Official Guitar, Amp, Pedals, and Gear Thread

[gomestar]

on the Fargen Mini Plex MK II, I know it has the ability to easily switch out the stock tubes for, as they put it, EL34/KT66 /KT77/6550/6L6/5881/6V6 without the need to re-bias. What's the deal with this? Is there a resister of some sorts in the chain?

Because it's a single ended Class-A amp with a fixed biased.

ok, so help me understand (srs question, my electrical knowledge pretty much ends with changing lightbulbs, and I still screwed that up a few weeks ago). We re-bias to regulate the power going into the tubes. EL34 require more power, wattage, voltage, or whatever compared to 6L6, which requiers less. Going from one to the other requires the re-bias to adjust for the natural input requirements of these individual tubes. How would a fixed bias help here? And on the Fargen, one isnt' necessarily going from simply a EL34 to a 6L6, but maybe even a KT88 with an EL34 in the pre-amp.

 

[gomestar]

also, I played a bunch with the 1965 Fender Princeton Reverb. 15 watts and plenty of power. I noticed that the volume seem to peak at level 6, and then 6-10 is where the tubes start to break up. Everything broke much quicker with the more powerful Lester.

The little thing has plenty of power to cut through un-mic'ed. Then I plugged it into a 2x12 cab. HUGE volume difference, as my bowels will tell you.


really, really, really beautiful sounding amp.

 

[ac_slater]

I can address all those questions Gomestar.

Let me explain how the Fargen works. It uses a circuit very similar to the THD amps that came out in the late 90's. The technology is really nothing new. Since it is a class A amp, this means it runs all tubes at full current all the time, no matter what. But that's not what's special about the Fargen. The Fargen uses tube sensing technology that very simply senses what bias the tube in each socket needs and supplies it appropriately. In the case of the Mini Plex II, each tube socket has its own individually supplied bias, so you can have two different tubes in each socket and they will be biased separately. Since the sockets are octal, you can use any octal tube you can get your hands on.

There is one big downside to the design. Since it's a class A amp and it runs tubes at full voltage all the time, you are obviously running a lot of these tubes way hotter than they might have been designed. As a result, they won't last as long. You can really burn through some 6L6's with the Mini Plex for sure. So in reality, the Fargen is only fixed in its output voltage, not its bias voltage.

There are some other inherent problems with an amp like this, but they are too complex and numerous to get into.

All old Fender amps peak at 4-6 on the volume. What's happening is the PT stage is not powerful enough to handle the volume being fed into it by the output tubes and you are hearing the entire output section of the amp bottleneck itself by distorting.

Literally when you hear an old Fender break up, it's because of an inherent flaw in the design where the amp can't handle the volume it's trying to create, so it distorts. Luckily everyone seems to love the sound of this "flawed design."

 

[gomestar]

ah, gotchya. I wasn't fully aware of the technology there. Cool that each socket will re-bias automatically according to the tube.

Would the full voltage mean that it's effectively running each tube as if pre-amp was turned to 11 and power amp was turned to 11 at all times? I can see that eating up tubes like crazy, even if you only play 1-2 hours a day. On the flip side, you don't need 4 6L6's just to get the thing back to running.

It sounds like, and correct me if i'm wrong, it's taking the signal from the guitar, amplifying it to the fullest via pre-amp tubes and then power tubes, and then taking this very full singnal and cutting the output without altering the tone like an attenuator would.

 

[ac_slater]



ah, gotchya. I wasn't fully aware of the technology there. Cool that each socket will re-bias automatically according to the tube.

Would the full voltage mean that it's effectively running each tube as if pre-amp was turned to 11 and power amp was turned to 11 at all times? I can see that eating up tubes like crazy, even if you only play 1-2 hours a day. On the flip side, you don't need 4 6L6's just to get the thing back to running.

It sounds like, and correct me if i'm wrong, it's taking the signal from the guitar, amplifying it to the fullest via pre-amp tubes and then power tubes, and then taking this very full singnal and cutting the output without altering the tone like an attenuator would.

To answer this I need to explain some things about bias in amps.

Fixed bias does not = a constant bias for all tubes. It merely means that the bias on the amp is meant to stay the same unless you change output tubes. In this case, the Fargen is fixed bias. It does technically change the bias for different tubes. A lot of Marshall amps are technically fixed bias. They contain a bias adjustment pot for when you change tube brands.

An actual fixed bias where the bias never changes in the amp is called Cathode biased. A Vox AC30 for example is cathode biased. The idea here is you take an EL84 tube and run it hot as hell ALL THE TIME and it will sound amazing from all the bloom and breakup.

The Fargen is advertised as Class A but that is sort of misleading because a lot of people when they think Class A also think Cathode biased, and the Fargen is not. However, the Fargen (I would assume) does set its bias points on each tube to run fairly hot, because if you are going to use the attenuator on it, you want your tubes to stay hot and keep that saturation.

Any time you have an amp running an unusual wattage for its tube (12 watts for a KT66 is marsupialed) or using a power attenuator, you are going to damage the tubes to some degree, one way or another.

Basically with the Fargen, you can turn the volume all the way up to get that super crunch and then lower the volume with the attenuator...but it's still running on full and burning the hell out of those tubes.

 

[gomestar]



Fixed bias does not = a constant bias for all tubes.

noted. I assumed it meant a constant bias that couldn't be adjusted. In retrospect, that would make no sense since each tube is naturally different and a constant bias would be no good.



I watched my father re-bias the 800 over the holiday break. I forget what the anode voltage was and what the MA was that we had to turn it to.

 

[Piobaire]



Basically with the Fargen, you can turn the volume all the way up to get that super crunch and then lower the volume with the attenuator...but it's still running on full and burning the hell out of those tubes.

But isn't this sort of meaningless? I mean, the idea is to get this sound at low volumes, not to build an amp that extends tube life. The whole concept of that amp is to get the sound at bedroom volume so it seems trivial to me this is not best for the tubes.

 

[ac_slater]



I watched my father re-bias the 800 over the holiday break. I forget what the anode voltage was and what the MA was that we had to turn it to.

Funny you mentioned it, the JCM 800 is probably one of the most interesting amps when you're talking about bias. For one thing, there are so many different models of 800's that you can cause a headache just talking about different revisions. Most people agree the 2203's from the early 80's are the best ones. But it's highly debatable.

Anyway, something most people don't talk about when you bring up the 800 is a couple of things really. Firstly, that amp gets a lot of hate because people don't understand how to use it or bias it properly. It was designed to be ran quite loud and simply doesn't sound goo at anything less than paint peeling levels of volume. A lot of people get one and complain that the treble is unbearable. There are two reasons for this. Firstly, most of the amp's bass comes from the power section, which doesn't get cooking at all until you're at half volume. Most people never get the amp past 25%. So of course the treble is going to be god awful. The other reason for this is the bias. People seem to think that the optimum bias point for power tubes in an 800 is 70% voltage. This might be true if it weren't for 2 things. Marshall designed the amp to run on NOS Mullard tubes, and they also forgot to put screen grid resistors in the amp, which basically means that this amp eats tubes for breakfast. The trick is to set it to 50% or 60% and run it like that. A lot of people freak out when I say this because they assume it's going to jack up the tone, but in reality you are going to get better tone at lower volumes because the tubes are gonna work harder to push the OT. In theory this is kind of backwards thinking, but I promise it's true.

But isn't this sort of meaningless? I mean, the idea is to get this sound at low volumes, not to build an amp that extends tube life. The whole concept of that amp is to get the sound at bedroom volume so it seems trivial to me this is not best for the tubes.

It's only meaningless if you are rich and can afford an endless supply of high end NOS tubes. Some people like to know how often they'll be buying new bottles when they get an amp.

There ARE amps out there that can get low wattage without burning tubes at such a high rate. The Fargen was not designed to do that, and keeping its price in mind, I doubt people who own one are crying daily about tube costs. I myself have a Matchless Chieftain that blows through EL34's pretty fast if I run it cranked. So this is nothing new to me.

 

[gomestar]



But isn't this sort of meaningless? I mean, the idea is to get this sound at low volumes, not to build an amp that extends tube life. The whole concept of that amp is to get the sound at bedroom volume so it seems trivial to me this is not best for the tubes.

what doesn't kill you makes you stronger. Unless you're a tube and can be easily replaced. Tone is key.

 

[patrickBOOTH]

I don't think the majority of us here are even playing enough where tube life is ever an issue.

 

[ac_slater]

Assuming you play around 3 hours a week you are looking at new tubes every 4 years depending on how hot you run your amp, hot they are biased, and what tube brand.

 

[patrickBOOTH]

Ehhh… Back when I was playing 3-4 hours a day I never ever, ever had to replace tubes except when I physically damaged them by an accident. Then again replacing two tubes every four years is a non-issue. To me it is like buying a Ferrari and complaining about putting gasoline in it.

 

[Piobaire]

Also, if I understand the Fargen correctly, it is three distinct power circuits, so if you played each "decade" equally, you're looking at new tubes every 12 years, based on four years per power circuit.

 

[gomestar]

i think tube use is still tube use regardless of the circuitry. They'd still be under the "4 year" assumption, though maybe a little less given how much the amp pushes the tubes.

 

[ac_slater]

He was being sarcastic.