OT Plate Load

[Wilder Amplification]

Why all of a sudden does it seem that none of the big name OEMs along with lots of the boutique shops think that OT plate load doesn't matter anymore? Hell we even have Randall Smith encouraging people to mismatch while others think it's OK to run any type of tube in any type of amp as long as you compensate the bias range, regardless of what tube the amp was originally designed to run without swapping out the OT for the OT with the proper plate load for the tube they're wanting to run.

Just curious as I always considered OT plate load a pretty damn critical spec to adhere to if you wanted your amp to live a long time. Thoughts?

[MWJB]

It just depends where your datum is.

If you had a 6L6/EL34 amp with a 2.5K primary you would be very unwise to reduce the speaker load and bring the primary down further.

Conversely, if the same tubes are working to 7.5-8K you would be equally unwise to double the speaker load, bringing up the primary further.

So if you aim for 3.6-4.5K you get some flexibility.

Tubes will operate reliably over quite a range (-50%/+100% of centre), other factors will include things like the use of parallel speaker jacks (halving rated load).

To all intents and purposes, plate current allowing, 6L6/5881/EL34/KT66/6550/KT88/KT90 CAN be considered intercangeable in that respect. Likewise 6V6/EL84. Even if "optimal" loads on data sheets suggest otherwise.

In reality, many amp manufacturers who used off the shelf OTs ended up with something that would "function" at a reasonable price, rather than an ideal. Hence you ended up with 6L6 amps that might be run into 2.5K, or as much as 7.5K...or 6V6 PP amps running as low as 4.5K.

Later designs might well emulate the proven, reliable, empirically arrived at values.

Bear in mind, that in many respects, guitar amp designs have been bucking the accepted tube manfacturers data since the late 50's...so it hasn't really happened "all of a sudden".

[Enzo]

I am trying to think of a time when it was considered critical. Leo Fender was busting specs 50 years ago. SO I don;t know about all of a sudden.

If we want to get just so and have our curves follow exactly our theory, then I guess things are critical. But if I have learned anything working in guitar amp land, it is that darn little is "critical." Folks obsess over half a milliamp in bias current settings, or a tenth of a watt dissipation. People worry about the 100 watt peak portion of their curves while ignoring that 99% of the time the circuit is not operating there. And so on. These people busting free of the recommended guidelines don;t seem to be making incendiary amplifiers, they seem to be working.

[Wilder Amplification]

Well I guess more so what I was getting at was kinda what MWJB stated about it depending on where your datum is. Most of what he says makes perfect sense...if you're plate load is spec'ed somewhere in the middle, plate current allowing, you have some wiggle room either way, but too far to one extreme or the other and you won't wanna mismatch one way or the other depending on which extreme you're closer to.

The problem is that you tell guitarists that it's OK to mismatch a certain amp due to the fact that the Z plate-plate spec allows for it on that certain amp, and they think it's OK to mismatch on ANY amp, when truth be known I wouldn't wanna try it unless I know exactly where in the acceptable range the OT plate load is at, plate current allowing, and nobody has a way of finding that out unless they have a signal generator of some sort (I use wall voltage on the primary) and take the voltage measurements, then calculate the Z ratio, which is something most guitarist not only don't know how to do but try to explain it to them and they're like a deer in the headlights. Hence to be on the safe side of things I just preach not to do it at all if it can be avoided.

The "all of a sudden" came about because 15 years ago when I was first learning amps I was always taught that you always avoid mismatching your impedance. When I was 14 my first tube amp believe it or not was this old Bogen PA amp with 7027As, and I smoked the OT within a week of owning it all because I wired the speaker cabinet completely wrong. That and it seems that I personally have always had bad experiences with mismatching. Too far up and plate current exceeds dissipation whereas too high and you're taking out screen resistors...hence the other reason why I always preach against it.

[jrfrond]

As much as some people would like to beat the specs to death, there is definitely a lot of latitude with tube power amp designs. Guitar amps aren't rocket science, and really never have been, save for a very few designs.

For me, as long as it all specs out in the end, looks good on the scope, isn't overheating or going up in smoke, and most importantly SOUNDS good, then it IS good. Designing guitar amps is generally a more organic process than hi-fidelity amp design. Now granted, there are many people who have the resources and brains to sweat the numbers, specs and minutia. I respect those who can, because even though I am pretty good at this, they can run technical rings around me. However, I submit to you that in the bulk of the cases, it is highly unnecessary, and things like off-the-shelf transformers together with a small dose off burying one's head in the sand and overlooking hard-core tech specs is sufficient for designing good-sounding guitar amps.

[Steve Conner]

Designing guitar amps is generally a more organic process than hi-fidelity amp design.

I totally agree! The bottom line is that musicians are usually broke, so guitar amps have to be cheap. So there isn't enough money in the market to support really good R&D. If you were a proper rocket scientist, you could make a lot more money designing things for industry or defence. The classic guitar amp designers in the 50s and 60s were the guys who didn't make the grade to get into that.

So, I'm pretty sure the amps we now consider classics just used whatever transformers were on special offer that week. "Organic design process" would be a polite way of putting it.

Look at the old Marshalls that used the RS universal output transformer. That was a general purpose replacement thing sold by Radiospares in the UK, with a selection of taps to suit most push-pull output stages.

Now, maybe it was what Clapton had in the amp that he recorded the Beano album with, or something. But the original now sells for something scary like $500, and I think Mercury Magnetics, or Marstran or someone, even have a clone of it.

http://www.marstran.com/JTM45%20RS%2...ransformer.htm

But it was just a jellybean thing at the time, like a Hammond now. The bottom line is that you can do the same! Just get it in the ballpark so things aren't burning up, crank it up and enjoy. The math really doesn't matter. I bet you money that if we travelled back in time and swapped out Clapton's OT for a Hammond, nobody would ever notice.

The trouble starts when you try to wring every last ounce of performance out of some part. The ballpark you're aiming for gets smaller, and there's less room for mismatching, abuse and so on. A case in point would be the post-CBS Fenders, trying to get 300W out of four 6550s and so on. You can do it if everything is set up just right, but along comes some guy and plugs a 16 ohm cabinet into the 4 ohm socket, and foom! I think those amps are a case of rocket science where it's not needed. They were making space rockets, and the market wanted tractors.

Don't even get me started on hi-fi designers, I think they're even worse than the guitar amp guys. Not only are they crazy, but they don't have a bottom line to keep them down to earth! No matter what whacked-out junk they come up with, some rich audiofule somewhere will buy it.

[Robert M. Martinelli]

So, I'm pretty sure the amps we now consider classics just used whatever transformers were on special offer that week. "Organic design process" would be a polite way of putting it.

Look at the old Marshalls that used the RS universal output transformer. That was a general purpose replacement thing sold by Radiospares in the UK, with a selection of taps to suit most push-pull output stages.

Now, maybe it was what Clapton had in the amp that he recorded the Beano album with, or something. But the original now sells for something scary like $500, and I think Mercury Magnetics, or Marstran or someone, even have a clone of it.

Absolutely true!

The same applies to guitars too.....I love vintage guitars, I own some too, but my memory is still good enough to remind me that Jimi played a (then) contemporary Strat at Woodstock, and that, back in '69, a '54 or a '57 Strat was only considered ....er.....I think there's no gentler way to say it.....simply old....and Strats were designed and geared for mass production, a vast number of them being made of cheap Alder, and "conceptually" aimed at the working musician. Now that the vintage craze has taken off we have people making their living out of cloning Jimi's "arctic white" Strat and relic-ing current production guitars sold at unrealistic prices....as if all the magic was in the guitar and not in Jimi's ( or Eric's ) fingers ( and heart and brain )......sheesh!

(some die-hard vintage addict will have his flamethrower lit and aimed at me already )

Truth can hurt

Sorry, maybe a little off-topic, but I couldn't hold myself back

Cheers

Bob

[Old Tele man]

Absolutely true!

The same applies to guitars too.....I love vintage guitars, I own some too, but my memory is still good enough to remind me that Jimi played a (then) contemporary Strat at Woodstock, and that, back in '69, a '54 or a '57 Strat was only considered ....er.....I think there's no gentler way to say it.....simply old....and Strats were designed and geared for mass production, a vast number of them being made of cheap Alder, and "conceptually" aimed at the working musician. Now that the vintage craze has taken off we have people making their living out of cloning Jimi's "arctic white" Strat and relic-ing current production guitars sold at unrealistic prices....as if all the magic was in the guitar and not in Jimi's ( or Eric's ) fingers ( and heart and brain )......sheesh!

(some die-hard vintage addict will have his flamethrower lit and aimed at me already )

Truth can hurt

Sorry, maybe a little off-topic, but I couldn't hold myself back

Cheers

Bob

...+1 and paraphrasing: "...what's good for the GOOSE is good for the GANDER" and likewise: "...what applied to AMPS also applied to the GUITARS."

[R.G.]

Well I guess more so what I was getting at was kinda what MWJB stated about it depending on where your datum is. Most of what he says makes perfect sense...if you're plate load is spec'ed somewhere in the middle, plate current allowing, you have some wiggle room either way, but too far to one extreme or the other and you won't wanna mismatch one way or the other depending on which extreme you're closer to.

I think that more to the point is the concept that tubes are, over a broad range, inherently power limited. You can burn them out, but they do not usually fail in a puff of smoke like the much higher power density solid state parts. You have to cook them to death, and they're physically large enough for that to take time.

Back in the Golden Age, the techie designers of output tube stages tested impedance loading on tubes over very, very wide ranges. They were looking for two things in general - maximum power out and lowest distortion. And they found that for most power tubes, there are specific loads for maximum power, and for minimum distortion. They also found that those two desirable items do not happen at the same loading, and that the curves which show those two things are not touchy, peaky, sensitive adjustments. Power tubes have very, very broad humps in the power out versus loading curves and also very, very wide minima in distortion.

The net result is that loading matters, but it isn't critical. It's just not a big deal. Yes, you can burn out your tubes by running them under conditions where they will overheat. But you can do that with an ill-considered bias adjustment probably easier that mismatching them to death.

... when truth be known I wouldn't wanna try it unless I know exactly where in the acceptable range the OT plate load is at, plate current allowing,

The acceptable range is really a huge range, the kind of thing you can drive a truck through, at lease in terms of damage.

and nobody has a way of finding that out unless they have a signal generator of some sort

I see a number of issues with the underlying assumptions here. First, it's not a delicate, fragile, sensitive alignment. Two, it's not a mystery, just not something that's all that commonly bandied about. Three, signal generators are not a big a requirement IMHO, and four, if a signal generator were needed, a suitable one takes one or two $0.05 transistors and maybe a dozen parts total; there's a schemo of such a generator on my web site.

(I use wall voltage on the primary) and take the voltage measurements, then calculate the Z ratio, which is something most guitarist not only don't know how to do but try to explain it to them and they're like a deer in the headlights.

The whole reason to have techies is to make it unnecessary for guitarists to have to learn that too. They have enough to do to make music.

Hence to be on the safe side of things I just preach not to do it at all if it can be avoided.

IMHO, you're being way overcautious.

The "all of a sudden" came about because 15 years ago when I was first learning amps I was always taught that you always avoid mismatching your impedance. When I was 14 my first tube amp believe it or not was this old Bogen PA amp with 7027As, and I smoked the OT within a week of owning it all because I wired the speaker cabinet completely wrong. That and it seems that I personally have always had bad experiences with mismatching. Too far up and plate current exceeds dissipation whereas too high and you're taking out screen resistors...hence the other reason why I always preach against it.

We are all products of our experiences, aren't we? I've tried to kill things by shorting outputs, open circuiting outputs, and so on. I've never managed to actually kill an OT, although I did overheat some tubes when I left them running into an abusive load. The skeptic in me also suspects that the OT on that Bogen PA may already have been having some problems and you might not have noticed since it was your first tube amp. It may not have been entirely (or at all) that you wired the speaker cab wrong.

I think that the operating conditions inside the amp have more to do with burning out tubes and OTs than is commonly recognized. A class A biased amp, for instance, is designed with a relatively low B+, and runs both the output tubes and OT at what amounts to full current and voltage all the time. It is quite difficult to overheat a class A amp (any more than it already is!) just by changing loads. That's very different from a deeply B-biased class AB amp. In a class AB amp the power supply is quite high and the standing bias is very low. The power supply current (that's where all the power comes from...) is very low when it's doing nothing, and gets much, much higher when you drive it. Loading probably matters more on deep AB amps than on class A, if only because the range of power available is bigger with the higher B+.

So I don't think it's a cut-and-dried issue at all. My experience has been that tube amps are very tolerant of loading as long as you don't let them get into puncturing their OT insulation or arcing by underloading them. And I've tried to kill a couple of conservatively designed tube amps by opening the output. Never could get them to die.

[mooreamps]

I totally agree! The bottom line is that musicians are usually broke, so guitar amps have to be cheap. So there isn't enough money in the market to support really good R&D.

Many claim to be broke. They just don't want to spend the money for a good amplifier. But, when it comes to making "their tone", they just want the moon, and not have to pay anything for it. There was some other guy, if actually forget who, was posting threads on other forum insisting an 18 watt amp could be built for just a couple hundred dollars. With 200 bucks, would just about cover the cost of all the transformers.

No, it's not quite rocket science, but it does require some amount of "horse sense" and really good assembly/woodworking skills to make a really good looking and good sounding amplifier. But what I do, I draw a line in the sand. If they won't fork over the bread, they don't get fed.


-g

[Enzo]

My favorite story from the History of Marshall book was Ken Bran, one of Marshall's designers, telling of the time he had some Fender transformers and decided to stick them in one of his Marshall amps. "Still sounded like a Marshall."

[jrfrond]

I totally agree with Steve and Rob. First of all, designs that DO squeeze every ounce of juice out of a tube rarely sound good for guitar, with the SVT and Marshal Major being two rare exceptions. These typically ultralinear are generally found in bass amps, but I have also found empirically that overdesigned hi-fi sounding bass amps are not necessarily the best approach, as some harmonic dirt yields more "growl" and allows them to peek out of the mix better. An as far as audiofools are concerned, I worked in that industry when I spent time as the service manager and senior tech for Tandberg USA, and I dealt with a lot of knuckleheads who swear they could hear 1dB deviations in audio. Yeah, right!

It is my personal opinion that the best guitar and amp designs have already been realized, and we've hit a virtual brick wall. Most new "designers", including most of us here, are really modders and recyclers, regardless of how knowledgeable one might be. Now, it's really up to the PLAYERS to realize the equipment potential and stop chasing ghosts and the elusive gear that will magically turn them into better musicians.

I am not saying that new, cool gear will never be produced or burst anyone's bubble, but again, we are not talking rocket science here. Making music is (or should be anyway) an organic process, so perhaps the "organic" design process for instruments often yields the best tools for the job.

[Joey Voltage]

Now, it's really up to the PLAYERS to realize the equipment potential and stop chasing ghosts and the elusive gear that will magically turn them into better musicians.

I agree, I said this on another forum, I guess I will say it again although I have a strong feeling everybody here is in agreement. Musicians are just kinda terrible people in general mand ost of them really want to attribute a good portion of musical ability to gear, and thats where the ghost chasing, (and businesses trying to sell the ghost) comes from. When a musician is trying to buy all the gear that his/her favorite player uses, what they are really asking for is how to play, phrase, and feel like them. The player is the first thing in the signal path, and am extraordinary amplifier is only as good as the asshat sitting or standing in front of it.

I totally agree with Steve and Rob. First of all, designs that DO squeeze every ounce of juice out of a tube rarely sound good for guitar, with the SVT and Marshal Major being two rare exceptions. These typically ultralinear are generally found in bass amps, but I have also found empirically that overdesigned hi-fi sounding bass amps are not necessarily the best approach, as some harmonic dirt yields more "growl" and allows them to peek out of the mix better. An as far as audiofools are concerned, I worked in that industry when I spent time as the service manager and senior tech for Tandberg USA, and I dealt with a lot of knuckleheads who swear they could hear 1dB deviations in audio. Yeah, right!

Well I agree, and disagree at the same time. To me it really depends on the context in which this PA will be used. I really feel a sweeping statement like that isn't necessarily true. It certainly may not suit the needs of a classic rocker who is trying to juice the PA a bit, and is trying to get a super harmonically complex sound, but may work very well for a modern metal head trying to get a very stiff percussive attack.

a 1db deviation is pretty silly though

[Wilder Amplification]

a 1db deviation is pretty silly though

But, that's "1 louda" i'nt it?

That's something I stated on another post on this forum. It seems that everyone assumes that we all use power amp overdrive and that we all want that creamy bluesy "woman tone", and how could you want anything else? To me, the bluesy creamy woman tone only cuts it for one thing and one thing only...BLUES. Try to play metal through it...the low end farts out and falls apart and the amp feels very sloppy.

Times have changed...only a small percentage of us actually run cranked/overdriven power tubes anymore. Most now are wanting a more versatile rig that allows you to seamlessly switch from crystal clean to flat out high gain without having to use the guitar's volume control, that requires an FX loop and a clean/tight PA. However, preamp overdrive has gotten much better than it used to be. A properly voiced preamp can sound every bit as great as an overdriven output section. But a high gain channel switcher, when used as designed, doesn't sound very good with a power amp that doesn't stay clean and tight at loud volumes. I'm not saying "solid state clean/tight". I'm saying tight, but still with lots more "girth" than a solid state amp could ever have.

Then as JV mentioned you get into the metal guys who want that tight and heavy percussive attack transient, and they want it LOUD. Some tube power amps will start falling apart in the low end when you push them to a certain point and while this may work for blues, it certainly does not work for metal. Just as a tight metal amp wouldn't work for blues (although it might work for that country twang & "spank" when ran clean).

[Steve Conner]

Well, you're talking about "the metal guys"... Sure you're not a closet mosher yourself? \m/

I am, when learning to play guitar I wore out Metallica records the same way a previous generation would have done with Zeppelin. I kind of regret that now, because in the scheme of things they really weren't that good. And my guitar tone preferences were pretty "modern", though I got into the blues end of things later on, partly on account of my interest in tube amps.

I pretty much agree with Wilder's observations there, except for one thing. I also found that it's easy to get that big, thumping low end at medium volumes, but when you try to make it at gig volumes it just falls apart and mushes up, unless you have a huge power amp and lots of powerful speakers like EVM12Ls.

So instead of lugging some gigantic stack, the fix I humbly suggest is to leave the percussive attack and thumping low end to the drummer and bassist. That's what they're there for! You can use a reasonably sized amp (though "reasonable" isn't a very metal concept!) and instead of using the EQ to scoop out the midrange, use it to focus whatever power the amp has into a really violent buzzsaw grind. You also need less gain once the power tubes start distorting.

There's nothing to stop you cranking up the bass and getting that heavy, thumping tone from the same amp when you're practicing on your own. But I think it's an important concept, this idea that there's no single "ultimate tone". There's just the right tone for the situation, and it can be completely different depending on how loud you're playing, what style of music you're playing, what size of venue you're playing, and who you're playing with.

And I think a good modern amp design should be able to rise to that challenge. You should be able to get lots of juicy distortion at any volume if you want it, from the preamp alone, or if you're playing loud, from the power amp too. And enough tone and EQ knobs to shape the sound in the way I described.

But, when it comes to making "their tone", they just want the moon, and not have to pay anything for it.

"They want the f***ing moon on a stick" as an old colleague of mine used to say.