohm tap, in the new amp, it has an 8 ohm tap. Is this correct?

Thanks for all the help, by the way. I know it must be annoying and/or frustrating having someone contradict you all the time. Especially someone who has 50 years less knowledge about the topic than you. I'm just trying to understand, and trying to get things right.

[sumgai]

Apr 10, 2010 16:35:39 GMT -5 minions said:

.... I feel I've learned an important lesson in the process!

Hey, anytime you learn something, even by accident, in my book that qualifies as a good day's work!

Now someone else told me that when using an OT typically found in an amp with 6L6s in an amp equipped with 6V6s, you double the output impedance. In other words, if the amp normally has a 4 ohm tap, in the new amp, it has an 8 ohm tap. Is this correct?

This is correct for one condition - that the 6L6 amp had 4 (four) tubes, and not just 2 (two) tubes. If there were 6 tubes (a Super Twin Reverb, or a Marshall 200watter, or a Hiwatt....), then the impedance would triple to 12Ω in your example. But for an equal number of tubes, it stays roughly the same.

And for the record, there are several boutique amp makers that are stuffing in 4 6V6's instead of 2 or 4 6L6's. It's all about the tone, doncha know?

And just because I'm highly caffeinated at the moment.....

I'll let you do the backup research, but you should recognize that when it comes to impedance, we can never get closer than "ballpark". By that I mean, impedance is a measure of the resistance to the flow of an alternating current. And what's that mean? Why of course it means 'frequency'. And at that point, you have to make a whole raft of assumptions, like what frequency spectrum is most likely to be present (if we're not talking about a hi-fi setup), and just how much power is going to be running around in here, etc. etc.

So the upshot is that, for speakers, we have a standard that is pretty universal. Simply stated, it goes like this: Since the impedance shown by the speaker to the output of the amp (tube or ss) varies by frequency, the median impedance point shall be the lowest frequency in the first dip after the first peak (as you start measuring the response from 20Hz on upwards). For some speakers, that might be as low as 30 or 40 Hz, or it might be as high as 1 to 3KHz. (For tweeters of course, it'll be much higher. In fact, for piezo tweeters the impedance is considered infinite - we don't even bother to factor it into our calculations.)

Now, recall that a transformer is inviolate - it can't change any of its parameters on the fly (which is why eddy currents can play merry hell with our signal). So, if the speaker is bouncing back and forth, up and down and all over the map vis-a-vis the impedance it reflects to the secondary winding, what do you suppose is happening over on the primary side of said transformer?

Yeppers, you guessed it - it's flying all over the map too! And that means, the typical values that the tube is rated for will be encountered only once in awhile, and not all the time. Only at some certain frequency, or a small number of them, will the tube and the OT and the speaker all be in sync with each other. And if you paid attention earlier in class, you can now understand why it wouldn't do any good to try and match an OT to a set of tubes - the speaker can (and will) make a big difference, and the frequencies encountered (including all the harmonics, don't forget them) all conspire to wreak havoc with an Engineer's gastronomic system.

The bottom line so far: I'd weigh some factors into my equation before I summarily jumped into this puddle. If I had a transformer on hand, or could get one for dirt cheap, then I'd go ahead and experiment. But if it's gonna take a month's worth of lunch money, then I'd probably skip the whole idea. I personally don't think the potential for benefit weighs enough to trump the cost and effort, but then again, that's just me. Other folks will not have the same priorities as I do, and will thus make different decisions.

I trust you can make up your own mind on that score. ;D

Thanks for all the help, by the way.

No problem!

I know it must be annoying and/or frustrating having someone contradict you all the time. Especially someone who has 50 years less knowledge about the topic than you.

I perceived no contradictions, from you or from anyone else. I did however detect the distinct odor of "lack-of-education-itis". Different thing altogether.

I'm just trying to understand, and trying to get things right.

Well, as the old saying goes, "If you've stopped learning, then you've stopped living". But don't get hung up on what's exactly "right", that's always up for debate! Just store everything away somewhere in your memory banks, and push onwards. Trust me on this.

HTH



sumgai