B52 AT100 Rectifier Classes

[bay4hotrod]

I'm going blind from all the reading to no avail, so maybe somebody that actually works on these can answer it?

I realize this amp has it's issues, and it endures significant bashing, but for 200 bucks it really does sound good.

Here is what I think I know:

1) Rectifier modes:
Class A: tube rectified
Class A/B: tube + ss rectified
Class SS: solid state

2) changing the mode, changes the plate voltage.(hearsay) actual SS: 475v @ 45ma w/30w tubes

Now what am I trying to ask?....

1) is a single tube capable of rectifying 100 watts?
2) if the plate voltage changes with the mode selection, would it be logical to choose a mode, set bias, and stay there?
3) I've heard these amps run quite hot in class A mode; is that why eurotubes recommends biasing/running in class A/B?
4) What the devil am I trying to ask here
5) Exactly what is the purpose of combining tube/ss rectification from a signal standpoint vs. running in a pure tube or ss circuit OR are the mode descriptions actually lost in Chinese translation and have nothing to do with actual operation?

I think (5) is my question, but the rest apply (including #4) - any thoughts?

[Chuck H]

You missed on the operating class and rectifier functions available from the switch. I think it'll be more like:

Class A tube rectified
Class AB tube rectified
Class AB solid state rectified

There is no such thing as 'tube and solid state rectified' (No pedantic jags need to tell me about the diodes feeding the rectifier tube. Thanks). And there's no such thing as Class SS.

Class A and class AB are, for the purposes of this discussion, bias controlled tube operating conditions. So to be able to choose between class A and class AB you need two bias modes. On your amp the class AB mode uses fixed bias. Set this however you like within safe operating parameters. You don't need to worry about how it will affect the class A mode BECAUSE the class A mode switches to cathode bias. This bias circuit sets the grids at a fixed 0V and elevates the cathodes from ground. Because of this resistive elevation a positive voltage is present on the cathodes. As far as tube operation is concerned the tube doesn't much care how it gets it's bias, whether the grid is negative WRT the cathode or the cathode is positive WRT the grid. Same thing to a tube. You can't adjust the cathode bias (class A mode) very easily in your amp. That's alright because it shouldn't need adjusting. Cathode bias is somewhat self adjusting, but see note below*

Class A operating class never allows the tubes into cutoff. The tube operates at full current full time and the signal is conducting from a mid point between saturation and cutoff determined by the bias. The result is lower power, more asymmetrical clipping and no crossover distortion (or it's happier counterpart, swirl)

Class AB only operates each tube for half the wave form and spends the other half of it's time in cut off not conducting at all. By doing this the tubes can conduct almost twice the power because they're only passing current half the time. The bias sets the crossover point so that there is very little audible changing of the guard from one tube to the other.

Class A sounds rounder and harrier at the same time because there is more 2nd order harmonics and less symmetry to the wave form. Class AB is louder and more dynamic. "Harder" sounding, if you don't mind the vague adjective.

The rectifier basically changes the amount of resistance in your power supply before any current devices. The rectifier tube has resistance and diodes (SS) don't. Adding resistance to the power supply does two things. It drops voltage, which results in a softer, less dynamic tone. Especially on the attack. And the resistance makes additional voltage dropping dynamic with current. The more current the amp tries to draw the greater the voltage drop across the resistance. This has a compression effect most players and amp guys call 'sag'. SS rectifiers don't do this (but there is still some sag from other resistances in the power supply).

*Your class A, cathode bias circuit is deigned to work with the tubes the amp was shipped with. As I mentioned, it's not an easy adjustment to make in your amp. I know you changed tube type in your amp so there is a possibility that the different tubes will be less compatible with the class A mode. You can check your bias for this mode in the following way:

With the amp off standby, in class A mode and all volumes at zero measure the DC voltage across R95. Divide that number by R95's resistance value (spec'd as 110 ohms). This is your current. Now measure the DC voltage from pin 3 to pin 8 of any power tube socket. This is the tubes operating voltage. Multiply the operating voltage and current to find watts. Divide that number by four (you want the approximate single tube figure). If all is well it should be between 20 and 24 watts or so. Less is ok if it sounds fine. More could be dangerous to the amp and/or tubes.

[bay4hotrod]

That's the best concise explanation I've ever heard. That makes sense.

Thanks for the correction on the rectifier classes.
Don't mind vague adjectives - I understand what you mean.


Let's see if I can stay corrected:

I'm under the impression that:

1) Class A is the most demanding on amplifiers... higher applied resistance combined with full wave utilization? fact or ignorance?
2) The added resistance in class A, creates a higher plate voltage over AB? ture/nonsense/go back to ckt analysis 101 ?



If any of the above is true, then:

it would seem that B52 opted for a middle-of-the-road bias approach, leaving the fringes to be a mite less efficient. Ideally then, the amp should be biased for a specific player, on a specific channel to yield maximum results?
and
Running this amp hard in class A is courting disaster with a known shaky power section.

If anybody ever caught that discontinued series 'dollhouse' then you'll remember the line "Would you like another treatment?". I keep hearing 'correction' subbed in there somewhere lol!

[Chuck H]

Class A is the most demanding on amplifiers... higher applied resistance combined with full wave utilization? fact or ignorance?

Yes! And no. Class A is probably the most demanding at idle because the tubes must remain in a high current state to pass signal with any efficiency. But when conducting the class A amp will usually be drawing a bit less current than an AB amp. The real enemy for class A designs is heat. Because most music amps spend more time in a low or no conduction state the class A amp does draw more net current than an AB in real world conditions. Making watts isn't that efficient and all that current results in a large heat byproduct. Some classic class A designs have a reputation for it. Some even catch fire occasionally. That's an exaggeration, but a small one.

The added resistance in class A, creates a higher plate voltage over AB? ture/nonsense/go back to ckt analysis 101 ?

False. Class A doesn't add resistance. The use of a rectifier tube does that. And in that case the resistance is up stream of any current. So any current that passes through the amp must first pass through the rectifiers resistance. The voltage rises across that resistance before it gets to the amp so the amp gets a lowered voltage. And the greater the current, the greater the voltage across the rectifier and the lower the voltage on the other amp circuits.

it would seem that B52 opted for a middle-of-the-road bias approach, leaving the fringes to be a mite less efficient. Ideally then, the amp should be biased for a specific player, on a specific channel to yield maximum results?

The amp switches bias circuits entirely for class A or class AB operation. Class A bias is 0V grids and a resistance is between the cathodes and ground. This is somewhat self adjusting and a typical circuit for class A guitar amps. That's probably why it was chosen. Class AB switches to a grid bias where a negative voltage is applied to the grids and the cathodes are held at 0V. This circuit is adjustable and is not in use when the amp is in class A mode. So... You can adjust the bias to your preferences for both AB modes and the class A mode is (ideally) idealized already. No inefficient fringes or middle of the road bias approaches.

Running this amp hard in class A is courting disaster with a known shaky power section.

Running an amp with a shaky power section operating in any bias mode is courting disaster But to address the question as I interpret it, running a class A amp that is too hot may be worse than running a class AB amp that is too hot because the class A amp draws more idle current. In actual use (when making sound) it's probably a wash. Class A amps that use cathode bias do have the advantage of the bias being somewhat self correcting.

*You didn't ask but* Using the different tube brand in your amp is probably fine. I gave the info on how to check just to be sure. The limitations of the class A cathode bias arrangement do not apply to the amp when it's operating in any AB mode and you have an adjustable grid bias for that.

EDIT: I can hardly wait for the ration I'm going to get for going all SGM on this thread (inside joke)

[Jazz P Bass]

B52 AT100 Schematic

Here is the schematic.

[potatofarmer]

If you plot the load lines, you'll see that there's no way these tubes could be "class A" when you flip that switch. They're just cathode biased, which has come to be equivalent to "class A" in guitar amp marketing speak. The amp is going to be in class AB1 no matter where that switch is flipped. Class A requires much lower plate voltage than what you can use in class AB (and a different primary impedance OT as well).

A more honest way to label the switch positions:
"Class A" = cathode bias, tube rectified
"Class AB" = fixed bias, tube rectified
"Class SS" = fixed bias, solid state rectifier.

as for this question:

1) is a single tube capable of rectifying 100 watts?

Should be rephrased as: 1) is a single 5AR4 rectifier tube capable of rectifying enough current for the power amp to deliver 100 watts safely and reliably?

And the answer is no! Absolutely not at full output. I don't see any voltages listed on the schematic, but just compare the data sheets for the 5AR4 and the 6L6GC. You can typically get ~250mA DC through a single 5AR4, which is about as much as a single pair of 6L6GC will draw at full tilt (including the peanuts that the preamp draws). This is why tube-rectified "hundred-watt" amps (typically) have two rectifier tubes.

[Chuck H]

What is "class SS"? And you still have "class A" in your 'honesty switch' positions.?.

Indeed, at 110R for the cathode resistor the amp is almost certainly not in true class A. I've always sort of thought class A push/pull was a bit of a misnomer anyhow. I've never read a good explanation on how these two functions don't automatically negate each other (like jumbo shrimp). Even the mighty AC30 has been shown to be operating in class AB at full power. It's possible then that with a lower drive signal this AT100 is operating class A push/pull every bit as much as the AC 30. And FWIW I made some effort to specifically NOT imply that cathode bias means class A.

Also true that a single 5ar4 isn't going to handle all the watts those tubes can make. I expect the amp doesn't actually make all those watts. Clearly it isn't or rectifier tubes would be popping like corn in this model. Just because it has four big bottle tubes and the model designation includes the number 100 doesn't necessarily mean it's a 100 watt amp. Just as cathode bias has come to mean class A I think four big bottles has come to mean 100. It's not a 100 watt amp any more than it's class A.

I decided not to raise these arguments in the interest of simplicity since I don't think the OP will benefit from them for the purposes of this thread.

[potatofarmer]

What is "class SS"? And you still have "class A" in your 'honesty switch' positions.?.

That's what they're labeled on the amp. I indicated what they should've been labeled on the right of the equals sign.

Indeed, at 110R for the cathode resistor the amp is almost certainly not in true class A. I've always sort of thought class A push/pull was a bit of a misnomer anyhow. I've never read a good explanation on how these two functions don't automatically negate each other (like jumbo shrimp). Even the mighty AC30 has been shown to be operating in class AB at full power. It's possible then that with a lower drive signal this AT100 is operating class A push/pull every bit as much as the AC 30. And FWIW I made some effort to specifically NOT imply that cathode bias means class A.

There isn't any inherent contradiction in "push-pull class A" - there's a phase inverter that inverts the phase delivered to one half of the power tubes, but neither of them reach cut off at full output over the full 360 degrees of the input signal. Basically parallel single-ended, but since they're push-pull they'll also cancel even-order harmonics to some extent. The "every amp is class A at low power" argument is flawed because you determine class at full output. Technically it's class B or D or whatever with the cord unplugged as the tubes are conducting 0% of the time.

Also true that a single 5ar4 isn't going to handle all the watts those tubes can make. I expect the amp doesn't actually make all those watts. Clearly it isn't or rectifier tubes would be popping like corn in this model. Just because it has four big bottle tubes and the model designation includes the number 100 doesn't necessarily mean it's a 100 watt amp. Just as cathode bias has come to mean class A I think four big bottles has come to mean 100. It's not a 100 watt amp any more than it's class A.

If you look in the instruction manual, it shows a diagram of the back plate of the amp that specifies "100W RMS" above the speaker jacks. http://www.b-52stealthseries.com/manuals/AT100.pdf

Of course, they don't specify at what percent distortion the amp makes those 100W. Also I put "hundred watt" in quotes for this reason; it's probably closer to 80W (properly measured) but it's close enough that it's a moot point. They are pretty good about not putting that claim in writing anywhere else though!

And yeah, these have a reputation for being unreliable and breaking down frequently. I don't know how much of that is related to the tube rectifier though; it could be something else entirely.

I suspect that since this is a high (higher?) gain amp that the typical user is going to keep the master volume low and crank the preamp volume(s) so the overall current draw is going to be low. This kind of begs the question of why the tube rectifier is even installed, as I would imagine it has little effect with the master rolled down. That's probably the safest way of operating it, if the OP wants to use either of the tube rectified settings. Or install a sag resistor inside the amp and remove the rectifier socket. IIRC Weber sells one of their Copper Cap rectifier substitutes that's designed to give similar to a 5AR4 but with double the current handling. No idea if those are safe to use though.

Personally, if you're expecting to crank the amp, I'd only use the "Class SS" setting.

I decided not to raise these arguments in the interest of simplicity since I don't think the OP will benefit from them for the purposes of this thread.

Well, I mean, he did ask. Also Dollhouse was great.

But since this isn't Class A in any sense of the term, I don't think we can state that the "class A" setting is any harder on the amp than the other settings, simply by virtue of it being cathode biased. Sure, it's probably biased warmer to reduce the crossover distortion at higher outputs, but it's the warmer bias itself that is harder on the power supply, not how the tubes are biased.

Eurotubes' advice about "amp runs hot in class A" could also be related to all the waste heat the cathode bias resistor has to dissipate, which will be significant. If there isn't a good way to get that heat out of the chassis, it's going to negatively impact the life expectancy of the components.

For peace of mind, it'd be worthwhile to check the bias (well, idle current) at each of the three settings.

[Chuck H]

While your post did clear up some things for me I'm still not getting where class A push/pull is pushing or pulling anything

Thinking about it again it seems like a 110r resistor with a quad of 5881's is going to be damn close to typical a class A bias point. Lack of a proper OT design not withstanding. And I'm not really hung up on it either. Nor am I hung up on whether an amp that operates class A only to a point below it's maximum output is actually class A. It seems that so many terms have been used incorrectly for so long that you sometimes need to use a term incorrectly to be understood. If that makes any sense. There's also the rub that I never studied electronics. So I couldn't have told anyone that class A is only class A if the amp produces it's full power in class A. I would have been much more likely to concede that an amp operates class A to a point and then there is a shift in operating class. I know there are proper and correct usages for many of the parameters and operations of amps that are ignored by the guitar amp ilk. It's just been happening for so long that fact checking it becomes an exercise in semantics.

But I don't mind that either. Someone must carry the flame lest we all walk in the dark

[bay4hotrod]

Some classic class A designs have a reputation for it. Some even catch fire occasionally. That's an exaggeration, but a small one.

From what I've heard, this AT100 might be a good example, but I don't own a suitable soap-box

Thank you Jazz for the print; I usually remember to include that!

What is "class SS"? And you still have "class A" in your 'honesty switch' positions.?.

That made me chuckle, but I understand what you both mean!

I suspect that since this is a high (higher?) gain amp that the typical user is going to keep the master volume low and crank the preamp volume(s)

I can't speak for everybody, but some, including myself have noticed we get a better attack, better harmonics..... well just better everything with the master at 50-60% or so, and then dial channels to suit.
Although I like loud, I don't gig, and this amp is in my not-too-big studio, so it averages a comfortable 40% output. My guess, but I'm sure that will change as my hearing loss improves

It seems that so many terms have been used incorrectly for so long that you sometimes need to use a term incorrectly to be understood. If that makes any sense.

In my shop anything that doesn't work must be 'shorted' ; I don't even bother ejamacatin folks anymore, I just say " we'll check it out" !

My "shop" is auto-mechanics (and whatever else lately) and from experience I've noticed that all cars are built fairly decent. If they weren't, the company would cease to exist with a quickness. BUT, I have also noticed two things

1) Some models, and in some cases, whole makes weren't intended to be repaired. This is how the Kia/Hyundai idea started. They were meant to drive 100-150K miles, then get [I]another one[I]- hence the low initial cost. putting them back together again is often akin to re-gluing a tightly wound rubber band. (no offense to those owners at all)
2) Every engine has its characteristic weak points. Example: GM 3100/3400 had coolant pipes [I]above[I] the engine, which had it's uses, but was prone to air pockets, which creates continual stress leading to blown radiators, heater cores and head gaskets over time. These engines are die-hard under a watchful eye, but a good deal of owners are anything BUT. so this is one of my staples.

My point: B52 seems to have taken a great design and gone the low-road with the part quality and assembly. Potatofarmer mentioned high heat at idle, and possible 'cranking the amp' issues an Class A. Even though I might never fully understand the intricate details of tube circuits, I like to at least know, in a fundamental way, about the equipment I own! (and how I am less likely to have to keep pestering you guy on how to revive the thing!).
According to my use, it seems to me that I should stick to .... wait here.. the 'labeled' class ss, or A/B at any rate!
I know I need practice on my soldering, but I'd much rather drive it than work on it..

I suppose after all this my real question was: What are the weak points in the amp (so many) and how to best prevent failure. Should have been a better buyer, and bought another one instead? hah! There, I said it first!

I wouldn't know how to bring up half the arguments discussed in this forum, but I've never researched a service manual for something I already knew either!