Would you wire your 16 ohm series 2x12 8 ohm parallel to match your 8 ohm 4x12?

[WRGKMC]

It all depends on how much tolerance the transformer has and how loud you crank it. If this was a Marshall Plexi for example I know those amps and don't suggest running in between impedances on them. You may get away with it on other amps and to your ears it may sound fine, but I spent enough years repairing amps to know better. You may run it for ten years with an oddball impedance but when the amp gets older the tolerance of parts drift and all of a sudden you blow tubes or even a transformer.

 

Even with new amps you have components rated with tolerances of 1, 5, 10 & 20% tolerances. Your best gear is made with the lowest tolerances. God knows want a mass produced amp consists of. Without eyeballing all the parts, looking at the bands on the resistors , testing voltages etc. there's really not much advice you can give on how well and oddball impedance will fare.

 

Then you have the speakers tolerances too. 6 speakers being off by a half ohm each can be within tolerance even with good speakers. The question is whether that tolerance is less or more ohms and whether they icombine to push the transformers maximum load.

 

We really don't have any way of knowing these are going to be factors or not without some high quality testing with a scope, meter and stress testing the amp. Personally I don't own the amp and don't know it well enough to advise others to abuse it. If it were something like My old Fender Bassman I know those amps like the back of my hand. Its rated for 4 ohms, runs fine at 8 but it will run maybe a month at 16 ohms before the tubes cook and blow the grid resistors.

 

Tubes run best with balanced bias and balanced loads. If the power isn't consumed by the load it doesn't just disappear into thin air, it backs up and creates heat someplace and it usually affects the sound quality in the process.

[WRGKMC]

^i stand corrected, thx to the support

 

@WRGMC: my search for the schematic was not successfull, google didn't show me

i downloaded it, but especially the section with the switch is hard to read. i'm not used to these symbols, where the lines have crosses and the small circles, is this now a connection or not? will the extension speaker jacks short anything if something is plugged in etc...

this schematic is very confusing for me.

 

some other schematics are clearer drawn, but also some are confusing (at least for me as i'm not a pro schematic guru) when it comes to how switches drawn or sometimes pots or dual pots, and it gets worse if there is only a bad scan of a hand drawn schematic

 

Allot of newer schematics are like that. You should see some of the high tech gear schematics I work on. You can have ten 20X20 fold outs and the wires end with some coding and you have to switch to another schematic to see where they wind up going. The Connectors may have a --< as an input signal and a --> and an outgoing signal. Problem is not all engineers use the same connector codes.

 

The ones in that diagram are pretty easy though. The diagram is missing the actual transformer on that page. It just shows the connectors to it. They likely have a separate breakout with the detailed voltages and wire color codes on another page in the service manual. This would make sense because its is a component that will likely require a substitute some day and having those details can be highly beneficial for a tech finding a compatible transformer.

 

 

The diagram doesn't show the actual wires going to the internal speakers which is also typical. The schematic may apply to a head as well as a combo and they just solder the internal wires to the extension jack. Many times they reuse older schematics to build other newer heads too. They can just take their cad program and redo the areas they want to modify and leave other parts alone. I wouldn't doubt that schematic started off as a head and they just stuck it in a combo cab. It also explains why they didn't have those internal speaker wires on that page. As a tech you get used to seeing that kind of stuff.

 

If it were a head and you did stick it in a cab, and you did wire the speakers in direct the most obvious place to wire the speakers in would be to the same jack the extensions use. Its open and easy to get to. .

[t_e_l_e]

 

Allot of newer schematics are like that. You should see some of the high tech gear schematics I work on. You can have ten 20X20 fold outs and the wires end with some coding and you have to switch to another schematic to see where they wind up going. The Connectors may have a --< as an input signal and a --> and an outgoing signal. Problem is not all engineers use the same connector codes.

 

The ones in that diagram are pretty easy though. The diagram is missing the actual transformer on that page. It just shows the connectors to it. They likely have a separate breakout with the detailed voltages and wire color codes on another page in the service manual. This would make sense because its is a component that will likely require a substitute some day and having those details can be highly beneficial for a tech finding a compatible transformer.

 

 

The diagram doesn't show the actual wires going to the internal speakers which is also typical. The schematic may apply to a head as well as a combo and they just solder the internal wires to the extension jack. Many times they reuse older schematics to build other newer heads too. They can just take their cad program and redo the areas they want to modify and leave other parts alone. I wouldn't doubt that schematic started off as a head and they just stuck it in a combo cab. It also explains why they didn't have those internal speaker wires on that page. As a tech you get used to seeing that kind of stuff.

 

If it were a head and you did stick it in a cab, and you did wire the speakers in direct the most obvious place to wire the speakers in would be to the same jack the extensions use. Its open and easy to get to. .

 

thx for sharing information. yep thats what i'm mostly missing -> experience. mostly experience in trouble shooting. i know how things work in principle can read schematics to some extend and built something up from scratch with "good" instructions (like BYOC fx pedals i did, and their tweed royal amp i now mainly use).

 

but damn you, if something doesn't work. to find that cold solder joint, or the broken part if it isn't visibly burned is extremely hard without a problem solving tactic and experience

 

and unfortunately this they were not able to teach us in electrical engineering school 20 years ago, problem solving competence and experience

 

but instead in theory i know how an amplifier or a tv does work, or a digital signal processor...

[WRGKMC]

^^^ Learning the theory then putting it to practical use is a must to become good at troubleshooting. You do learn allot from other techs both good an bad too.

 

Setting up a systematic approach and using deductive reasoning and testing to rule out areas is how it usually goes. In many ways its detective work and you have to be good at tying together bits an pieces to the most practical conclusions. You may not always be right of course, but its much better then using a Symptomatic troubleshooting. As a service manager and tech, I found the guys who have the worst callback ratios are the guys who fix symptoms. Its a band aid approach to fixing an immediate problem without ever finding that actual cause which has good chance of reoccurring because the cause of the fault is never diagnosed.

 

You do have to look at all causes of a symptom not the symptom itself and find why that symptom occurred. The better you know the basics and the piece of gear you're working on the easier the troubleshooting becomes. But even then its all about the long term fix and quality of work. Most of the companies I work for track the quality of a fix in how many days the repair goes, costs parts used and all that kind of stuff. It makes it pretty easy to spot the guys who have it and those who don't.

 

If a tech does something long enough he may learn to do it well but it comes down to motivation just like any trade. You have some guitarists that are just great and others that aren't so hot. The techs who become the best, usually try and out do themselves by taking on the most difficult challenges and never giving up. Believe me I've had some challenges that would absolutely boggle the mind. You go through a piece of gear and rule out every single component and still it wont work right. An what winds up kicking your ass is something so stupid simple its not even on your radar screen as being a possibility.

 

As far as help and bailing out on such challenges the words give up aren't even in your dictionary. It can become economically unproductive to continue working on something due to the hours spent but giving up because you cant figure it out is a losers perspective you just don't accept. Winners don't give up. If they need to retrain, they do it, If they need to take a break and rest the mind, then find a new approach they do it. They may ask others for opinions, dig through manuals or ask engineers. Worse comes to worse you swap assemblies or boards to try and localize the problem and is they are down to the chassis and harness.

 

Then when you fail with all of that that you pull a revolver out and shoot yourself. (just kidding of course) I been there though and had to start the entire process over again, more then one time. Someplace you missed a step. You cant even rule out new parts being bad or blowing instantly. The whole idea is to force a change and hopefully force the cause of the problem into the open.

 

Of course some things have become too difficult to repair like you did in the past. I grew up repairing boards to component level. I was pretty darn good at it too. As mass production of boards became cheaper it became more efficient and less costly to just swap boards then send those into a lab and just have a computer analyze them. You could have one good tech analyze the problem and dozens of trained trolls just changing the bad parts out.

 

This is the way of nearly all electronics now. Even engineers have to be ultra competent to stay at the top of the pack where the creative stuff happens, otherwise they may wind up being given the menial task of how to save five cents in raw material by figuring out how to reuse scrap chassis metal. Spending a little tome on practical hands on work can give an engineer a whole different perspective on how dumb some designs are.

[DeepEnd]

Once again, the taps are for nominal impedances only. Real speakers are all over the map. Here's the frequency response/impedance curve for a Jensen C10Q, nominally an 8 Ohm speaker. http://www.jensentone.com/vintage_ceramic/c10q. DC resistance is specified as 6 Ohms. But if you look at the chart, the actual impedance (the lower curve) is around 7.5 Ohms minimum and there's a peak of about 65 Ohms at around 90 Hz (the resonance frequency is 96 Hz).

[eeddings]

As I mentioned previously, you can do it. Speaker impedance is kind of an average anyway with higher and lower values at different frequencies. But I believe you'll end up with roughly twice as much power going to the external cab as you will to the internal speakers because of the lower (by 1/2) impedance of the cab. As long as both sets of speakers (internal and external) can handle it, you should be fine. With any luck, someone who knows more about this stuff will either confirm or disprove this. Personally, I'd match impedances on principle but that's me.

 

I think matching impedances is my best bet. Really I was surprised that Bugera customer care said I could mismatch, when it says in the manual not to. I certainly don't want to do anything that will jeopardize the health of the amplifier, long or short term. I do enjoy reading all of these insights on this piece of gear and on amplifiers and speakers in general. WRGKMC wrote some keen interesting inside views into the art of troubleshooting amps that I find fascinating.

 

You can always recognize the people who really know their stuff just by how they write about it. Many thanks t_e_l_e, WRGKMC, and DeepEnd for your responses in this thread and for the help offered. I know it takes time and I know how precious time can be.

 

[WRGKMC]

Bugera is into selling amps and they know once you swap the speakers from the originals your warrantee is void so why would they care if its the best advice.

 

I doubt you even contacted a tech however. I'm not sure how big the company is but its highly unlikely you'd have a Tech or engineer responding to emails. They may have some office worker doing this and blowing off customers with questions like this. Their job is to make people feel good and attempt to fix as many problems as they can without them having to take the amp in and do free warrantee repairs.

 

I've done tech support work with many companies but in my case it was mostly for technicians doing repairs. That is a big step up from Customer support but you still get allot of dumb questions on stuff the techs already know shouldn't be done. Customers are just a hundred times worse.

 

I suggest you save that email correspondence. It will come in very handy if your amp ever has problems with the power section. They have advised you its ok to abuse your amp and are thereby responsible for any damages that occur in a court of law including the amp catching on fire, burning down a building and killing someone. Not to say this would happen, but an experienced tech who knew the legal ramifications of such advise would never give it.

 

There have been amps that have caught on file by the way. Look at what happened to Music Man Amps before they were modified. With todays lawyers that company would have been bankrupt the next day and Leo Fender would have died a pauper.

[eeddings]

Oh, I didn't send them any correspondence regarding swapping out the speakers -- I asked those questions here only. I only asked Bugera about running an external cab in addition to the internal speakers, which it sounds to me like the internal speakers are basically just treated like an external cab based on how they plug in with the 1/4" jack.

[DeepEnd]

You mentioned previously building a 2X12 cab. Your best bet is to make it a 16 Ohm cab so that your amp sees two 16 Ohm loads in parallel or a single 8 Ohm load. That means two 8 Ohm speakers in series since 32 Ohm speakers are rare. Since the full output of your amp will be going to each speaker rather than being split the way a parallel connection would, you'll need individual speakers that can handle the full output of your Bugera, 120 Watts RMS. Once again, find out what's in the cab you like and buy the same brand and--if possible--series in 8 Ohms. If you still want to use Celestions, the G12T "Hot 100" should work and will run you around $200 for a pair: http://www.musiciansfriend.com/accessories/celestion-g12t-hot-100-100w-12-guitar-speaker.