Guitar store experience/ tube question

Ax · 09-27-2006, 12:05 PM

now, what's really cool is the Tunderverb...

**thesteve** · 09-27-2006, 01:57 PM

So I sent that email to B-52: Here's what I said,
"Hello,
I was wondering if it would be possible to get a hold on a copy of the expanded AT-212 manual that comes with the amp. I've seen the one online, but a friend of mine told me that his AT-212 came with a more extensive manual. I can handle PDF files if that is an option. Thanks.
-Steve"

And their response (which was surprisingly fast considering I emailed about 4 hours ago)
"Steve:

Thank you for your support.
This is the only manual we provide.

Best Regards,
Avi El-Kiss"

Ax · 09-27-2006, 04:10 PM

mine has 4 pages, the one on the PDF has 2...

**thesteve** · 09-27-2006, 04:19 PM

is this the one that came with the amp?
http://www.b-52stealthseries.com/manuals/AT212.pdf

What5647 · 09-27-2006, 04:31 PM

now i'm really puzzled about that amp's rectifiers, that manual really doen't answer our questions. would different output voltage levels be enough to create the tonal differences described there? i think things boil donw to either that switch changeing the voltage ou on the tube rectifier, or it alters something in the signal chain as well.

**thesteve** · 09-27-2006, 04:47 PM

Quote:

Originally Posted by What5647

now i'm really puzzled about that amp's rectifiers, that manual really doen't answer our questions. would different output voltage levels be enough to create the tonal differences described there? i think things boil donw to either that switch changeing the voltage ou on the tube rectifier, or it alters something in the signal chain as well.

Changing the output voltage would cause a bias shift from hot to cold, which might be significant enough to effect the tonality of the amp.

Ax · 09-27-2006, 05:10 PM

maybe that is it... it doens't look right, but if that's what they say they give out, then that must be what they give out...

and i can hear subtle differences, i usually use A/B or SS...

What5647 · 09-27-2006, 08:09 PM

which makes sense, since you (from what i remember) tend to paly stuff with alot of distortion, and the "A" setting would (presumably) be the most "saggy." changing the rectifier output voltage would certainly affect sag.

**thesteve** · 09-28-2006, 12:50 AM

Quote:

Originally Posted by What5647

which makes sense, since you (from what i remember) tend to paly stuff with alot of distortion, and the "A" setting would (presumably) be the most "saggy." changing the rectifier output voltage would certainly affect sag.

Ahh...I should clarify. What I meant with the output voltage variation was in reference to the power tube biasing. On my Fender Hot Rod Deluxe, there definitely was a tonal difference in the amp when I compared the tubes biased at the recommended spec (68mV) compared to a cold spec (45mV).

Ax · 09-28-2006, 07:46 AM

i mean, it's not a huge difference, not enough to compromise tone, but, it's like it's got a different "feel" to the sound... that's why i generally prefer A/B... A has a better feel, but it doesn't sound as good... it's like the mix from quality of sound to great feel, so i choose the medium... it's hard to explain...

Major Tom · 09-28-2006, 10:03 AM

I don't understand the rectifier switch on the B-52 either.... A couple thoughts - if it has 2 rectifier tubes, there may be an actual tube engineering class A and class AB rectifier circuit that would yield different amounts of current or whatever that could yield some variation of overall tone.... It could also be that they just call it that, after all, "Class A" is a term that conjures positive feelings in players. Either way, any class a, ab, x, y, or z rectifier circuit has nothing to do with how the power tube circuit, which is the actual signal path, is configured (class a, ab, etc.).

I researched this a bit a couple years back when I was doing some extensive mods on a couple amps, reading stuff from a lot of "tube gurus". I am not any tube expert....

Amps with a single power tube are by nature class a. The overwhelming majority of guitar amps that claim to be class a are not. Amps that are called class a by the mfr are usually cathode biased, as opposed to the more common fixed bias circuitry (Fenders, Marshalls, etc.). It is my understanding that cathode biased amps do typically operate in class a mode at low volumes, but when you turn them up a bit, they morph into class ab. The bias circuit is not the signal path, its like a control circuit, controlling how the power tubes will work. Fixed bias is when the bias voltage is set, it doesn't change - fixed bias amps are the ones that need to have this bias voltage/current adjusted when new power tubes are installed due to the fact the current draw of power tubes vary a bit, and this is a very sensitive/precise circuit. Cathode biasing is a circuit where the bias voltage/current varies, I don't understand it all that well, but its dependent on how much current the power tubes are "processing". Cathode biased amps do not need any "re-biasing" when changing power tubes; the power tubes themselves govern the bias circuit. Its my understanding that in general, all else being equal, cathode biased amps are harder on power tubes, they won't last quite as long.

One of the mods I did on my Peavey Bravos was a switchable cathode bias. If it had come like this from the factory, I'm sure Peavey would have labeled this switch "Class A / Class AB"

With the way I set the amp's eq and gain controls, I don't really hear any significant difference, although I haven't done extensive a/b testing. It may be my imagination, but I do think the cathode setting is just a bit more compressed, what some may call more "vintage" in tone; not enough to really worry about though.

I did read from one "tube guru" who basically stated that the difference between class a and ab will make a bit of difference in a high fidelity amp, but would not be audible in a guitar amp.

The grandfather of the whole "class a" amp thing is the Vox AC30 (and other Vox amps too). There are a lot of things about the Vox amps that give it its unique tone - no negative feedback loop, the speakers, unique preamp circuit, etc. IMO, the cathode biasing may be a part of it, but a very small part.

Bottom line, don't concern yourself with whether an amp is class a, or ab or whatever. That whole "class A" thing is like a marketing term that really means nothing, and is pretty much a joke among tube amp techs.