Fender BXR 400 Dual Bass Bridged ?

[dogface]

 Hey really enjoying this forum, already got some great advice and can't believe what a great place this is.

Here's my latest brainstorm. I have  BXR 400 I used as a gigging amp for several years. I've always wondered if this two power amp Bi-amp for two different speakers setup could be readily bridged so it is a sho-nuff 400 watt amp feeding a single cabinet with all the watts.

Is this possible or just a crazy idea ?  :duh   It's a really well made and powerful amp, I hate to just let it slip away. I would like to make it into a more useable configuration. Thanks for any info.

[Roly]

Hey really enjoying this forum, already got some great advice and can't believe what a great place this is.

And worth every penny.   8|

BXR 400 I used as a gigging amp for several years. I've always wondered if this two power amp Bi-amp for two different speakers setup could be readily bridged

Don't know about the BXR 400 specifically (and I can't find a circuit) but generally bi-amps differ from stereo rack amps in that the two power amplifiers are not identical, each crafted to its intended bandwidth and speaker load, so they would not normally be bridge-able - they need to be identical, and even then it can be problematic.

A circuit will tell all.

[Enzo]

as far as I know, this is a straight stereo power amp, with biamp crossover inputs available.  To bridge a stereo amp, you need some way to invert the signal at one power amp input, and then you take the output from the positive pole of each channel to the speaker.  Without binding posts, that is hard to do.  This amp is 25-30 years old, so predates speakon connectors.

I think the schematic will fit on this forum

[g1]

  These are nice amps for servicing and quite reliable.
I'll start from the other angle and ask whether this is practical.  You will need a cab that is rated for 400watts at double the impedance of what the minimum is for each channel.  I'm pretty sure the minimum is 4ohms per channel.  So you need an 8ohm cab rated for 400watts or better.
  Now what's the chance it's a single speaker in that cab?  If it isn't, the easiest thing is to rewired the cab into 2 banks of speakers with 2 inputs.  Now you can get full power into a single cab.
  Otherwise, it's bi-amp capable, but also has a switch for mono, so the 2 amps are the same as Enzo mentioned.
  No big deal to make an adapter cable for the outputs, but you still need an inversion of one of the input signals.  This would probably be possible by reconfiguring the biamp mode.

[dogface]

I never really gave this a lot of thought because I ran it with the original bi-amp spectrum bottom, or just one side into a 1x15 later. I have since gone to a more conventional head as my main rig.

Do I understand correctly that in mono mode, into two cabinets, I'm getting the same wattage and frequencies into the two cabs ? I just never did it that way. In that case, I just forget my brainstorm.   :duh

If that's the case, I'll just leave it alone and keep it for a back up. I'll get a couple of fridges for it hehe.   :lmao:

It does have hum issues but I'm going to do the solder joint ground connection filter cap inspection to see to that first .
There is a balance pot on it that got the shaft and knob broken off by some knucklehead little brothers of a band member who "borrowed" our gear one day. Can someone reference me a part number for that ? , It's about the only thing really broken on the amp.

Many thanks !

[g1]

  Yes, you can get the full power of the amp into a pair of cabs, both getting full frequencies.
It will be 200W per side at 4 ohms for a total of 400W.
So no advantage to going bridged unless you need to get the full 400W into a single 8 ohm speaker.
The balance pot is 0026416, knob is 0026402.  They may not be available anymore so you may need to contact Fender and ask if they have any numbers for substitutes.
  Worst case, if you are going to use mono with balance centred anyway, you can set what's left of the pot, or use resistors.

[dogface]

Thanks G1. Think I'll just keep it as is, look for the obvious age issues, and not sweat the balance pot if it's not available. Many Thanks !

[J M Fahey]

LOVED it, very good old skool design, down to the *taped*  PCB designs, (no real PCs way back then), bring lots of old memories back.

Autotrax and Tango PCB were launched around '88 , one year later, go figure.

It must have been the flagship of the company, still a heavyweight (literally)  today.

Use it as-is: 200W RMS per side into 4 ohms.

Driving 2 fridges? ....... excellent idea

Or 2 old school large 2x15", think Peavey.

Just for fun, put it side by side with *any*  modern Class D "500W" 2x10" combo which seems to be everybody's offer today, and let it loose.

[dogface]

I opened the critter up today and it was really clean, everything in nice shape. All solder joints and grounds seem to be good. I was able to determine that the hum is a mechanical hum from the power tranny. Does this indicate anything perhaps on the horizon or is this just the nature of the beast ?

Many Thanks.

[Enzo]

Lots of transformers hum or buzz.  If any mounting hardware is loose, that can contribute, but the windings sometimes buzz against the laminations.  Usually no big deal.

[Roly]

Does this indicate anything perhaps on the horizon or is this just the nature of the beast ?

Transformers buzz (or hum).  They can super clamp the laminations, they can vacuum pot the windings, but the bottom line is magetostriction - the core changes size by a tiny amount with magnetisation, and like thermal expansion and contraction it's an irresistible force that can never be entirely stopped.

Try stopping next to your local electricity sub-station or distribution switch yard on a quiet night and you will hear the big trannies quietly, or not so quietly, humming to themselves.  At high load times like early evening they can make quite a racket.  In fact complaints about hum noise from sub-station transformers is a significant problem for all electricity distributors.

A tranny with a loose lamination can make quite buzzing racket, and you can often make these better by tightening the clamps or a spot of glue, but a humming tranny is a happy tranny (but a growling tranny is overloaded).

Note that output transformers in valve amps also "talk".  You can not only hear a clean sinewave (when you are running into a dummy load, naturally), you can also pick the clipping point by the onset of rasp on the clean note.  I use this all the time.


{ed: typo}

[dogface]

Yes interesting I read a bit about the how the alternating cycle does this. You'll probably laugh I was using this beast as a practice amp.Never was an issue at band levels of course.I may look again just for giggles to see if all those years of low frequency pounding  have loosed something. I read about potting just like pickups very interesting. I'll stick to my combos for practice :-)

[Roly]

There is no real reason why you can't use your Flamethrower 1000 for practice (apart from the risk of dropping your guitar and having to replaster your bedroom), but there is something to keep in mind, and that is that amp specs are generally referenced to maximum power output.

If the spec sheet says "Hum and noise less than 60dB ref full output" it should be apparent that the absolute residual noise level will be quite a bit higher for a 1000 watt amp than it would be for a 10 watt amp.

In a typical on-stage situation the hum and noise of a 1000 watt amp won't be audible in the front row, say 30 feet from the amp, but sitting next to it in a quiet bedroom it will be unacceptably loud, while 60dB below 10 watts might be almost inaudible.

That said, big amp builders generally take a bit more care and try to get a better hum and noise spec, say -90dB ref full power, so that the amp residual noise is acceptable in absolute terms even in a quiet environment.  Even if it is insignificant in intended use it will be auditioned in a much quieter music shop and an audible noise level is not a good selling point.

{JMF makes the point elsewhere contrasting Fender who short in the preamp input when the instrument is unplugged, to Marshall who mute the output of the preamp to remove this source of residual noise.}

[Enzo]

Well, amps are not monoliths.  I'd wager the majority of that noise that is 60db down comes not from the power amp, but mainly the preamp.  SO in my weary state this evening, it makes me think that turning the master way down for bedroom use will be turning down the noise too.  Consider that the preamp in the 10 watt amp and the 1000 watt amp ought to be more or less the same, at least academically.  So turning the power amp down to 10 watt levels ought to turn the noise down to similar levels.

or in other words, the noise of a 1000 watt amp set for stage levels (and no input signal just for this comparison) might be objectionably loud, but turning the master to 1 would reduce that noise level considerably.