speaker protection 25 watt Blackbacks Idea

[ckpop]

I wanted to throw an idea out there and get some input if there is a better way. I recently put together a Marshall 4 x12 cab with original Celestion 25 watt Blackbacks. My concern is pushing them to hard with a 100 watt Marshall. While I know there is no guarantee on blowing a speaker because some guys never have a problem using a 100 watt head with one 4x12 and other guys do.
My Idea is to get 4 16ohm speaker motors from weber and mount them in a box to act as a second cab.
The purpose is only to dissipate power from the one 4x12 for a safety leash.
Here is some other facts for the big picture

This is for recording purposes only in a major studio were high volume levels are not uncommon. For live yes a attenuator would do but thats not what I am looking.

Pulling two two's is not what I want.


How does the speaker motor idea sound to you guys.

[Cantplay]

4 12's wired in series/parallel gives 16 ohm total.

If you plug it to the 8 ohm tap on the amp it will only draw half the power.

John

[ckpop]

Thats what I had in mind 4 16ohm speaker motors in series/parallel exactly wired like the 16 ohm 4x12 running the amp at 8 ohm. Weber does have a single 100 watt speaker motor but I was think of using 4, 50 watt wired so the power dissipates to each motor like a regular 4 x 12. So this setup in mind the amp is seperating what what would be 2 4x12's or 8 speakers.

seems like work to do all this but I want the amp to see what looks like 2 4x12's without a actual second 4x12. I want the dummy load to be reactive.
Am I overthinking on this one folks ? To me this seems like a real power soak

[Cantplay]

No.

Just plug your one 4/12 into the 8 ohm tap.

That will put 16 ohm load on the 8 ohm tap and only draw half power.

I= V/R

John

[ckpop]

Hey John,
I thought of that but I want to avoid the whole impedance mismatch issue. Once you do that the sound starts to change and it's different then seeing the correct speaker load. Not to say it won't work but if the amp is getting pushed hard the 8 to 16 is much tougher on the tubes and transformer.

Carl

[Cantplay]

The idea is to push the amp to the distorted sound you like, but at reasonable volume that won't crash a ribbon mic, or get into too many room issues when you record, and your original thought of protecting the speakers.

Its actually easier on the transformer.

You can always use a higher impedance speaker on a lower ohm tap. Its when you do the reverse that things fry because you draw double the current.

John

[selloutrr]

Keep in mind your 100W head is only putting out ~85W. The speakers are 25W x4 =100W and can handle easily more.

The weber Idea is ok.. but it's going to give you the same effect as running at 8ohms.

The weber baskets are more prone to failure then your speakers. If you use them.. make sure you keep them cool and give them breaks from heat.

I'd just run your head directly into the cabinet. Keep the head in control room. put the cabinet in an iso room. Use an SPL meter to adjust volumes with in 70% of the ribbon microphones Max SPL. Then before you place the ribbon microphone try all your pedal settings and play a couple times to make sure no volume surges max out the SPL limits of the microphone. if all is good place microphone and hit record.

[Cantplay]

Look at it as resistors, what wattage do you get?


|---16---| |---16---|
------------| |---| |-----------
|---16---| |---16---|

If those are all 25W resistors, its a 50w total, no? in series you add the wattage so you have 2 50W resistors in series for 50w total at 16 ohms

John

[sliberty]

When I was a teen, I bought a used HiWatt Custom 100, and a used Marshall 4x12 with whatever was original at the time (probably G12M-25's). After about 1 week of cranking my new rig, I blew three of the four speakers. I couldn't understand how this was possible. Afterall, I had a 100 watt cabinet and a 100 watt head. Why wouldn't the cabinet handle the power????

Anyway....

So, I bought a second cabinet - a blue Univox 2x12. I used it along with the original 4x12 (with 3 recones and 1 original speaker), and never had another problem.

The idea of using Weber speaker motors has occurred to me as well. But I opted not to go there for some reason that I cannot remember. If you do this, I'd recommend wiring the 4 motors as though they were a second 4x12, and make it all switchable so that you can use 1 or 2 4x12's at your descretion. When using smaller amps (50 watts for example), you'll want to eliminate the "2nd cabinet". But when playing your 100 watter, you will want the protection of using your virtual full stack.

[kdmay]

Two possible solutions IMO:

1/ Pull 2 tubes and run it on 50w

2/ Buy some Scumback BM75s (or other G12M Blackback equivalent)

I wouldn't run a 100w plexi into a 100w cab. Done it before and have taken out speakers. A plexi at around 8 (which is where I like to pay them) is probably pushing around 150w, so it's luck if you don't take them out.....

[selloutrr]

It's sounds like your ultimate goal in this is to
A: use a single cabinet laoded with 4x25W speakers
B: to run your 100W head hot to force some power tube compression

You best compromise if you are not willing to run it direct one to one. Is to add a Power soak (or similar device). push your amp into the power attenuator giving you control over the voicing/compression. Then dial the attenuation volume to work for the microphone/room.

Keep in mind every adaptation will in someway compromise / change the tone. You will need to dial the amplifier in after you setup for the tone you desire. It's no different then playing live gigs and balancing to the room.

If it's a large room consider running multiple cabinets to help load the room. this will solve your wattage issue. Then simply only mic the speakers you want as the focus in your mix.

[Cliff Schecht]

Look at it as resistors, what wattage do you get?


|---16---| |---16---|
------------| |---| |-----------
|---16---| |---16---|

If those are all 25W resistors, its a 50w total, no? in series you add the wattage so you have 2 50W resistors in series for 50w total at 16 ohms

John

This is wrong. The wattage doesn't scale like resistance. The wattage adds up whether resistors are in series or parallel, it's the resistance that is doubled by series and halved by parallel connections.

[ckpop]

Thanks guys for your input and suggestions, they help alot........I am going to have to think about this one a bit before I figure the best way to go.